Re: AGC circuit to try?

ajparent1/KB1GMX
 

Lawrence,

So happens I'have all the mobile manuals and the SSB for radio amateurs since they were printed...

Yes its a good circuit, low noise and wide agc range.  HyCas is a variation of that as well as others.
It can easily be done with a pair of J310s just as well.  The 3n128 disappeared in the 70s but it was
used back then as it had a gm far higher than Jfets of the day.  Now we have better.

Its weaknesses are that the input match is nonexistant.  It does not make for a good match for
any filter unless loaded with a resistor and matched with a step up transformer.  Its output match is
also uncontrolled so  buffering is needed between it and filters note the source follower for that. 
Things we learned since then.  The reason it was done with preselectors is SWBC, short wave
broadcasters were far more common back then.  So selectivity as close to the antenna was the 
design methodology of the day (usually meant the crystal filter) and often the front ends were
doubled tuned for selectivity within the band.

I have 4 radios from the early era of transistors I still use.  Two the Tentec 505(Argonaut) and 340(Triton)
used circuits like that with  tuned preselectors.  They are both very good but current radios are better.
A receiver that has a front end using two fets cascade  is the AX190 I have, also does well on the bands
40+ years after I got it.

It would be good in front of the Minima receiver but for uBitx switching it out would be a pain never mind
the tuning requirements over the full working range of the uBitx.

Allison

Join BITX20@groups.io to automatically receive all group messages.