Re: SDR-Console Power dBm Accuracy


jdow
 

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.

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 averaging noise.

{^_^}

On 20210624 07:36:36, Gedas wrote:

Hi Larry. 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.

The same 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.

I generated a simple Youtube video illustrating the procedure if you think it is applicable to your needs. 73

https://www.youtube.com/watch?v=qPWXbodjVIA

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 wrote:
jdow,
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. 
Thanks,
Larry

On Jun 23, 2021, at 9:49 PM, jdow <jdow@...> wrote:

 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 Dodd wrote:
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.
Thanks,
Larry, K4LED


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