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Any good VNA can be used to evaluate preamps. However, remember to
consider IP3 and the max input that may yield 1 dB compression at the
output. When driven too hard, the device will not perform as it would with
a weak signal, which is your goal. Also, and I won't belabor the point as
it has been previously mentioned, best NF is generally not best match.
Rely on the published noise S-Parameters.

While not an instrument to measure absolute noise figure (but it can be
calibrated), the gated noise source is good for setting gain, bandwidth,
and noise figure. Zener diodes make wonderful noise sources when biased
near or at the knee. You can 'gate' them using the square wave output from
any signal generator and view the output on a spectrum analyzer. The
'noise output' impedance can be stabilized by installing a 50-ohm loss pad
between the Zener and the noise output. 5 dB of attenuation will assure a
worst case match (or known source impedance) of 2:1 SWR, or a return loss
of 10 dB. Best noise figure will occur when the noise on vs. noise off is
at a maximum. Using a broadband noise source, you will also be able to
view the bandwidth and, to an extent, the gain whether for max, best match,
and/or best NF. Even without a spectrum analyzer, you can set the preamp
for best NF using an o'scope with adequate bandwidth. If you can find or
buy a calibrated noise source (which I gave $5.00 for at a local ham
swapmeet several years ago), you will have an absolute measurement of NF.
When I found it on the table, the guy selling it commented, "Who wants more
noise"?! Little did he know the multiple uses of a calibrated noise
source!!

Dave - W?LEV

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*Dave - W?LEV*
*Just Let Darwin Work*