On Sun, 2007-05-20 at 14:00 -0700, Kevin Purcell wrote:
> I know that feeling!
>
> Another question: Is it still a MOSFET (like the Omni C) or did they
> make the change to a higher emitter current BJT such as the 2N3886
> (like the Argosy 1 second revision) or 2N5109 (like the Argosy 2)?
>
> It seems like TT were fiddling with the designs in the early 1980s
> making incremental improvements.
>
> 73
> Kevin
>
The bipolar RF (and first IF) stage uses transformer feedback with a
three winding transformer. Its a circuit developed for the output stages
of cable TV distribution amplifiers where acceptable intermod is 70 dB
down and in Ulrich Rohde's book on receiver design he sees nothing close
to having its performance. A double gate MOSFET doesn't come close. I've
run Kenwood with MOSFETs and those MOSFETS don't come close to the
strong signal handling of my Corsair II.
That circuit is covered by an Anzac patent or two and that may be why
Tentec went to the grounded gate JFET circuit in the Omni V and VI.
Those use four JFETS in parallel with individual source resistors to
make them balance better. I've not seem their performance compared
directly to the Anzac circuit, but the Omni V and VI seem to do quite
well in the intermod department.
Schottky ring mixers are hard to beat and the higher the LO power (so
long as the mixer is designed for it) the better the strong signal
performance. Its also important that at least two of the three ports of
the ring mixer be terminated over a wide range of frequencies, else NF,
mixer loss, and intermod can be a lot worse. The mixer output often sees
a crystal filter and that is the worst thing that can be done, one book
on mixers says that can cost 30 dB intermod range. The Corsair II, Omni
V and VI use a broad band IF stage with lots of dynamic range to provide
that IF port broad band load and it shows. It works.
Don't know about the Omni V and VI, but the "attenuator" switch on the
Corsair II actually removes the RF stage from the circuit. There is no
attenuator.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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