PICASTAR First LO Squarer

Warning

If you decide to use any of the information here, you will make your PICASTAR non-standard. Even if you wish to do the changes on a 'new-build' STAR, please build it first using all the standard build information, with no changes from the published design. Also you need to calibrate it using all the excellent BASIC programmes, to verify that it works correctly, before you modify it. This is what I did myself.

If you do not 'RTFM' (read and use all the PICASTAR documentation in the Yahoo! Groups Pic-a-Project group) before you modify your build in any way, then no-one will be able to help you to get your own build working. You must join this group in order to obtain (and be permitted to use) the design information that you need. If you then need help (with an un-modified STAR) you will get it by posting your problem on the picastar-users group. If your STAR is modified in any way, then the very helpful people on the picastar-users group will probably not be able to help you.

If things work so badly that you are unable to make it operate, then consider asking first on the picastar-users group. But read my suggestions anyway...

Introduction

When I set about commissioning my newly built ComboStar (PICASTAR using Glenn's excellent 'all-in-one' PCB) I found that the first mixer didn't work properly, especially at the higher LO frequencies. I began by looking at the logic drives to the H-bridge bus switches.

What I found was that, at low LO frequencies, the squarer (IC503) buffer outputs on pins 8 and 11 were far from square waves; poor duty cycle with 'feathers' on the transitions due to oscillations somewhere. At high LO frequencies, there was nothing but a low-frequency relaxation waveform at both outputs. This was clearly not any use, so the process of investigation began. Had I used an incorrect component, or left something unsoldered?
To avoid any suggestion that I had 'built it wrongly' I replaced every component of the squarer and the LO and its output filter, and measured the new parts before fitting them. I also used a Fairchild rather than T.I. version of the 74AC86. It was exactly as before.

The purpose of the squarer is to produce a square wave at LO frequency for operating an 'H-mode mixer' which is just 4 FETS in a H-bridge chopping the incoming RF at LO frequency to produce the IF.
This whole thing (squarer plus H-mode switches) is a replacement for the original diode double-balanced mixer, since switch-based mixers have (it seems) better intercept & lower distortion. The original modular diode mixer had 50R +7dBm LO port spec, which therefore became the input for the squarer, but in reality the squarer needs volts, not power, since it is a FET stage.

I present two levels of improvement here. The first is possible for anyone to do on an existing PCB. The second requires changes to board layout for good results and, although spectacular, cannot be done retrospectively.

Findings

As a PICASTAR novice, I had not expected that there would be problems in this area; it seemed as if no-one else had this problem. As time has passed, I know that many constructors have had the same or similar problems and have devised workarounds in many different ways, including wholesale substitution of different circuitry. But I felt that it was necessary to understand why this standard design didn't work for me, and to try to fix it as non-invasively as possible. As a long-stop I had in mind the possibility of using two of the amazing Analog Devices ADCMP601 'super-comparators', which can have internal hysteresis and can directly drive the bus switches, but this was not my preferred route since it would need board changes.

The original circuit, from Glenn's ComboBoard, is shown below for reference:
Original Squarer Circuit

When I refer to the SQUARER it is the two sections of IC503 with low-numbered pins (left-hand side of the symbol in the schematic), which is effectively an amplifier overdriven with the LO sine wave "Local Osc Input". The second part of IC503 (right-hand side of the the schematic symbol) BUFFERs the squarer output and produces anti-phase logic drives for the H-switches of the mixer.

Brief Description of Squarer Circuit

In the schematic above, the 470R R508 acts as a 'grid stopper' to isolate the LO Input coax from the IC503 XOR input pin (I believe the original design uses 270R). This reduces the likelihood of the track and coax acting as a resonator which can exacerbate oscillation by providing phase shift at IC503 input (pin 2). Any such resonance is heavily damped by the resistor. The 10k (R536) feeds back a voltage which is the averaged output of the squarer, presumably to provide some d.c. -ve feedback to stabilise the input switching point.
The "BALANCE" potentiometer can move this bias point somewhat (but only in one direction) and has been found useful by some constructors in order to minimise 'birdies'.
R503 (56R) nominally terminates the LO Buffer and the short coax from the DDS Filter, and C507 (1nF) provides the necessary d.c. blocking for the XOR with its roughly mid-rail threshold voltage.
Within IC503, two XOR sections (pins 1,2,3 and 4,5,6) are configured as inverting and non-inverting respectively by strapping pins 1 and 5 to opposite supplies. This gives the overall inverting configuration that puts the feedback of the d.c. operating point in the correct inversion to work.

Gate Thresholds

See more about Gate Thresholds in the Appendix
The averaging circuit R527, C533 at the squarer output (IC503 pin 6) is, I believe, intended to set the bias for the input gate (IC503 pin 2) by averaging the rectangular wave at the output and feeding back the mean level to the input pin via R526. The component values are chosen to remove the HF signal present at pin 5 of the XOR, just leaving a d.c. level that is nominally mid-rail for a square-wave, becoming less positive if the output spends too much of the cycle at 0v and vice versa. If the gate threshold were at mid-rail, then this could be a good way to maintain an equal mark:space at the output. We may expect the bias voltage to shift to reflect  the lack of squareness, thereby altering the response to the incoming sine wave in such a way that the slope of the sine wave moves relative to the CMOS threshold, nominally correcting the output squareness.

At least, this is the way it seems at first glance.

Of course, the CMOS gate threshold is only typically at mid-rail. It can be between one third and two thirds of the rail and is still within specification, according to the data-sheet.

So let us think about how the fed back bias is produced. It is an averaged version of the squarer output (on pin 6 of the XOR). This averaging is performed by the RC network consisting of R527 (10k) and C533 (100nF), which has a time constant of 1000us, much longer than the period of the lowest LO frequency (just over 10MHz, say 100ns).
This network feeds back the average value of the output waveform. But we need a square-wave at the output, and the only ways that the average can change are:
If the waveform is a square-wave, the averager feeds back exactly half the chip output excursion to bias the gate. For a nominal device, this sounds fine at face value, but what if the chip threshold is not at this level?

If this were a self-biased linear op-amp circuit (let us say a voltage follower), the negative feedback would act to hold the output at the same voltage as the input and would be a representation of the input. We have negative feedback - so what is different?
The difference is that the squarer is definitely not a linear circuit. It is strongly overdriven and the output is a clipped and amplified version of the input (the LO). The only way that the fed-back threshold voltage can vary dynamically is for the waveform to change from being a square-wave, i.e. the mark:space ratio changes. In other words, in order to produce the threshold voltage of the gate, which it must do if we are to produce a square-wave, the output will not be a square-wave unless the threshold just happens to be at one specific level.

There is a provision for a further modification of the fed-back voltage in the presence of the Pot RV501 (500k) which forms a potential divider at the averaging capacitor node. However, as designed, this adjuster can only serve to reduce the node voltage; it cannot increase it.

Because our desired output is a square-wave, we must adjust the pot RV501 until the voltage fed back is equal to the gate input threshold, so that the incoming a.c. coupled sine-wave is symmetrically clipped by the circuit. Any bias voltage other than this will not produce a square-wave at the output.
Since we can only use RV501 to reduce the level produced by averaging the output, there is no way that a square-wave will be produced if the threshold of the chip is above the nominal mid-rail. It must then produce a non-1:1 mark:space, which is not at all what we want.

Steve G7WAS found that he could not obtain a square-wave at the output, and modified his circuit as I describe below, to use RV501 across the whole supply in order to allow any bias level to be set. With this he was then easily able to set the potentiometer to produce a square-wave. Since the pot setting was now at the device threshold, he was able to measure the pot voltage and tell me the threshold of his own chip.
 Steve said:  "I modified the MR squarer by using the supply rail and balance pot. It cured the asymmetry see attached image; it is quite typical of all the bands, some small variation with frequency. But much better than original circuit which showed asymmetry when fully adjusted".
Steve G7WAS squarer outputs
This is what Steve now obtains as bus-switch drives (Note that one trace is inverted for clarity).

We might expect, then, that Steve's chip had a threshold above the typical mid-rail level, which is impossible to compensate with the original pot configuration.

He made some measurements:
Chip supply:    5.015V
Pot setting:    2.425V
So Steve's chip threshold was a fair bit lower than the mid-rail.
Why didn't the original pot configuration provide adjustment, as we have expected?

The output waveform is averaged to provide the bias feedback, as described. But remember - it is the output waveform, not a swing between rails, that is averaged.

The apparent discrepancy is actually produced by the 'official mod' resistor RSQ1 (470R) which loads the 'high' condition of the CMOS output and, together with the 'on'-resistance of the PMOS FET that is trying to pull a high level, reduces the high level at the pin to somewhat less than the supply voltage. The low level is a good 0V, both from the chip and the resistor.

Taking the manufacturer's published data-sheet figures (for high output level at 24mA for supplies of 4.5V and 5.5V), there is a voltage drop in the PFET of 0.74V for 24mA sink, at both supply values (4.5V and 5.5V).
Adjusting this linearly (the PFET is resistive in saturation) for a load current of 10mA (approximate value for a 470R load RSQ1 and 5V supply) we expect a drop of up to 0.31V in the upper (PMOS) output FET in the gate.

This means, for Steve's case, with a supply of 5.015V, the output at pin 6 would swing between 0V and (5.015 - 0.31)V, giving a mean value for a square-wave output of 2.35V
This 2.35V feedback is already below the gate threshold of Steve's chip (2.425V) so no amount of adjusting the pot could ever cause the threshold to be reached; it can only reduce the voltage in the standard configuration. Therefore, because the input is biassed below threshold, the input LO waveform (a.c. coupled sine wave) spends more time above than below the threshold. Because the squarer is inverting overall, this means the output is 'low' for longer than it is 'high', which although it allows a stable bias to be fed back, this is not as a result of having a square-wave output.

So for Steve the pot connection 'as a potentiometer' between the supplies is the only way to obtain the square-wave he expects at pin 6.
This will apply to most constructors with chips that have even below the typical 'mid-rail' threshold.

RSQ1 may have reduced the tendency for the stage to oscillate, but it also reduces the chances of getting a square-wave out.
The solution is to re-deploy the Potentiometer as described below, with the pot across the supply-ground, to provide an adjustable voltage that tracks the chip threshold (both are a proportion of the supply) and permits the full range of possible chip threshold to be compensated.

New connection for RV501

A much better bias arrangement would be to put RV501 as a potential divider across the supply rail, and use it as the means of setting the chip bias voltage (as a part of the supply). Forget using the output signal in feedback - leave it open loop. With an AC86, the  threshold tracks the supply voltage anyway (it a percentage of the supply), which is what the potentiometer also does.
The only 'better' way would be to derive a die-dependent threshold voltage from an otherwise unused gate on the same die, which is often done when single-stage inverters are used. Sadly, the chances of linearly self-biasing a multi-stage gate (like the XOR, or a 'buffered' inverter) are zero, it simply won't happen; the stage simply oscillates at high frequency, which is not what we want. In any case, the XOR is a highly complex gate with both inverting and non-inverting paths; it is simply unsuitable for threshold generation (or any other linear biassing). In the sketch below I have not drawn R529 (22k) which is in series with the pot wiper; this can remain in circuit between the wiper and the C533/R526 junction:
Revised connection for RV501 allows full range

This ('open-loop' bias using RV501) is probably the best we can do.
It may be 'open-loop', but still tracks the supply in the same was as does the chip threshold - both are a proportion of the supply voltage, and when the pot is set to produce the chip threshold they are both at the identical proportion).

Physically the change is not ridiculously difficult. One end of RV510 track is at ground, and by removing R529 we can link the other end of the pot track to the 5v supply. But at present the wiper is connected to ground. We must remove this connection, to be able to use it as the variable bias voltage feed. Remove the original feedback resistor from the pin-6 output (R527, 10k) completely,  since we don't want the output squareness to set the bias point. Then connect the liberated pot wiper to C533, to inject the bias. Perhaps the easiest practical way is to remove the pot RV510 and glue it to the board surface to give access to the lead-out wires, then connect them appropriately.

 All we need do then is to adjust RV501 for a symmetrical square wave from IC503 pin 6. How? Maybe using a timer-counter to measure both high & low intervals. Maybe by using a 'scope to get close and then tuning RV501 for 'best results'. How did you intend verifying that it was 50:50 before you read this? Do it like that. Steve G7WAS used a scope that displays duty-cycle.
Beware, though, of trying to use an RC averager to set duty-cycle for half-rail - the load of RSQ1 causes an incorrect result unless it is done at either pin 8 or pin 11, where the logic level is buffered.

Note that this method is functionally little different to the present design. There is absolutely no 'auto corrective feedback' such as might just possibly have existed with the original (if we permit the output duty cycle to vary, which we don't), otherwise instead of using the output half-rail averaged signal as the pot  feed, we use the full supply. This allows us to adjust the circuit for almost any device threshold voltage (but at extremes, beware that the tips of the LO sine wave must not exceed the supply/ground voltages or non-linearity and poor performance must result as the protection diodes conduct. Please do not be tempted to use a 74ACT86, which can have an input threshold voltage as low as 0.8V).

Once you have done this change, there is no longer any negative feedback from the squarer output (pin 6) to the input (pin 2) through R527 and R526. This removes the unstable feedback path that previously provided a low-frequency rectangular wave oscillation at the output if the LO is absent. You should expect that the residual difference between the bias from the pot and the chip threshold will be amplified by the cascaded XOR stages and will drive the output to a supply rail (if it drives high at the output, remember that RSQ1 will cause the level to fall short of Vcc by up to 0.3V, as described earlier). There is no feedback, so no unstable feedback loop and no L.F. instability as there was before.
Steve G7WAS confirms that his own Picastar, now with the pot modification, behaves exactly like this. He has adjusted for the best square-wave and now, in the absence of LO input, the pin 6 output sits at 4.917V for a chip supply of 5.019V, which is a shortfall of 0.1V as a result of driving RSQ1 (the chip spec worst case would give a drop of 0.3V). In this condition of "no-LO", RSQ1 (470R) is dissipating 53mW, which is safely within the 100mW rating of most 0603-body resistors, although some low-cost parts. e.g. from Multicomp, are rated at 63mW which is a bit close (you will be OK with 0805 or 1206 style).

Steve has kindly measured the XOR body temperature using a thermocouple meter, with the following result:

IC503 Temperature with and without LO signal - 'Pot Mod' done.
Condition
 Temperature after 15mins
Normal LO present
 41°C
LO absent
 36°C

This lower temperature with no LO signal indicates that there is no serious effect from internal instability when the RV510 pot mod is done as described.

Input Sensitivity

 The Squarer receives input sine-waves from the LO. Its task is to form the sine-wave into a logic level capable of operating the next stages. In order to do this, it must produce clean logic signals from the incoming sine-wave at all frequencies of interest. For a standard Picastar the LO is always higher than the signal frequency and the IF output from the first mixer is usually close to 10.7MHz, so the LO can be considered to produce signals from about 12MHz to 40MHz. We are told that an LO level of +7dBm is required. Certainly, my original LO level of 2 to 3 dBm did not result in proper operation of the squarer, but once I had solved that and obtained +7dBm it did seem to work - but with what margin?

There was still a tendency for the squarer to produce slight feathery oscillations at the output transitions; maybe there were even more significant internal device oscillations that could not be seen at the output. These may account for the 'birdies' that are alterable by adjusting RV501 - also the 'later addition' of RSQ1 as a heavy load on the squarer (pin 6) acts both to reduce the gain of the second gate and shift  its input threshold slightly, both of which will help prevent parasitics at transitions.

The residual parasitics were reduced by some simple changes, as follows:
The circuit behaves as before, but now the new 100R directly at pin 2 of IC 503 provides isolation of the entire long track. The fed back bias level from R526 now feeds through this new resistor, but the function is identical. 100R effectively in series with 10k (R526) is insignificant. Otherwise, the circuit is exactly as before (but the value of input resistor is reduced).

A value of 100R at IC503 pin 2 still gives good isolation of the long connection from the LO, but has the following advantage over the original 470R (R508):
The input capacitance of IC503 is specified as <6pF (from the Philips data sheet). This capacitance, plus tracking strays, together with R508 acts as a low-pass element. With the original 470R, this is -3dB at <38MHz, which is probably not a good idea for an LO that produces frequencies up to 40MHz and which is already struggling at the high frequencies. Replacing the 470R with 100R raises the corner to 160MHz, which is perfectly fine.
This applies to the revision above, with the new 100R right in series with the IC  pin. If this low value were used as a plain replacement for the 470R R508 then it would probably make things worse.

'No Signal' condition

From earlier observations, it was apparent that there was never a 'no output' situation, since the squarer self-oscillated at a low frequency with no input and produced a corresponding square wave output. It happened for Glenn VK3PE in just the same way; here is his 'scope trace at pin 6 of the squarer with no LO signal:
Glenn's trace of squarer self-oscillation with no LO

I have suggested implementing an "RV501 Pot Mod" due to inability to adjust for a 50:50 duty cycle with the original configuration. Because this no longer uses negative feedback from output to input of the squarer in order to provide bias, the unstable feedback mechanism that causes the oscillation with no LO is absent. The output sits at either a high or low static logic level in the absence of LO (for Steve G7WAS it is a high level).

This leads me to believe that (for my STAR at least) the official modifications that a.c. couple the squarer to the buffers (CSQ1 and CSQ2) are not needed, since there was never a condition of 'no signal from the squarer' and so the XOR sections with outputs on pins 8 and 11 could never see threshold inputs, which had apparently caused overheating for some people and was the reason for the change. None of the squarers for which I have measurements provides such threshold output with no LO input. However, there was no noticeable problem as a result of leaving these components present, so I retained them. The AC86 input protection diodes on pins 10 and 12 behaved as intended and d.c. restored the coupled signals.

It is worth a mention that not everyone has had success with the a.c. coupled buffers that drive the bus switch inputs.

These buffers are the remaining two XOR gates in IC503, on pins 8,9,10 and 11,12,13 with the lower numbered gate strapped to be non-inverting and the upper strapped as inverting, by taking one input pin to Gnd or Vcc in each case. The AC86 has the same specified propagation delay in both configurations, making it ideal as a single-ended to complementary drive converter.
The a.c. coupling 'official mod' was introduced in order to avoid these sections over-dissipating when fed with a threshold level from the squarer part; the pull-up/down resistors on pins 12,10 ensuring a static logic  level if there is no alternating signal provided by pin 6. The chip's input protection diodes serve to d.c. restore the a.c. coupled signals when they are present, to make it all work. I already mentioned that my own squarer self-oscillates at a low frequency in the absence of input, and does not produce a stable 'threshold-voltage' level on pin 6, but it may be that some constructors have experienced such a static level.

Others have fitted the CSQ modification, only to find that their buffers then do not work correctly. Bart Schrijver has had this trouble (he used a 74HCT86 rather than 74HC86 XOR). Bart says:
"I found on both of mine I had to add a 3.9k resistor to ground from pin 12 to make that arm switch. The other arm switches fine. This circuit also suffers from asymmetries, I think due to the logic switching levels that do not happen at mid rail. The suggested modifications, which add a resistor and capacitor may make things worse.  In my PICASTAR I used the 74AHCT86 versions which may be worse in this respect. I could not find the specified AC version, since these are becoming obsolete."

On the ComboBoard, Glenn has made provision for a resistor to be added by others who have this problem (RSQ4).However, the proper answer is not to try using the 74ACT device in place of the 74AC part.
I suspect that the obsolescence of the 74AC part that Bart reports is because the part number he used is that of a 'lead-enriched' part; all of which are now replaced by compliant lead-free parts with corresponding distinct part-name suffix.

Waveforms

The eventual drive waveforms from the 74AC86 pins 8 and 11, as shown on my 100MHz oscilloscope looked as good as could be expected. I used a short earth-clip lead, but the perturbations due to it are clearly visible on both traces.

This is the squarer output on 30m band setting
squarer buffer on 30m band

And below is the same measurement on the 10m band
squarer on 10m

The edge rates are similar to the 'scope rise times. Note that the "X10 MAG ON" button is pressed, so the timebase is 10ns/div on the 0.1 microseconds/div timebase setting. The trace shows a rise-time of around 2.5ns; the 'scope rise-time is "less than 3.5ns".  I wish I had a better 'scope - and a cleaner one! The rise & fall times of the signal are therefore 'very fast'; less than 2.5ns!

You have already seen the waveform obtained by Steve G7WAS. Steve's indicated rise and fall times are about 3ns. No doubt his scope also has probes with limited bandwidth, so his rise and fall times will be considerably less than 3ns also.

Board Layout

The commutation 'feathers' due to VHF instability at the signal transitions were exacerbated by the layout of the PCB.
Peter's original layout has very short track on the input (IC503 pin 1) and no other signal tracks run close. But Glenn's layout introduced significant length at the input, as well as close proximity to the output signals, which must increase the possibility of instability. Note that Glenn used the same layout for all his PCBs ('bricks' and Combo).

In the images below, the upper layout (Peter's original PCB) has very short tracking on pin 2, isolated by R8 and R26 immediately and with no other tracks adjacent. The output on pin 6 is well away from the input pin, partly because coupling capacitors are mounted over the chip body.
Peters Squarer layout

The lower layout is from my own ComboBoard (Glenn's layout) before any modifications. I have coloured two tracks for clarity. It is clear that not only is the track on pin 2 about as long as it could be, but also that the track to pin 6 (which is the stage output signal) passes close by as it connects to the following circuitry. Remember that Glenn has run the part numbers on this section of the Combo from 500, so R8 and R26 on Peter's layout become R508 and R526 respectively on Glenn's.
Glenn's Squarer layout

The conclusion that I came to is that Peter's original layout did the things that would assist stability, but Glenn's layout did not.
However, the modification presented earlier to reduce the parasitic oscillations is easily done on the ComboBoard and isolates the long track (red in the picture) from the chip input, which is what is needed. It is easy to do on the board and is also easily reversible. There is no easy way (you could consider cutting the track and using wire for a different route) to avoid the track from pin 6 (orange above) passing under the chip and therefore close to the pins and beneath the die.

Measurements of sensitivity

Glenn VK3PE has kindly made measurements of the minimum LO level that he found necessary in order to produce a useful output from the squarer. In these measurements, a lower input level requirement is better, of course.
He did not use a complete Picastar, but made up a test PCB containing little more than the squarer plus output buffers:

Glenn's test PCB - Combo layout

Glenn then measured the LO input level (from a signal generator) that was needed in order to give clean output signals with no observable parasitics or instability:

Graph of Signal to operate vs Frequency

The reduction in LO level needed in order to obtain a clean output is noticeable. At higher frequencies there is still a bit of a kink, but in general it is better behaved. The smallest LO level needed for the original is 4.5dBm; for the revision is 3.5dBm.

Although I don't have data to support it, Glenn achieved a maximum frequency of well over 100MHz with the revised version ("Change R508 to 0R and add 100R right at Pin 2" in the plot above), still with less than +7dBm input.

Do the mod; it is simple and is easily reversible should you wish.

100R at lifted pin 2
Bob's Combo with R508 as a 0R link and a 100R 0603 at the lifted pin 2. Lots of blobby soldering as the result of many experimental changes!

Schematic showing Squarer Mod

Recommended Changes to ComboBoard

For the pin 2 mod:

For the RV501 Pot mod:

This is more difficult to perform.
You can now adjust RV501 for best symmetry of the squarer output at pin 6, on the band of your choice.
It is OK to link the wiper to either end of R529, whatever is convenient (see schematic above)


Other Things

Following this investigation, Glenn and I devised and tested several alternative PCB layouts. One of these produced excellent results, with good sensitivity and freedom from instability on the transitions. The operating frequencies easily extended well beyond the range that would be needed. Because the PCB changes, these are not suitable for modifying the Combo (unless Glenn produces a new ComboBoard!). I have detailed them as an Appendix

Capacitively Coupled Drivers
The output of the Squarer is capacitively coupled to the two XOR buffer stages that produce the true and inverted drives to the Bus Switch device that is the H-Mode Mixer. These were provided (CSQ1 and CSQ2) in order that the buffers do not sit with their inputs at threshold in the absence of LO input. I had found that this did not happen; my own squarer oscillated at a low frequency with no LO input and still produced proper logic levels. Others have found the same. Nevertheless, rather than removing (shorting) the capacitors, it is OK to retain them. The Appendix looks at some considerations.
XOR slew-rate
It was suggested to me that the XOR used in the squarer and for the Bus Switch buffers does not have adequate slew-rate. I studied this in an Appendix and concluded that even at worst-case it is perfectly adequate. From the various oscilloscope traces you can see anyway that the rise-time achieved is less that 3ns. What more could you ask?

Appendix: More about CMOS gate thresholds

The saturated swing of any logic gate output or internal intermediate stage, being an amplified version of the input, has finite slew-rate, depending partly on the input amplitude and the stage's voltage gain (which is usually between 10 and 100 for a single CMOS stage of those within one XOR gate - such as one only of the pairs of FETS labelled 'p' and 'n' in the circuit below). During this slew, both the P & N-channel FETs of the stage will normally conduct, drawing a fairly large current through themselves from the supply to ground. This may cause instability, perhaps by the current in the ground bond-wire causing the source of the N-channel FETs to move slightly positive; equivalent to a feedback to the input (which is between the chip input pin and an external ground). This can cause instability during this time, often at VHF or UHF, and is caused by the inductance of the chip metallisation and bond wire. In normal digital operation, the input slews very rapidly through this threshold, but our incoming sine wave has finite slew-rate...

In our squarer, the output of this first stage then feeds the second (the XOR has four CMOS stages in cascade per gate) and the external connection between pins 3 and 4 feeds it on to the next gate and thence to the output on pin 6.

Obviously, as more such gain stages are cascaded, the later stages tend to switch more suddenly because the slew-rates get progressively higher (for our rather slowly slewing sine-wave input). At some point on the input slew, many stages are switching simultaneously and rapidly, all gulping supply current as they do so. That supply current all flows in common ground paths (such as the chip bond wire) and yanks up the internal ground relative to the external (PCB) ground. This can be oscillatory at VHF/UHF due to ground inductances, and represents a signal effectively between the input (pin 2) and it's N-channel source, which happens just as the FET pair is in the linear region and has gain... The result is oscillatory feedback until the input signal moves the input pin away from the linear region of the stage. The more complex the gate, and the more sections within the chip that are involved, the larger the ground perturbation will be, hence the more likely is instability. This was the source of my observed 'feathers' on output transitions. Since it is happening within a sensitive part of the receiver, it can give rise to all manner of squeaks and squawks. The best way to minimise the time spent with the input stage in the linear region is to drive it with a very fast-slewing signal. Obviously this is what a logic circuit does, but we are using a sine-wave and can only make it slew more rapidly through threshold by increasing the amplitude. This is why the amplitude corresponding to +7dBm is needed at the input. Even the 1.44vpp is not over-generous - if the first stage has a voltage gain of 10, then for the stage output to saturate (and therefore have no gain to input changes) then the stage must see at least a 0.5V 'chunk' of the sine-wave input; you can work out how long the input stays within this +-0.25V window (and hence is susceptible).

I have no internal circuit for the 74AC86, but below is the circuit of a single XOR gate within the TI CD4030, a similar part (but different pin-out and speed; it's a rather old part). As you can see, it is a very complex gate with oodles of PN 'gain stages' per gate!

Internal logic of one XOR gate

Philips very kindly give even more insight into the use of an unbuffered CMOS inverter in the linear mode (just a single stage per section), even showing how it can be used as an amplifier or oscillator. The most relevant items for us right now are the gate transfer characteristics, which I reproduce below:
circuit of a single-stage inverter

You can clearly see the single PN pair that is one inverter.
Below is the transfer characteristic of just this single stage:

74HCU04 transfer characteristic

The gulp of supply current (up to 12mA) as the input passes through the threshold is painfully apparent. The slow rise and fall of current as the input sweeps from 0 to Vcc is the result of each of the FETs progressively drawing more current through its saturated partner. At the point where the output passes through mid-rail they are each equally enhanced (and the current is at maximum).

Multiply the above peak current by the number of stages in an XOR gate to get a very rough idea of what the peak current drawn by that gate might be. We have 4 XOR gates cascaded, nominally biassed at threshold (peak current) and each gate contains 4 active PN pairs (per pin) so we could have 16 of the above elements all drawing their peak current of 12mA simultaneously through the metallisation and bond-wire; 192mA potentially.

Circuit designers place a low-ESR capacitor as directly between the supply pins of the chip, in order to provide this gulp of current so that it does not flow through the power tracks and affect other devices. It also hold the chip power pins at a reasonably steady voltage in the inevitable presence of supply-feed inductance. But not surprisingly there is no such capacitor within the IC, on the die, so the gulp of current does flow in the impedance of the die metallisation, bond wires and pins also. At this time, the supply voltages on the die can vary considerably. Remembering that the ground impedance is in series with the LO input signal, this transient can momentarily alter the apparent input signal and give unwanted VHF feedback. This is in addition to any output signal fed back by capacitance, and may produce transient instability (modern CMOS logic devices have very high internal bandwidth).

I wondered if Philips' data-sheet for their 74AC86 showed such helpful characteristics of the supply current vs input voltage. But they clearly don't think anyone would use it in a manner where this would be important (only as a digital gate).

Appendix: XOR Output Slew Rates

It has been suggested to me that the 74AC86 chip has inadequate rise and fall times for use on the 10m band. When this was suggested on 'the group', the response was to 'build it and see how it works', which is not entirely helpful.

So let's see.

For reference, I am using the Fairchild 74AC86 data sheet dated September 1988, Revised February 2005.

The input-to-output propagation delay affects symmetry of an output when fed by an input. For this device the range of both high-low and low-high delay is  1.5 to 8.5ns at Vcc=5V, with the typical for both being 4.5ns. This looks good. If they are always the same (e.g. both at minimum) for a given device, this is not a source of distortion. It is likely that they will be the same for a given chip (which is fortunately the one we used [sad attempt at humour, sorry]).

Sadly, for this data-sheet, this is the only dynamic characteristic specified. I guess we will need to look at either another manufacturer's data-sheet, or at ACMOS family characteristics.

The Fairchild 74AC Family Parameters data sheet "FACT(tm) Descriptions and Family Characteristics", November 1988 Revised January 2000, gives some generic parameters. On page 13 is a pair of curves showing rise & fall times with various load capacitances at a supply of 5V, for a normal gate (not a bus-buffer).  This shows that, with 10pF load, the rise-time is about 1.8ns and the fall-time about 1.4ns. Rise-time increases to just over 2ns with 20pF, while fall-time remains below 2ns. Of course, these are for a typical device at Vcc=5V, 25C, but they are the best we will  find.  Here they are:

ACMOS Rise time vs load capacitanceFairchild ACMOS Fall time vs load capacitance

Remember to look above at the curve for FACT AC, not FACT QS.

In the Picastar design, the input capacitance of the pairs of bus switches forming the H-mode mixer is isolated somewhat by the small (56R) resistors in series with the drivers, R501 and R502, which can just be seen in the scrap circuit well above. The input capacitance of a single Fairchild bus switch is typically 3pF (no limits given), so two in parallel give 6pF. If we allow a couple of pF for the PCB track and a bit in hand, we get 10pF. This is at the low end of the curves for AC device rise/fall times in the graphs above and will give less than 2ns rise-time. The series 56R resistors will add some, but with a time-constant of half a ns (56R, 10pF) it should not be much (the eventual value is 'root sum of squares' of the rise times, which will be a tad more than 2ns).
I would certainly expect the bus switch inputs to see a signal rise-time of around 2ns to 2.5ns.

With my 100MHz 'scope and x10 probe I measure around 2.5ns on my own STAR, but bear in mind that this is not only below the specified maximum 'scope rise-time, but also I have the extra capacitive load of the 'scope probe present. Nevertheless, I'm happy to call it 2.5ns drive rise-time and fall-time. It isn't more than this. Steve G7WAS also measured this with closely similar results.
At 10m the LO is roughly 40MHz, so a half-cycle takes 12ns.   Within this time, a rise-time of 2.5ns is a small proportion.

Bear in mind also that the bus switches have internal drive buffers, so the actual part of the rise-time over which switching occurs within the bus switch is much less than the 10% to 90% transition that is rise time. It may actually happen over the central 1V or less of the waveform, which takes only about half a ns to slew. Worrying about the slew rate of the 74AC86 outputs is therefore not something we need do.

Also, provided that the rise and fall times are similar, the receiving device (bus switch input for us) will see both edges occur slightly later than the transmitting gate (the 74AC86) began sending it - but these compensate and the resulting width distortion is small. It appears as a slight signal delay rather than width distortion, so long as the waveform has time to complete the logic transition before the next transition occurs.

This input slew (through logic threshold) is not the same as the operation time of the bus switch, which has its own delays and internal slews, but that's another story (the FST3125 spec is 1ns to 5ns enable time, and 1.5ns to 5.3ns disable time, but it isn't so easy to measure and is something we should simply live with).

If, on observing this waveform, a significantly slower slew is seen, you should ensure that the measuring equipment is entirely capable, with adequate 'scope and probe bandwidth and a properly compensated X10 probe used with near-zero length tip & ground connections.

From this, therefore, I believe that the drives to the bus switches have perfectly adequate rise-time and fall-time.

Appendix: A.C. coupled buffer stages

The high-numbered pin gates (pins 8 to 13) of the 74AC86 are the ones used to drive the bus switches that constitute the H-Mode Mixer. Both are driven with the same output of the squarer, from pin 6. One gate is strapped as inverting; the other as non-inverting, thereby producing the complementary drive waveforms required for the bus switches. This arrangement is good, since both gates are on the same die and will therefore have closely similar characteristics (mainly propagation delay and output drives) within the worst-case specification. This way we get accurately matched complementary signals, needed for best mixer operation.

In the past, an 'official modification' was introduced which a.c. couples the squarer to each of the buffers. A pull-up or pull-down resistor (as appropriate for 'anti-phaseness') gives a definite 'no-signal' condition, and the chip input protection diodes are used to d.c. restore the coupled drives and keep them within the input range (the components are CSQ1, CSQ2, RSQ2 and RSQ3).
This modification was intended to help prevent over-dissipation by ensuring that all 4 gates did not sit at input threshold in the absence of an LO signal. On my own build (and at least three others where I have been given results) this did not happen; the squarer self-oscillates at a low frequency and would provide good logic-level drives to the output buffers in this case, but just because I don't experience it doesn't mean it can't happen (it may actually be that each stage internally oscillates at a high frequency, with internal non-linearities producing a near d.c. output level, since these gates are complex and unlikely to sit stably at threshold).
Given that the a.c. coupled waveform is DC-restored, it must exceed the conduction threshold of the diode (the input protection diode). For each gate, this means that the resulting signal will swing one way beyond the rail (for diode conduction) and the other way correspondingly less than the rail. For a 74AC86, the clamp diode will conduct at around 0.6V to 0.9V (it is an esd clamp, not a precision diode!).
Bear in mind here that RSQ1 loads the high level of the squarer output (pin 6) and causes a shortfall of 0.3V in the high level out.

One buffer will therefore see an input signal of (Vdd-0.9-0.3)V to -0.9V; the other (Vdd+0.9V) to (+0.9+0.3)V.
The logic thresholds for 74AC86 are between 30% and 70% of Vdd; for a 5V supply this is 1.5V to 3.5V. The d.c. restored signal comfortably crosses these points, so all devices should work.

It is useful to consider the 74ACT86 here. I have already said not to use it for the squarer, but if we did...
The logic thresholds for 74ACT devices are 0.8V to 2.0V irrespective of Vdd (limited to 4.5 to 5.5V). We saw (above) that the logic swing after d.c. restoration on one driver could be (Vdd+0.9V) to (0.9+0.3)V - this is the driver with the resistor taken to Vdd, of course.  Now the swing is not comfortable, since the low level after the d.c. restoration is 1.2V, which is not cleanly below the low threshold (0.8V) of the device and will probably not produce a (satisfactory) output signal on that driver. The other one will be fine!

On the schematic is an optional resistor RSQ4, which reduces the aiming voltage of the pull-up RSQ3 and therefore modifies the clamp levels slightly. In the limit, if it were made equal to RSQ3 then an AC device would nominally be linearly biassed again, which is what we are trying to avoid, so it will be made larger than this - unless we have used a 74ACT86, with thresholds between 0.8V and 2.0V. This resistor will then better ensure that the low period of the d.c. restored signal actually crosses this rather low threshold.
Since the 74ACT86 is not good for either the squarer or the buffers, please avoid it. It is a reasonable substitute for the 74AC86 when used as a general logic device, but not for these highly threshold-centric uses.

Appendix: Physical Layout Changes to Squarer

Whilst musing on the squarer, some possible physical improvements appeared.
  1. The first thing is peculiar to the ComboBoard layout. A track on the IC side of the PCB passes from the squarer output and runs beneath the body of the IC to eventually connect to the capacitors CSQ1 and CSQ2. This track carries an inversion of the input and passes close to the input pin (and closely below the package die). When the input signal begins to cause a logic transition on this track, some of the edge energy is bound to feed back to the input (in anti-phase to the input that caused it. This not only opposes the input signal, but is also a known cause of high-frequency oscillation.  A better physical arrangement is needed.
  2. At present, the input to the XOR is on pin 2, with pin 1 as a supply rail setting the stage as inverting. Energy appearing at transition of pin 3 (the output of the first stage) is fed back to the input by the close proximity of these pins and the chip bond wires. This feedback, being in anti-phase with the LO signal, opposes the very input change that causes it. This is a source of instability. By swapping these pins, the input (now on pin 1) is screened from the first gate output on pin 3 by the rail now connected to pin 2. This happens even inside the device package, with the bond wires rather than outside proximity, and probably also happens on the die.  By arranging the pins like this, far less of the pin 3 signal is fed back to the input. This reduces the tendency of the stage to be unstable. In addition, a ground (or supply) 'Guard' between this track and the low numbered pins could be used in order to minimise fed-back signal. This is standard practice.
  3. Then the real humdinger hit me. At present, the first stage is configured as an inverter and the second as a non-inverter. The configuration is done by connecting the second XOR gate input to the appropriate supply rail in order to define the signal pin as inverting or non-inverting. The use of the first stage as inverting automatically gives an environment likely to produce VHF or UHF oscillation at transitions due to feedback, both on and off-chip.  Why not swap these 'inversions' and let any such feedback cause the short-term reinforcement of the input, rather than opposing it? This was worthy of investigation!
The Full Change
This change retains the better screening of the revisions above, but takes the feedback that was anti-socially opposing the input signal, and turns it on its head.
Over my cornflakes, I was musing idly on the fact that the stray capacitive negative feedback acts against the input transition, and wished it were positive feedback (hysteresis) so that the circuit would 'snap' over to the new condition instead of wallowing through the threshold. Then I realised how easy it is to arrange this!
In the original design, the first gate is arranged to be inverting; the second non-inverting, to give the required overall inversion.
But why not reverse this - make the first gate non-inverting and the second inverting! This is so easy with the XOR function, since it simply means swapping the static voltage levels on the 2nd pin of each gate. Then, provided that the feedback was predominantly within the first gate (which seemed likely after doing all that screening), the negative feedback that fights the switch-over and causes transient instability would become positive feedback, reinforcing the switch-over and defeating instability, so long as the time-constant lasts long enough. And we can always add a tiny capacitance to increase the hysteresis effect!

This onwards is cut & pasted from the original page and probably needs changing!

A few calculations later, it seemed well worth trying:

The amount of feedback we get from strays depends, naturally, on the strays.
For calculation, I assumed that there is a capacitance to ground on the input pin of typically 4.5pF due to the XOR gate (Philips data-sheet) and maybe another 1.5pF due to the PCB pad and the connection to the 100R input resistor; a total of 6pF.
The feedback capacitance is far more nebulous, but my wild guess is that it is 0.1pF with the revised layout. This forms a capacitive divider, so that the output step on pin 3 (of 5Vpp) appears on the input as a step of 5V * (0.1pF / 6.1pF) which is 81.96mV
This then decays due to the 125R (100R input resistor + two paralleled 50R source resistance), which has a time-constant with the 6.1pF of  only 75ps.  The deliberate LO a.c. coupling has a much longer time constant and does not affect the result.
The effect is therefore to provide an 81mV step feedback as the first gate responds to the change of LO signal (the sine wave transition into the gate threshold). This step is in phase with the LO signal and reinforces it, causing the output to 'snap over'.
Within 75ps the reinforcing feedback has decayed by 60% (an exponential decay of one time-constant); by three time-constants it has become negligible.

Let us check that the feedback works in our favour and has decayed soon enough not to upset behaviour:
The rate of change of the incoming LO sine wave as it passes through the gate threshold is approximately that of a sine wave passing through the zero-crossing, at which point it is slewing most rapidly. The approximation is due to the possibility that the adjustment potentiometer R has been used to offset the automatic threshold feedback.
Let us assume that the input is a 50MHz sine wave, which is the highest frequency we might intend to use.
The rate of change of this sine wave at the zero crossing is approximately 3.92V/ns * Vpk. For our nominally +7dBm LO input, the amplitude is 1.414Vpp, 0.707Vpk, giving a slew rate at the zero crossing of 0.707 * 3.92V/ns = 2.8V/ns (this assumes that the tangent to the zero-crossing waveform is very close to the actual sinusoid during this time, which is a reasonable assumption for a tiny voltage change).
So, as the capacitive hysteresis decays to 40% in a time-constant of 75ps, so the LO signal will have moved by about 200mV, reinforcing the logic state. Thereafter the LO sine wave will overdrive the input until eventually it crosses the zero-value 'the other way' and it all begins again.
At lower LO frequencies the slew rate through the zero crossing is, naturally, slower; this might mean that the input sine has not moved far enough to hold the stage in saturation after the hysteresis has decayed, but it will always be better than before.

To check that the hysteresis due to stray feedback capacitance will not prevent the subsequent zero-crossing being seen, we can see if the feedback effect is over within the prevailing half-cycle at 50MHz; this is 10ns duration.  Our capacitive feedback had a time-constant of 75ps; after 3 time-constants it will have decayed to roughly 1% of the original step, i.e. from 82mV to  0.82mV, which is wholly insignificant compared with the input of 0.707Vp - and after 10ns (an input half-cycle) it will have decayed for 133 time-constants and will be totally negligible, which is excellent.   

The above calculations show that there is a likelihood of obtaining really excellent results, with no possible disadvantages. As soon as the incoming LO sine wave 'tickles' the circuit enough to start the output shifting, the stray capacitive feedback reinforces the change, causing the output to move more rapidly - it snaps over to the new state.
This is in contrast to the original behaviour, where the negative feedback opposed the LO signal transition once the gate output began to change state, causing the whole commutation to take longer and promoting VHF oscillation until the incoming sine wave provided sufficient drive to swamp it.
So Glenn made further alterations to the layout.
This time he also did away with the connection beneath the IC taking the squarer output from pin 6 to the Official Mod capacitors on pins 10 & 12 in order entirely to do away with the track beneath the IC (another possibility would be to run it on the opposite side of the PCB, but this would result in a break in the ground fill on that side). As you may have gathered in earlier parts, I did not ever see a stable threshold state that would make the extra components worthwhile. Similarly, Glenn has built this circuit several times and has also only seen the low-frequency saturated oscillation that I mentioned earlier. However, in recognition of this mod, Glenn suggested that the caps should be fitted over the IC body as in the original mod and as is done on the mixadaptor for the second mixer; using plain wires (direct connection) first in order to see if the capacitors were needed.
An 0603 pad-pair connected to the IC pin 3 (output of non-inverting first stage) and pin 1 (input directly on the pin) would also permit the addition of a high-value resistor (several Megohms) to apply deliberate but small d.c. hysteresis if needed, or to be a place to solder two short pieces of wire to increase the capacitive feedback if this seemed advantageous. In the event, neither was used.

This arrangement works wonderfully well.
There was no sign of any disturbance on the rising or falling edges of the output, unlike the earlier attempts which all showed 'fuzz'. In addition, the sensitivity (or, at least, the smallest signal which gave a clean output) was improved considerably.
Of course, there is still an input level below which the circuit produces a rubbish output, but this is inevitable and only differs from the original circuit in that it still 'snaps' from state to state rather than producing frantic self-oscillation.

This, then, represents the most sensitive, least unstable configuration and is possible because we use an XOR gate and can readily make the stages invert or not by the static d.c. level on the second input. The only parts of the circuit where inverting feedback can occur is on the die or from pickup on the input of output signals.

Of course, this change in particular really needs proper PCB implementation, so it is not a ready mod for existing boards.  Glenn suggests the possibility of a small 'add-on' PCB in the manner of his test layouts; this could replace the present squarer entirely.
Glenn's results with and without the modification coupling capacitors showed no difference in any practical respect. With no LO input, the circuit oscillated at low frequency (as it always seems to do); with no coupling caps the drives to the bus switches follow this. But with the caps present, the short time-constant differentiation means that the buffers transit slowly through their thresholds in this condition. I am more concerned that, at this time, those stages may oscillate or at least draw supply current through the PN pairs, either of these things possibly affecting the input (which of course shares the same ground bond-wire and possibly some die metallisation). So it seems best to link these signals directly, without the CSQ additions - Glenn simply shorted the capacitors that he had fitted above the IC to do this!

The inevitable conclusion that I reach is in several parts:
Having ploughed through the words, you will want to see some pictures of these improvements.
The results below were obtained by Glenn for each of the above configurations; he has kindly allowed me to reproduce them here.

The principal inverting configurations are as in the picture below. These all require a new PCB layout and are not possible as modifications to an existing PCB.
Some test boards
 
Clockwise from the top these are:
  1. Signal input to pin 1 of XOR, improved ground screening, short input track
  2. Signal input to pin 2 of XOR, improved ground screening, short input track
  3. Signal input to pin 2 of XOR, original (Combo) layout, long input track on IC pin 2!
These, in reverse order, represent the three principal versions where the first XOR gate is inverting.

Final design

The final design changes the order of the inverting and non-inverting XOR sections to non-inverting, inverting. This retains the identical overall inverting configuration, but local stray feedback in the first stage and associated tracking is now +ve rather than -ve feedback. This provides regeneration that causes the output to change state rapidly once the input threshold is reached, rather than the original degenerative feedback that not only opposed the input (resulting in reduced slew-rate) but also provided the mechanism for VHF/UHF internal oscillation during the transition. Because any coupling capacitance is small, the regeneration does not persist long into the LO half-cycle.
Schematic of final test PCB

The photo below is of the fully modified test-layout, now with (stray-dependent) hysteresis and with the squarer output no longer passing below the IC but with capacitors taken above it. The capacitors have been linked out using wire, with no ill effect. The ground-plane is cleared from under the input-pin (XOR pin 1) and 100R resistor end, in order to minimise node capacitance.

This is the final test-layout. There may be small improvements possible, but this is now unlikely.

Final PCB layout

None of the physical changes described in this section can be done properly on an existing PCB, so they are not things you can try easily. That is why I have kept this to an appendix. If Glenn ever produces new artwork, he might be tempted to incorporate some or all of these changes - but there is no reason for this. In any case, the easy modification to the original layout (putting 100R right at the pin, as described a lot earlier) gives pretty good results and is painless.

So how does it perform?

Glenn kindly performed more measurements on his home-made PCBs.

Glenn's graph shows, as before, the input level (dBm) necessary in order to give a reasonable output - a lower input requirement is better:

Plots of 'inverting' versions

Note that these measurements show an odd shape for the swapping of pins 1 & 2 on the XOR, not at all what we might expect. Glenn later repeated all these measurements and included one without the pot RV501 (still in the original 'RTFM' configuration).
The "100R at pin 1" measurement is for the modification of a standard ComboBoard as described earlier.
"Bob's mod" is the swap of inverting & non-inverting gate functions as described above. In addition, the track lengths on "Bob's mod" and "Swap 1-2" have been reduced as much as possible, and the ground plane beneath pin 1 (now the input) and the resistor connected to it has been removed in order to minimise the input node capacitance to ground.

The final set of measurements are shown below.
These include one with the original layout having the 100R at the (lifted) input pin, but with the original resistor still present - this is just for information and does not represent a useful configuration.
Interestingly, sensitivity and flatness of slope are both very much better in the test without the pot RV501 fitted.

The various versions compared graphically

None of the results that I have presented show the highest frequency that the revisions can achieve. Here is something for the "Bob's Mod" inversion-swapped layout at up to 70MHz:

First result with hysteresis

There is no significant reduction in sensitivity, which is encouraging. Furthermore, Glenn was unable to see any sign of 'feathery edges' at the output; these are a sign of VHF instability at the transition point and were visible for all the "unswapped" versions.
From this, it seems as if the fully revised squarer will operate at over 70MHz with less than 4.5dBm of LO. In fact, Glenn recalls that this version operated at well over 100MHz without any problems. Maybe I can persuade him to see what the maximum frequency of reasonable operation is with, say, +7dBm of LO... Not that the MR or front-end will support this, though!

Glenn VK3PE has kindly measured the board response again, for frequencies up to 150MHz. Also, he patiently explored the strange slope around 100MHz (probably a resonance) by taking smaller frequency steps:
Fully revise squarer sensitivity up to 150MHz
 
These tests were all performed with the original feedback biasing configuration (RV510 not re-deployed between supplies and R527 still providing feedback). For the test above RV501 was not fitted, but otherwise the configuration was the same. The re-deployment of RV501 and removal of R527 is recommended, however, since it permits adjustment of mark:space for XOR devices regardless of their threshold voltage. This also provides maximum input sensitivity whilst permitting production of a square-wave.

Please bear in mind that the squarer benefits from being given the largest LO signal possible (so long as the input protection diodes of the XOR don't conduct). All the results presented here are for the smallest LO level that gives good output squaring as seen on an oscilloscope. They do not represent the level that you should use!