This approach is "workable" but it is not as
accurate because there are more sources of error in the
measurement. The Y-Factor approach simply requires a pair of noise
sources each at a well calibrated temperature roughly in line with
the expected noise figure you will be measuring.
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The MDS method depends on precision of the measurement noise
bandwidth. The Y-factor approach does not. The Y-factor approach
requires two easily measured temperatures. an RF relay, and a
means of measuring relative noise power between the two sources.
All bandwidth terms within the receiver used for measurement wash
out and you are left with only the actual receiver's noise figure
affecting the measurement. One more thing affects your reading -
how long is the noise power reading averaged? The longer the more
accurate the readings.
MDS measurements are simply a quick and dirty means of estimating
noise figure. The receiver noise bandwidth must be known. It must
report true RMS values on its signal meter, which is a very rare
thing outside of some SDRs. And the long term average readings are
required as noted above. Mark 1 eyeball does a damn poor job of
On 20210624 07:36:36, Gedas wrote:
Have you considered deriving the NF from the MDS ? This is
quite repeatable and accurate for detection systems that do
not have extremely low NF's.
caveats apply requiring a calibrated sig gen and external
step attenuator(s) with a calibrated true reading RMS meter.
Commonly done in my past life in tempest engineering.
a simple Youtube video illustrating the procedure if you
think it is applicable to your needs. 73
Gedas, W8BYA EN70JT
Gallery at http://w8bya.com (under repair)
Light travels faster than sound....
This is why some people appear bright until you hear them speak.
On 6/23/2021 9:57 PM, Larry Dodd
Thanks. We use a calibrated on/off noise source
to do a Y Factor Method for SDR noise figure measurements. I
have found just using the spectrum dBm measurements easier
than the dBm meter. It usually reads -140 dBm constantly with
low noise floors.
On Jun 23, 2021, at 9:49 PM, jdow <jdow@...>
That's a big subject.
To a first approximation the spectrum is very accurate for
differences in reading. It's theoretically perfect if
there is a perfect A/D converter involved. The A/D
converter's step to step linearity can mess up this
theoretically perfect relative calibration.
In any absolute sense any calibration present is rather
loosy goosy unless you provide a precision source and take
care getting your readings. Otherwise "loosy goosy" can
mean +/- 20 dB or worse.
I refuse to address Simon's S-Meter accuracy until I check
to see if I can still spoof it.
On 20210623 17:53:16, Larry
I have been asked the question, "How accurate are the
SDR-Console dBm spectrograph and dBm meter readings?" Can
anyone answer that question? We are interested in
measuring the noise floor of various SDR receivers.