[amsat-bb] Signal to Noise ratio

[Mike Seguin]

And here's a short text from Ian, GM3SEK about adjusting preamp gain. No 
test equipment needed. Too much gain can be a bad thing...

====

Here's a method that requires no test equipment at all. It comes from
G4DGU, who designed all the original muTek transverters and outboard
preamps to have adjustable gain. This method uses the sharp threshold
effect of FM detectors at low S/N ratios, and it allows you to optimize
the preamp/transverter gain for your local band noise conditions.

1. Turn the transverter/preamp gain well up.

2. Find a very weak but steady unmodulated carrier (off-air, not from a
signal generator or a local birdie). Rotate the antenna until you can
just detect the signal in FM mode.

3. Reduce the preamp/transverter gain until you hear the noise increase.
The FM threshold is sensitive to a small fraction of a dB in S/N.

4. Increase the gain just a little,to the point where you can't hear the
quieting improve much.

5. Switch back to a real DX mode.

Remember that every dB of unnecessary preamp/transverter gain will
probably subtract almost 1dB from your system intermod intercept!

The penalty of adjusting the gain correctly is that you're living just
above the "knee" where S/N will begin to deteriorate rapidly if
something changes. It's worthwhile to repeat this test every few months
- especially just before a contest.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                           'In Practice' columnist for RadCom (RSGB)

On 7/28/2020 9:28 AM, Leffke, Zachary via AMSAT-BB wrote:
> An EXCELLENT topic of conversation, and one I keep circling back to over and over and over.......
> 
> -Zach, KJ4QLP

-- 

73,
Mike, N1JEZ
"A closed mouth gathers no feet"