The graph (below) is a simulation of the original ComboSTAR circuit from input to output, including the pad and common-base amplifier
The shape of the output vs frequency is immediately apparent: Is this a response you would welcome? The rolloff below 20kHz is caused by the too-small original coupling capacitors.

The circuit for simulation is below:

The markers at approximately 15kHz (wanted 2nd IF) and 33kHz (quantisation aliasing frequency) show how little rolloff the original circuit has. Maybe the deep notch around 90kHz was intended to be at the alias frequency?

In the schematic below, I have shown the PI-pad values that I used; the graph above uses the standard values which give 3dB more attenuation.
This is a part-way solution and is not final.

Note that I have changed the coupling capacitors from the original (too small) values here, since this is required anyway. Although the scales for the responses have altered, it can be seen that distributing the 50R HF load between the 2 arms of the filter (R10, R11 above) results in a deeper trough due to the higher Q of the resonant series arm. These two configurations are the best I could manage with this topology. Note that the change to a high value of R1 (47k) in the above schematics is disabling the original HF arm of the diplexer. The HF termination is done with the new 47R (R10) or with the series combination (R10, R11).

The attenuation at 33kHz relative to 15kHz is approximately:

• Cauer-2 configuration, 7dB
• Cauer-1 configuration, 11dB (in spite of the shallower trough).
• Simple filter for comparison, 9dB.

So I conclude that the Cauer, once sanitized not to shunt the output capacitively at higher frequencies, does not provide any great benefit over the simpler, conventional 'L' filter.

In any event, we must make the identical circuit serve in the Transmit direction also.

Final Filter

After studying the Transmit Path, it seemed a good idea to use a 'T' low-pass filter rather than an 'L' configuration, mainly in order to avoid a capacitive load on the transmit-side buffer. The 'modified Cauer' (Cauer-2) above almost gets there, by presenting a 22R resistive load at high frequencies,  but there are other possibilities.

When studying across sheets of the schematics and drawing a scrap circuit of things, it became obvious that the output of the MixAdaptor section is biassed at 2.5V, so the diplexer receives this d.c. offset together with the signal. Then, following the filter, there is a capacitor to 'isolate' this d.c. component, followed immediately by reinstating it in order to bias the switch IC604. The implication is that there is no voltage across C632, so I measured it: 12mV! There is no need for C632, let alone a 10uF-value C632; it can be replaced with a short-circuit (and the bias divider R625, R626 removed or not fitted if desired).

With this in mind, I considered replacing C632 with an inductor rather than a short-circuit, thereby building a 'T' low-pass filter consisting of RFC612, C651 and the 'C632' inductor. This would allow not only the emitters of TR604, TR606 to see a load that increases at high frequency, but also gives the filter another element.  However, on looking for inductors of 'reasonable' value, hundreds of uH, in a surface-mount package small enough to fit in place of C632, it became apparent that they don't come this physically small. One thing we are avoiding is hacking the PCB...

Undeterred and raring to see if it was worth trying, I attempted to design a 'T' filter that would have something like 50R looking both ways in and out of it, a symmetrical filter, but with the bias towards the receive direction being good, since on Rx we are trying to pick signals out of received -um- stuff. On Tx the signal from the DSP is as good as it can be.  After a good bit of head-scratching and nearly wearing out Elsie <g>, I tried some 'handy' values and hit gold!  By using a larger value for C651, 200nF (it was 100nF for the 'L' filter) and adding 330uH in place of C631, I ended up with a filter in the receive direction with a sharper cutoff than before, and one in the transmit direction with a reasonably respectable behaviour!   Too good to ignore, I discovered that the axial 330uH chokes from Farnell would fit the surface mount footprint if the legs are bent appropriately! The body stands vertically, but who cares? It's a bit tight for space, because of adjacent components, but it fits (note that Glenn later used a different way of mounting it that may be more appropriate).

I'm sure that there are other implementations that will suffice, but at least this one meets with my personal aim of presenting inductors to the mixer and Tx buffer, and it also allows a diplexer to be constructed that properly terminates the 2nd mixer.
Henning DK5LV has used different component values in his implementation; you can choose. Henning has also used Wima MKS capacitors in order to avoid the poor behaviour of Hi-K ceramic capacitors; I have chosen to use 'non-Hi-K' NP0 ceramic capacitors instead.

I fitted two 100nF 1206-body 5% NP0 capacitors in parallel, one above the other, for C651; these are expensive at 90p each (Farnell 882-0210) but that is not a major problem in the scheme of things. They have a minimum order quantity of 10, so fitting two is also no problem! You could maybe use X7R dielectric at a pinch for cheapness, but see my note elsewhere on Farnell offerings. It is not worth putting anything worse than NP0 in any filter or tuned circuit.

The inductor fitted nicely, albeit a bit tricky to position in the space available.
In all of this, I have ignored group delay (as I believe did the original design). Since we only use a small part of the full bandwith it is likely but not certain that the effect of group delay will not be significant; please contact me if you would like to comment on this (or anything else).








Well, that filter looks pretty good. Please note that this is with reduced attenuation in the PI-pad (120-51-120R). I found that the original 82-100-82R, with 4dB more attenuation, gave adequate signal with the output pot near minimum, better for achieving low distortion and intermodulation by running at lower level in the amplifiers.

The extra pole on Receive due to the capacitor across R620 (5k6) is further 'improving' the slope, but the 'T' LPF is alone is amazingly effective. The increase to 100nF from 10nF of C638 has maintained a 50R load except for the onset of the amplitude rolloff, as can be seen by the slight 'kick' upwards of the input amplitude trace (in green above) at just over 20kHz. This is difficult to avoid without considerable extra poles or by changing the original design criteria (possibly relying entirely on the CODEC filters) and is not a problem.

I think that this is getting close to the best we can achieve without adding a daughter board. Although it would obviously be inviting to have a fresh start with the Rx-side 2nd IF, the performance of this 'low intrusion' diplexer/filter is more than adequate to ensure correct termination and to provide some alias rejection.

The above schematic represents my modification to the design. I have been persuaded that it would be a Good Idea to increase C638 (C1 on the simulation) from 10nF to 100nF in order to minimise the 'gap' before the HF arm 'comes in'. This will reduce the high termination impedance (visible on the simulation plot as an input amplitude increase) that otherwise occurs between 30kHz and 300kHz. For the sake of one more change we may as well get it right. This is shown on the simulation above. It is best to use another of the NP0 capacitors.
 
A better input impedance over the full frequency range is obtained with C638 at 200nF, but because the input becomes shunted by the HF arm at 15kHz, the gain and slope of the transmission characteristic are adversely affected also. Stick with 100nF!

Coupling Capacitor types

As we all know, Hi-K ceramic capacitors have poor initial tolerance, temperature coefficient of capacitance, voltage coefficient of capacitance, and have high losses dependent on frequency. The X7R and Y5V should not be used in tuned circuits of any sort. Preferred typed for tuned circuits are NP0 ceramics or, if their self-inductance is acceptable, plastic capacitors such as polystyrene, polyester or polycarbonate.

When used as decoupling capacitors, the characteristics of Hi-K ceramics are usually tolerable (sometimes paralleled with a smaller value capacitor better to handle high frequencies), but are they suitable for coupling signals (d.c. blocking)?

Consider the normal requirement for a coupling capacitor: it must pass the entire a.c. signal but block the d.c. level. If it actually does this, then the a.c. signal across it is zero and dielectric type not of major significance. If it has a significant a.c. signal appearing across it, then the dielectric characteristics determine whether that signal will be affected. Perhaps the most worrying of these effects would be voltage non-linearity of capacitance, where the capacitance and hence voltage drop would depend on the instantaneous voltage present across it. This would be manifest as non-linearity of signal drop with instantaneous amplitude, a source of distortion, which we clearly do not want!
Since this depends on the voltage across the capacitor, it is clear that (for a given frequency and circuit impedance) a larger capacitor value will produce a smaller effect overall than a smaller one, since it develops less voltage drop and therefore has less overall effect.

If our Hi-K capacitor has a value small enough to allow a significant a.c. voltage to appear across it, signal distortion is likely. However, if we use a Hi-K capacitor of sufficient value that any variations of voltage-drop due to the dielectric are insignificant compared with the full signal voltage, then not only is the Hi-K effect far less important, but also as much signal level as possible will be passed to the subsequent circuit.

The original Picastar uses several 220nF Hi-K coupling capacitors in nominal 50R paths in the 2nd IF stage, which operates at 15kHz +- 3kHz. At 15kHz, 220nF has an impedance of 48.23 Ohms. This is the same as the circuit impedance, so half the signal will be developed across the capacitor, giving not only signal attenuation but also the likelihood of distortion due to the dielectric behaviour. This is not good!

I have suggested using 10uF, or at a pinch 2u2. At 15kHz, 10uF has an impedance of 1.06 Ohms, which is as near zero as we might like. This means that not only will it hardly attenuate the signal in the 50R path, but also that any capacitance variation will have only a very small effect on the signal - it will not give significant distortion.
Clearly, even larger values would be better, but 10uF is a useful value and is obtainable in small SMT sizes that will fit the pads. Although, in most cases, the actual terminal voltage difference is small, it is wise to choose capacitors with 10V rating (this is the supply voltage) for safety. X7R is the best choice of dielectric, but Y5V might still not be that bad.
Farnell offers X7R 10uF 16V 1206 10% capacitors (1463368) for 47p each (singly). The higher voltage rating of these should provide an excellent margin. A 10V part is also available (1288264) for 19p each, but these are priced in tens. Either of these would be a good choice.

If it were physically possible, we could use plastic dielectric capacitors (polystyrene, polyester, polycarbonate) or mica (at a cost). But values of 10uF are not really practical; we need a large value in order to avoid the rolloff. Maybe you could consider using these, but bear in mind that we are trying to improve things, not achieve perfection. The capacitors mentioned in the previous paragraph will fit the existing PCB pads and, I believe, are a colossal improvement.

Comments

Actual response measurements for the suggested filter have been performed by others and show good conformance to the theoretical shape.

Henning DK5LV

Henning has built his diplexer/filter with his own chosen values. He says "I have changed the diplexer after the second mixer to a Tshebychef LP 0.05 dB ripple giving 20 dB return loss in the passband (two times 330 uh, 177 nF (150 nF + 22 nF in parallel; type Wima MKS) to ground in the middle".
These values give about 0.25dB less gain difference across 15kHz +- 1.2kHz, but considerably less attenuation (about 6dB less) at the image frequency.  Because the image and wanted frequencies are only an octave apart, it is a difficult compromise, but I am happy with the values I used. Henning has wisely considered my change philosphy and chosen his own filter preference.
You may prefer to use Henning's values, but remember to use NP0 or MKS capacitors for stability and low distortion.

Dave G3SUL

On the topic of anti-aliasing, Dave G3SUL has kindly pointed out (message #4345 on the picastar-users group) that the AC97 specification requires that compatible CODECs "have on-chip filtering incorporated which gives rejection of at least 74 dB at the Rx aliasing frequencies, and Tx anti-alias rejection of 40 dB for all output noise from 28.8kHz to 100kHz." Dave suggests that therefore "any additional anti-aliassing rejection provided by modifications to the 15 kHz LPF and buffer stage is of no practical consequence to the anti-aliassing performance."

Even though the modifications will not provide any eventual improvement to the anti-aliasing performance, it is important to bear in mind that the reasons for changing the filter are:

• The original Cauer filter topology mis-terminates the 2nd mixer (in the role of diplexer), so it must be altered to an inductor-input 'filter' anyway.
• The second inductor was added principally in order to avoid the Transmit buffer having a capacitive load, as it originally did - and as a 2-section LC filter still would. This is not quite so essential after finding defects in this Tx buffer anyway, and providing a real 50R load for it, but it is generally bad for the stability of a feedback amplifier to load it capacitively, so the output inductor of the (now) 'T' filter is still useful. The inductive output arm avoids the load dropping from 100R to 50R at higher frequencies as capacitor impedance drops, which is good even for the revised buffer. That this gives a further pole in the filter, allowing a better rolloff than a 2-section filter, is a bonus. 

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