You also have to know the noise bandwidth of the system to use this
method. If the response is dominated by a single, fairly steep-sided
filter, this is easy. But otherwise (such as if both IF filtering and AF
amplifier response shape the overall response), some calculation and/or
measurement is required. I've gotten what I believe are reasonable
results on HF and AF amplifiers using this method. But I haven't tried
it on a really low NF system, so would heed Jim's caution.

Roy Lewallen, W7EL

Jim wrote:

Steve,

There is another method of measuring noise figure that does not need a noise
source.

The reason that it is not used as often is that it is harder to measure low
noise figures accurately. Since there are still a few layers of rust on my
brain I will not geive the details here, since i do not want to say anything
that is too far off.

This other method involves measuring the gain of the device under test and
then measuring the noise power output with the input terminated properly.
The noise output is a combination of the DUT noise and the 'ambient' noise
from the input termination, times the gain of the receiver. Working
backwards you can then determine the DUT noise. As you can probably guess,
the reason it doesn't work so good on small noise figures is that any errors
in the measurement can easily outnumber the actual noise figure you are
trying to measure! And I have actually had to deal with this problem. That
is where half my gray hairs came from (the other half from being laid off).

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.

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

Jim
N8EE