EMRFD Message Archive 15170
Message Date From Subject 15170 2018-10-28 03:08:34 peter_dl8ov R2 Receiver Gain Distribution I know, I should know this stuff, but I want to make sure first before I make a fool of myself.
In the standard R2 receiver please could someone confirm that almost all of the 100 dB nominal gain takes place in the final audio amplifier stage? If true then this seems very odd as the all-pass filter network with all those opamps in series will be dealing with tiny signals that are perhaps buried in the noise.
I'm putting together a presentation on the R2/T2 and I need to be ready for the awkward questions.
15173 2018-10-28 08:07:54 John Levreault Re: R2 Receiver Gain Distribution Things aren't as bad as you seem to think. Although there are indeed losses in
the input mixers and band-pass diplexers, the 50-ohm audio preamps have a
sizable amount of gain. (I did a quick LTSpice simulation, and I'm seeing nearly
44dB of gain.) This gain boosts the signals going into the audio phase-shift
networks so that noise should not be a problem. The originally-chosen NE5514
opamps have decent noise performance; although there are better modern devices,
the input preamp gain insures that it's not necessary to go overboard with
expensive low-noise opamps in the phase-shift network.
Unless a front-end RF preamp is used, the overall system noise figure will be
modest at best. IIRC, the author was estimating somewhere in the 20dB range and
is determined by the losses in the mixers and diplexers, along with the noise
figure of the audio preamp. This should prove more than adequate for its
intended QRP application.
I've built a few of these over the years and really like the "sound". It's very
clear and clean, in contrast to many an 80's vintage transceiver. And that,
IIRC, was one of the author's intended goals, to provide clean, non-fatiguing audio.
15178 2018-11-01 15:08:08 peter_dl8ov Re: R2 Receiver Gain Distribution " the 50-ohm audio preamps have a sizable amount of gain. (I did a quick LTSpice simulation, and I'm seeing nearly 44dB of gain.) This gain boosts the signals going into the audio phase-shift
networks so that noise should not be a problem."
OK John, and thanks for such a detailed reply. I assume that by audio preamps you mean the common base stages and the opamps just after the two mixers but before the audio phase shift network? This is shown in my notes as the Downconverter PCB.
15179 2018-11-02 07:44:13 John Levreault Re: R2 Receiver Gain Distribution I meant the audio preamp, as identified in text on the schematic from the
original article. That would be the common-base stage(s). I don't have the
schematic in front of me at the moment, but don't the opamps run unity gain?