[amsat-bb] Re: fun with preamps

Stan, W1LE stanw1le at verizon.net
Mon Mar 15 17:02:54 PDT 2010

Hello Bob,

Does the preamplifier have a internal gain control ?

If it does, back it off at ~ 6 S units.

Excess gain only reduces the system dynamic range.

The real test is with a SINAD meter or a NF meter, injecting a test 
signal up at the preamplifier input,
and reducing gain until NF or SINAD sensitivity is just barely degraded.
That will be the point of best S/N, (best sensitivity).

Most of the S7 reading you have is just noise, not signal.

The preamplifier could be defective and performance is out of spec.
Whatever that spec is, I do not recall ever seeing a ICOM AG-25/35 
performance spec.

If the preamplifier is a multi stage unit, the first stage which sets 
the system noise figure
could be defective/soft and the second stage just amplifies the noise 
ahead of it.

Most convincing would be a NF test with professional meter.

Send it to me and I will test for free. I will NOT repair. I will NOT 
open it.
Include return postage if you want it returned (HI).

I will provide you a plot of the NF and gain performance using
a HP-8970B Noise Figure Meter and HP-346C Noise source.

Of course, this offer is open to any other AMSAT member.
E-mail me for details.

What do you say ?

Stan, W1LE    Cape Cod     FN41sr

Bob- W7LRD wrote:
> Hello 
> Just installed a AG-25 (Icom preamp) on my 2M cp yagi.  No preamp noise Smeter=0, preamp on Smeter=7.  Signals "appear" better after backing down the volume with the preamp.  I have 8-10 feet of LMR400uf from antenna to preamp, 50 feet of LMR400 to shack.  I suppose I was expecting more "dramatic" results, I realize the AG-25 isn't the greatest and best, but it works well with the 910.  Are these real world results for what I have.  S7 noise level seems a bit high. 
> as always-thanks 
> 73 Bob W7LRD 
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