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Re: DEM 13LNA Preamp



In a message dated 12/20/01 6:36:33 AM Eastern Standard Time, 
Jonathan.Naylor@socgen.com writes:

> I remember reading in Technical Topics in RadCom (the RSGB's comic) a tip 
> from
>  G4DGU (of Mutek fame) about adjusting for minimum noise.

While not the original article, here's a note from G3SEK:
=========================
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)

======================

73,
Mike, N1JEZ
AMSAT #29649
Local Area Coordinator
"A closed mouth gathers no feet."
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