Summary of changes
The list of modifications I now have incorporated is:
- Add 1nF NP0 5% across R620 to provide a further pole of rolloff,
- Change C633 from 220nF to 10uF to avoid operating below the low -3dB
- Change the 'T' pad back to 82R-100R-82R 1% (5% would suffice) to
give roughly the
original path attenuation
whilst regaining a 50R path (the original values were 470-100-82R, so
only the input resistor is changed). See
- Change CZ3 from 1uF electrolytic to 10uF ceramic to avoid
operating at low -3dB cutoff.
Common path to Rx & Tx:
- Replace C632 (220nF) with 2.2uF or 10uF ceramic in series with
avoid operating at the low
-3dB point and to give a properly defined
source-resistance for the filter.
- Change Tx buffer, alter two and add 4 resistors to give linear
gain, no -ve peak clipping, no crossover distortion, no chance of
runaway. R604 & R615 both change to 3k9; see text for remaining changes,
which provide Class AB biassing to minimise distortion & prevent
thermal problems; Class AB is recommended but optional.
- parallel a 5k6 over R631 to correct the biassing on the Bus
Switch to avoid +ve peak clipping &
- Remove redundant C631 coupling capacitor and fit 330uH 5% axial
choke delicately to the pads,
- Change C651 for two 100nF 5% NP0 ceramic caps in parallel. See
notes for Farnell part,
- Remove C639 completely,
- Change RFC612 from 330uH to 680uH 5% (I used 2 of 330uH in series
- Change C638 from 10nF to 100nF X7R [e.g. Farnell 1740665] (or NP0
as above), better to align
of high & low arms.
- It is wise to measure your inductors (680uH and 330uH) to verify
that they really are within 5%. I have seen marked inductors whose
marked value was purely advisory, with an actual value grossly
different to that marked. So far the worst has been +30% on a marked
10% inductor. Cheap Chinese parts need checking! I used
Farnell Epcos parts 608579 (330uH 5%) and 608592 (680uH 5%) which were
- The C651 capacitors are specified as 100nF NP0 5% [e.g. Farnell
8820210]. These are costly, but other dielectrics are not suitable for
this position. Even if you select from X7R types, the temperature
coefficient and dielectric losses are poor.
- The 10uF ceramics (e.g Farnell 1762635 Murata 10uF 16V 10%) could
be replaced with electrolytics, even 4u7 if absolutely essential, but I
can see no reason why you would choose to do this. The dielectric is
less important since these are not part of a resonant circuit.
'PI' Pad alterations:
If the pot RV604 does not give enough signal at minimum resistance,
preferably re-calculate the PI network values for less attenuation, or
can increase RZ3 (not
higher level. Do not alter for higher level than
necessary, which would worsen distortion and intermodulation in TR603.
The output level pot RV604 should preferably be near (but not at) the
minimum resistance end when the IF setup is complete. If it is at the
end-stop, a feedback pole with C692 and op-amp internal resistors will
probably cause instability; back RV604 off a fraction to avoid this.
That's it! Not too bad really.
Photographs of the IF2 modifications on my ComboSTAR
The photos below are of my
ComboSTAR with the 'T' filter above incorporated. I have performed the
IF calibration successfully on Receive, but have not yet been able to
perform meaningful antenna sensitivity checks. It is no worse than
before, anyway. The two 330uH inductors are in series in the place of
RFC612 (680uH) and measure almost
exactly 680uH. The nearer 330uH inductor is mounted on the pads vacated
by C631. The PI pad
now has 82-100-82R. All my receive side coupling capacitors (now only
CZ3 and C633) are now 10uF ceramic X7R, and I have modified the
transmit coupling (C632) to be a 10uF and 47R in series in
the first photo. The repair to a pad on IC604 from changing the
chip (I soldered it mis-orientated) is clearly visible. I subsequently
made a 'both sides in parallel' desolderer! The Tx gain
trim RX is not yet set.
Since I am not party to any of the original circuit design or
the 2nd IF, it is possible that some of the things about which I
comment are actually deliberate ways of tailoring performance. For
example, the LF rolloff at
15kHz may be a deliberate way of making the response look more like
that of a tuned circuit, with a peak 'at' the 2nd IF (15kHz). On
reading the description accompanying the original design from which the
circuit was cloned (DSP-10) it is apparent that this
is not so; it is just a mistake. In the 1999 QST article by Bob W7PUA
he says "No
active supply decoupling is needed because the lowest frequency
amplified (set by C32 [which is our C633]) is a few kilohertz", but it
is actually above the 2nd IF.
is my personal opinion that the circuitry in this
area has gone a bit wild; it works but not as we know it (to paraphrase
from Star Trek). If you
disagree with or dispute what I have said & done, please contact me
and put me right.
My own results
In the next weeks I aim to complete fully my own STAR incorporating all
these changes and any others that seem necessary. I have already
modified the DDS filter & buffer as previously
. In the fullness of time
I will let you know the results, which will mainly be 'works' or
'doesn't work' since I will
not have "before and after" performance measurements. For me it will
suffice that several peculiarities have been dealt with.
So far I have performed the full scope of modifications that are
described above and verified the Receive path.
The IF calibration BASIC programme has no issues with the
I have no comparison data to see if the overall performance has
changed, in terms of MDS or IP3. Maybe someone can do this
someday, but for me it is sufficient to have exorcised the 2nd IF
stages of so many
wrongs. My personal 'take' is that mismatches and filter errors
in any part of STAR, especially those which are so easily remedied, are
simply not welcome. Correcting them will help me to be happy that
things are not just 'working', but 'working properly'.
Has all this done any good?
There are loads and loads of changes to almost every part of the 2nd IF
These changes have now been incorporated into an original PICASTAR
(modular) and a ComboSTAR.
Measurements have been made by an
independent Amateur of MDS on each unit 'before and after',
carefully averaging readings and performing them conscientiously, which
Measured on 20 metres in a 3 kHz BW using Hewlett Packard test
MDS improves by 1dB, from -127 dBm to -128 dBm, in 'Best NF'
MDS improves by 1dB, from -122 dBm to -123 dBm, in 'Best IP3'
(I apologise to the kind Amateur who conducted these measurements for
my careless error in presentation of his 'Best IP3' figure, and thanks
to the reader who pointed it out).
Measured on 20 metres in a 3 kHz BW using Hewlett Packard test
MDS improves by 1dB, from -128 to -129 dBm, in 'Best NF' mode.
This result was clearly a
comfortable 1dB; it came with the comment "I was seeing less 'wobble'
on the meter needle and it looked like nearer 1.5 but I'm still not
going to add any decimal points to my quoted result".
This is probably the easiest 1dB improvement you can get, since it
comes free of charge by
correcting the original design errors. It may not be earth-shattering,
but since it comes as a result
of correcting the design it is nevertheless welcomed!
received a comment that a mere 1dB improvement is not significant, but
someone has also said that if it had become 1dB worse rather than
better then it would have been heavily criticised...
At least it shows that putting things right in the 2nd IF does not make
it worse! I am very happy with
this result, albeit not earth-shattering - after all, this series of
corrections was never done with MDS improvements in mind, but to remove
unsound design elements.
Are all these changes necessary?
Only if you wish to remove a host of errors from the circuit.
know that it is difficult to obtain a perfect design.
The silly thing
is that by performing proper calculations and using a bit of ingenuity,
it is possible not only to remove all of these problems, but to do so
within the existing physical framework.
Many of the original components are retained, often only small
corrections being needed in order to ensure that the proper operation
Like me, you will be surprised at how many problems there are
in this relatively straightforward area. You may decide that I am
wrong; they are features of a good design. That is your prerogative. I
have provided full explanations, plots, analyses, measurements and even
told you what to do in order to remove these design errors from your
STAR. It is entirely your choice if you do any or all of it.
the changes I have shown
are needed in order to purge the design of
errors, some serious.
But you choose.
These mods provide:
- Correct Diplexer operation.
- Functional antialiasing filter for Rx, with first alias frequency
reduction by about 12dB instead of 1dB. This is not strictly necessary,
since there is little but noise at the aliased frequency and the AC97
CODEC has built-in rejection.
- More correct reconstruction filter for Tx, leaving far less
splatter to be removed by the roofing filter. Again, the CODEC has
considerable anti-aliasing provision, but crossover distortion in the
buffer amp produced unwanted frequencies.
- Proper matching throughout, not casual mismatch or simply no
- Removal of any capacitive load from Transmit buffer output,
only limited by the fortuitous parasitic on-resistance of the bus
obviates any possible unwanted buffer oscillation due to a spurious
- Distortion, serious clipping and possibility of thermal runaway
Should you incorporate the changes?
Existing STAR users would be wasting their time. They have a pretty
transceiver. Access is probably difficult. The changes won't give
earth-shattering improvements in Rx sensitivity and will very likely
not be discernable.
New constructors, who have easy access to the components and who are
prepared to use the new values should decide for themselves what they
wish to do. Remember - these are still Unofficial Modifications.
my opinion, I would say that sanitizing the couplings and matchings,
together with removing distortion, is
beneficial! The 2nd IF will be working as you expected it to. You
decide, I have provided all the information to the best of my knowledge
and ability. Compromising the signal just for the sake of it seems
strange to me, especially when it is easy to get it right.
know what you think, especially if you
decide to go ahead!
As always, I can offer no guarantees of
OK, my approach has initially been theoretical, with calculations and
simulations. So what do things really look like?
For me, it was not easy to perform measurements, especially on the
'before' which no longer exists on my ComboSTAR. Fortunately,
others have leapt in and bridged the gap.
Both Roderick VK3YC and Glenn VK3PE have built an N2PK VNA; Glenn
discovered that these will operate down to about 10kHz (the internal
transformers seriously distort waveforms below this). This is good
enough to plot the actual behaviour of chunks of the circuit. Both
Glenn and Roderick have kindly lashed up the original and modified
filter sections and measured their responses.
Luckily Glenn is constructing yet another STAR (!) and has measured the
whole 2nd IF response with the original components, then has adapted it
and re-measured with the revised parts.
See the actual results.
contact me with your thoughts.
Thanks to the following people, without whom this would probably have
fallen by the wayside:
- Peter G3XJP for bringing
us the PICASTAR in the first place,
- Numerous Others without
whose efforts and dedication there may have been no PICASTAR,
- Glenn VK3PE for his
excellent PCBs and contributions and for listening to my waffle,
- Robert G3WKU for his
- Roderick VK3YC for his
- Henning DK5LV for his
comments and suggestions,
- Dave G3SUL for pointing
out to me the anti-aliasing capability of the CODEC,
- Others who wish to remain
- All of you who have
bothered to read what I have to say!