"Stewart Bryant" wrote in message
news:1097795826.35817@sj-nntpcache-3...
Can you measure the noise power of the noise source by comparing
the output (through the RX) with the power from a signal
generator (again through the RX) of known output power?
Presumably using some form of averaging of multiple readings
across the RX passband.

- Stewart G3YSX

Steve Kavanagh wrote:

Some assorted comments and follow-on questions on your suggestions:

- (from bviel) The Elecraft noise generator is not a calibrated unit.
They give a typical ENR but specifically state that it varies from
unit to unit. Their low-level signal generator is calibrated (though
perhaps not quite accurately enough) but this would bring in the issue
of measurement errors between noise and sine-wave powers. And it only
works at one frequency.

- (from bviel) Which MMIC did you find had flat noise figure to 1 GHz
? My experience is that MMIC NF specs are usually at a frequency
above the flat part of the spectrum, as this is more indicative of the
high frequency performance. Are there any with noise figure also
specified, or at least well characterized, at low frequencies (within
the spectral region where the NF is flat) ? But the other question
with MMICs is the unit-to-unit variation in noise figure which I don't
believe is ever specified. If all units of a given part have, say,
between 2 and 3 dB noise figure then a MMIC amp could be a fairly good
standard for amateur use, but if the variation is from 1 to 4 dB then
the usefulness is limited, in my opinion.

- (from bviel) I don't have the JT-44 software but I did look at the
(full) manual. On page 35 it describes the "Measure Sub-Mode" which
allows noise levels to be compared. However it does not appear to
support absolute noise figure measurements (unless you use a
calibrated noise source). Is there more in the online help ?

- (from Mike Andrews) Terry Ritter's stuff seems mostly concerned with
the degree of randomness rather than absolute output power, which
makes sense as he is concerned with cryptography. But I did find,
elsewhere on his web page,

http://www.ciphersbyritter.com/REALRAND/92102201.HTM

a low frequency noise generator based on a diode source followed by a
limiting amp. I imagine this concept could be extended to HF by
suitable choice of amplifier. I am not sure of the significance of
the very different output waveform (pulses of random width and fixed
amplitude) compared to the true random (Gaussian) noise from the diode
itself. I am not sure if I am willing to tackle the math needed to
understand this or to calculate the ENR - can anyone help ?

73,
Steve VE3SMA


The answer is: Sort of.

Another post I made mentions that if you know the gain of your DUT you can
calculate the noise figure by measuring the noise power output with the
input terminated properly. Likewise, I think you could determine the ENR of
a noise source by the same method.

Someone else mentioned calculating the noise power of the ENR from the
current used. This would give the total noise power, but what is needed is
the noise power at a very narrow band of frequencies. If you are trying to
measure a receiver, the receiver determines the bandwidth, but with an LNA
you need a narrow band detector.

When I was doing the afore mentioned software, I used a FFT power spectrum
and made four measurements (DUT with noise source, DUT with out noise
source, test receiver with noise source, test receiver without noise
source). The FFT provided the narrow band filtering.

Of course, watch out for double sideband fold over in the mixer!

Jim
N8EE