"Jim" wrote in message ...

This other method involves measuring the gain of the device under test and
then measuring the noise power output with the input terminated
properly

....any errors
in the measurement can easily outnumber the actual noise figure

Thanks for reminding me of that one, Jim. But I see your point about
errors. For example, in measuring the gain one needs a standard. One
of the few pieces of real test gear I have is a bolometer-type RF
power meter which can measure about -13 dBm accurately. If the
measurement bandwidth is 1 MHz (suitable for VHF, perhaps) then
thermal noise is -114 dBm. So I need about 100 dB gain for a very low
noise figure DUT. To measure that I might need five 20 dB attenuators
as a standard, each with perhaps +/-0.5 dB accuracy if I am lucky...so
there's +/-2.5 dB error (well, I suppose I could cross my fingers and
RSS the numbers). Or I have a diode-type power meter that will
measure lower power, which leads to issues of how the detector
responds to noise. And then there's the problem of knowing the noise
bandwidth precisely...

is where half my gray hairs came from (the other half from being laid off).

I think about half of mine come from the latter factor too !

If you are measuring an entire receiver there are a few things you have to
be careful with. The receiver must be a linear receiver (no FM, AM diode
detector, etc.---basically just SSB). There should be a filter to pick just
one sideband. Turn the AGC off. Make sure you measure the gain in the
linear region, which also applies to a simple amplifier.

Definitely. Though, with my method the input never gets more than 3
dB above the receiver noise floor and in most cases a well designed
receiver will have no AGC response at that level. But with a preamp
in front it usually will activate the AGC, so AGC has to be switched
off when comparing preamps - which of course is impossible to do in
most ham rigs !

If you have a DUT with a known noise figure, I think that this would be one
way of calibrating a homebrew noise source.

Hence the interest in NF repeatability of MMIC amps, since they are
hard to build wrong, are well matched over a wide bandwidth and don't
require tuning for best noise figure. The known DUT can also be the
standard itself (in association with a receiver of only roughly known
NF) to avoid issues of errors in calibrating the noise source ENR.

73,
Steve VE3SMA