PICASTAR Bidirectional Amplifier

Foreword

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 perfectly apparent 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?

Although the 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 Mixer 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 untuned 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]

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.

Design Considerations

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!).

Thanks,
M0RJD