Actually the two TenTec Corsairs did a decent job of getting the
selectivity close to the antenna. The RF stage is a bipolar power
transistor with RF feedback (the Anzac circuit), with a hefty double
balanced mixer and the first IF is a copy of the RF stage. Then the SSB
bandwidth crystal filter. I find that works well. Sure, a CW filter in
that first filter slot would be better for really tough situations. The
Omni V and VI use a gaggle of paralleled grounded gate JFETS for the RF
and IF stage and also perform decently.
The key to receiver selectivity performance and dynamic range is to keep
the stages before the filter from being driven into saturation by strong
signals. The Anzac circuit is one of the best (saith Ulrich Rohde), and
diode ring mixers can handle strong signals, if their local oscillator
signal is stronger. Its important (especially at VHF and up) that the
mixer loss be minimized so the RF stage gain required to overcome the
noise of the mixer is minimum. I've been rooting in the literature for
low conversion loss mixer circuits and their performance. One designer
at Collins Microwave claimed a double balanced mixer with only 1 or 2 dB
loss about 20 years ago, then it dropped out of sight. Steve Maas proves
that while such a low conversion loss mixer is possible, the
terminations that cause the low loss also cause the mixer noise to be
much higher and the third order intercept to be much poorer. Such as a 4
or 5 dB increase in noise figure over the conversion loss (typically a
diode ring mixer noise figure is essentially the conversion loss) and a
20 dB drop in IMD. So there's no free lunch. I'm not done but need to be
spending more time on the basics of housing than on exotic radio
circuits this spring and summer.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.
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