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G4RFR EME on 10GHz uses a 10GHz HP Spec Ann , on the 618MHz i.f. output of an Octagon LNB ; SDR14 running Spectraview , on a 28MHz down-converted , Continuum Mode and a G4JNT 144MHz diode detector noise meter system ( 2MHz filter bandwidth) .All downconversion stages use GPS referenced LO s .Typical Moon surface noise 1.8dB , Sun at 15.5dB .Echoes with 200W at feed "readable SSB" throughout the Lunar cycle .
Hope to be QRV again soon .
We're talking X-purposes. I'm interested in bandwidths captured by A/D converters, either in a soundcard for narrowish bands or a DS-SDR for wider . That way it's trivial to obtain an RMS value in the time domain; just square and sum every sample in a defined block length, then root the answer. Result, exact RMS of that set of samples. The QEX article does it in blocks of 50ms, then chooses the lowest sum to ensure the highest probability of getting a block with no spurious signals
For the FFT technique, do an FFT on a block, length suited to the frequency resolution of interest and convert the complex number in each bin to a power with Pythagorus. Then do the ordering and statistics on the result. That way any signals present get shoved up to the top of the ordered set and can be thrown away.
Practical measuring kit:
A thermal wattmeter will give you true RMS (well, power really) but averaging and time depends on the thermal mass of the detector and any electronic averaging put in later. I have a skepticism about "true RMS" meters. They are pretty universally designed for 50Hz mains, so I suspect their accuracy at audio frequencies until proven otherwise - they probably are OK, but they use an approximation / feedback method to measure RMS, relying on averaging in a capacitor to set a block / time constant for the RMS. Anyone recall the old Analog Devices AD590?
That's always the trouble with RMS measurement - when is a time period part of the RMS and when is it a varying quantity? And if a Cap is used in the averaging, where and how is the transition. (And why is a DC component never included in RMS measurement in any real kit)
Diode detectors may well give you a nice relative value for Sun and Moon peaking purposes, but they drift with temperature, DC offset etc. Also, if they're calibrated using a carrier then used with noise, there is at least 3:1 ratio for 99% of the noise peak to RMS. The detector has to remain properly square law to well over 10dB more dynamic range than the thing you're measuring. OK for Moon noise, but when it comes to sun noise with a large dish and good LNA, you end up needing over 25dB of perfect SQL linearity to make a really accurate measurement. I remember that problem playing with the FRARS EME system. The diode detector was a bit suspicious on sun noise measurements. I believe they've moved to SDR-IQ continuum measurement now
As you may gather - I've become quite a geek when it comes to measuring noise and S/N accurately - it's not an easy subject.
On Wed, 16 Sep 2020 at 14:06, Chris Bartram G4DGU <chris@...
Hello again, Andy,
I can certainly see that off-air HF noise could include artefacts
of the non-linear nature of the propagation medium - I'm old
enough to remember the 'Luxembourg Effect' on MW! IMD2 does
nowadays seem to be taken more-or-less seriously, particularly
since the advent of direct sampling receivers. But much VHF and
up equipment still seems to ignore it.
The use of FFT techniques could easily lead to measurement
inaccuracies/uncertainties in a situation where the 'noise'
spectrum was non-stochastic. I may be a bit of a dinosaur, but I
still use a zero-bias diode detector with proven square-law
response, a thermal wattmeter, or alternatively a wideband true
RMS voltmeter for making noise power measurements. True RMS
voltmeters like the Racal-Dana 9300 or 9303 are available surplus
at quite low prices, and with a simple downconverter can be used
as a back-end for noise ratio measurements. However that's getting
a bit off-topic.
question really came about after reading an
article in the latest QEX about noise monitoring
and WSPR, where they use two methods of
measuring noise, both of which I've adopted in
the past. The RMS of the time domain signal,
measured during the quiet period of no WSPR
transmissions. And the FFT techniques where
the bins are put into order of power and noise
deemed to be the level in the bin at the 30%
point ( I use the lower quartile plus 5dB -
thoughts on this differ but the delta is
that article they mention that often the two
measurement methods give significantly different
results, and at other times are very close.
This is put-down to the non-Gaussian nature of
noise at HF. Hence my thoughts at higher