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Andrea Baldoni · August 20th 06, 12:05 AM posted to rec.radio.amateur.homebrew

wrote:

: I'm researching about the matter and I just read that, in a BJT for
: instance, emitter current is inversely proportional to the noise. So,
: if AGC reduces the gain (so current), SNR degrade?

: Not necessarily true. Noise, true natural noise, in a bipolar
: ...
: of how many factors go into noise generation within the
: transistor. :-)

So, if you have to engineer a (let's start with HF) receiver, do you think
it may better to:

1)
find a way to insert automatically a stepped attenuation (maybe
using a diode switched resistor network) and leaving amplifiers without AGC,
thus optimizing them for a particular gain

2)
build circuits with so high dynamic range that's completely impossible to
have input signals overload them (what's the dynamic range one should
normally expect at the antenna input, excluding obvious limit-case situations
where the transmitting output is fed into the receiver input...?)

3)
use the usual AGC

....I'm thinking the 1 could be a good solution if the demodulator had to be
a digital one. That way, a calibrated attenuator simply add bits to the ADC.
Hovewer, the 2 is very attractive, providing that all is analog, or the
ADC dynamic range is better than the one that could come from the antenna...

I've read most use the 3, digitizing the AGC signal maybe with a second
ADC channel, to have anyway a sort of more bits of resolution.
So probably I'm wrong and the right solution is the 3... but only if adding
an AGC never ruin amplifiers performance.

: There isn't much FM on HF. What there is would be in
: narrow-band Data mode signals. Some of that Data is a
: combination of AM and PM similar to a wireline modem's
: modulation.

In fact I was receiving 144MHz using a converter.

: What is needed in an investigation of this is a reasonably-
: well-calibrated signal generator with a calibrated attenuator.

Unfortunately I have only a Instek function generator, and I'm not
very satisfied with any intrument I bought from this firm...

Anyway, sooner or later I'll build a dds one...

Ah, another question. I have a very precise digital voltmeter. Very
precise, 6.5digits (this time from Agilent)... Unfortunately, it's
absolutely unable to handle RF.
I would like to build a "RF" frontend for it... Any ideas?
I'm thinking to a precise rectifier built with an OP AMP followed by a
OP AMP integrator...

Ciao,
AB

.... Andrea Baldoni, 2002: messaggio non protetto da copyright.