tubes age 15
selectivity, then the following IF amps are only for gain and gain control. Just
before the detector its worth having another set of filters to remove the wide
band noise from the detector that's been added by the IFs. This is rarely ever
done. The distributed selectivity is one of the things that makes a 390 probably
have a little bit better signal to noise ratio on weak signals (including SSB and
CW) than the 390A.
The failings of the supposedly modern rice box tend to come from ignoring
phase noise from the synthesized local oscillators that lead to reciprocal
mixing (oscillator noise side bands mix strong unwanted signal components
into the IF bandpass where they can't be removed), neglect of the wide band
terminations of the mixers, and letting the first IF filter be wide enough for FM
reception which opens up the second mixer to close in unwanted strong
signals. Some I suspect also keep that first "roofing" filter wide because they
get the fine steps of the tuning at the second conversion...
TenTec comes closer to what I think is the best receiver, though their best
receivers are not general coverage. The TenTec Corsair II uses the Norton RF
stage (Ulrich Rohde's favorite) with a ring mixer feeding another Norton at the IF
and then the 9 MHz main selectivity... There's a second conversion to a 6.3 Mhz
second IF so there can be some bandpass tuning. Which I find very effective.
The later Tentecs use the same frequencies but a synthesized local oscillator.
They cut the phase noise by keeping the tuning range narrow and doing it at
VHF then dividing down to the 5 MHz range. I do find a low pass filter on the
Corsair II speaker beneficial in reducing hiss noises from the IFs.
With the slow but practical availability of low phase noise signal generators like
the HP8640B (I wish I had a pair to measure receiver intermod) I think a better
receiver might be made using one of them as the local oscillator with the
filtration right after the mixer. Bandpass filters at the input would be some
benefit, and needed for image rejection because the good RF stages and
mixers are good from LF through VHF. I do have an HP 5100/5110 synthesizer
that is very low noise, covers 0.1 Hz to 49.9999 Mhz. I should try that with a plain
IF. I've used it to mix general coverage to a hamband receiver with decent
results.
Replacing a shunt regulator with a series regulator (have to remove the series
resistor that fed the shunt regulator) should reduce power dissipation some.
But it can't be automatically plug in unless there are some wiring changes
made to the 0A2 socket to allow some separation of functions. Have to use the
jumper in the 0A2 to connect up the original series resistor and supply
unregulated to some other OA2 pin. Beware that applying a high voltage to an
unused 0A2 pin might lead to that pin glowing inside the 0A2.
I've not yet bread boarded my 0A2 circuit, but have worked out resistor values.
And tossed in the feedback capacitor to slow things down.
(14 pages)
(10 pages)
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