PICASTAR Bidirectional Amplifier
Recently retired, I chose PICASTAR as a first and only rig to build
mainly because I wanted to construct a mature design, completely 'done
and dusted'. I was particularly attracted to the use of DSP, it was
clearly the result of a great deal of work and is absolutely the right
way to go.
Although it would be possible to ignore the shortcomings
of the Bidirectional Amplifier, and 'carry on regardless', it is
to me that if the original design team had been aware of any problems,
they would have cleared them all up long ago. Therefore I can only
assume that they will be happy to see these gremlins put to rest and
not see my efforts as any form of criticism of their own excellent
work; it is not.
I can't think of any good reason to leave in place design problems that
were simply not noticed before.
There will be 'new' builders of PICASTAR, probably for many years to
come. After all, I am one of them! If they can build their STAR
confident that no-one is turning a blind eye to problems, no matter how
late in the day they are seen, this surely cannot be a bad thing?
STAR as a whole works well without this attention, it is likely
(but not inevitable) that purging it of obvious and correctable defects
and deficiencies will improve things. I have a personal
opportunity to address any such things as I
feel necessary now, while I am constructing my ComboSTAR (Thanks
Glenn!) before it is all shielded and cased.
The Bidirectional Amplifier
The Bidirectional Amplifier is used as a buffer amplifier between the
Roofing Filter and AD603 IF Amplifier (on Receive) and between the 2nd
and Roofing Filter (on Transmit). A cunning circuit arrangement
using diode gating changes the direction in which the signal flows
between Tx and Rx, hence the name Bidirectional Amplifier.
The circuit first appeared in the "Belthorn 4" some years before, and
like so many other areas of STAR, seems to have been 'cloned' without
any further investigation of suitability.
This amplifier uses a single J310 FET to provide amplification and some
input-output isolation. At the Roofer port, a transformer wound on an
iron dust core T37-2 acts both as a low-Q resonant load and a conjugate
match to the impedance of the crystal filter (900R//10pF or 910R//25pF,
depending on the filter used).
At the AD603 IF Amp / Mixer port, the amplifier is coupled using an
ferrite transformer of 10t:3t wound on an FT37-43 toroidal core.
Both ports are intended to show 560R, defined by resistors of this
value. This is borne out by the ferrite transformer, whose ratio
transforms the 50R mixer output by 102 / 32,
100/9 = 555.6R, very close to 560R.
This does not work out properly for the 2nd IF amplifier, IC 602
(AD603) because the input of the AD603 is a closely controlled
(laser-trimmed) 100R, not 50R, and no correction is performed.
This means that the bidirectional amplifier is mis-terminated by the
AD603 IF amplifier when operating in Receive; this needs consideration
since it must affect the bidirectional amplifier performance during
Receive. I deal with this later.
[add a schematic scrap here]
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
I am currently considering the modifications that I proposed previously.
Thanks to the advice of Henning-Christof Weddig I am able to see that
my earlier suggestions did not necessarily result in a better solution
than the original design, especially in respect of the modified
transformer that I proposed for coupling the FET to the 2nd mixer and
AD603 IF amplifier.
Once I have a better solution I will put it here.
The original page is available here, but please do not
perform the changes that I provided unless you are personally happy to
undo them again (this is not difficult!).