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Rx Amplitude do not show the real values

Tom1000
Posts: 2
Joined: Fri Feb 26, 2021 3:54 pm

Rx Amplitude do not show the real values

#1

Unread post by Tom1000 »

Hello Community,
I am still on the way to switch from SDR Uno to SDR console because of connecting my FT 817 to sdr console.
In order to see how sdrc works I made a simple setup.
1. Rspdx on sdr uno
Signal generator set to -40,2dBm, cable and connectors compensated to -40dBm real input power on rspdx. Frequency set to cw 435MHz, connected to port b of rspdx. Rspdx set to am modulation. Test was performed with bw of 500khz and 2mhz.
Test with different gain and bw settings. Read out amplitude between -39.1 and -40.7dBm =>everything fine.

2. Start with Sdr-console and bw of 2mhz and 500Khz. Measured the same signal with different gain and if gain settings. Readout amplitude between -59 and -22dBm, always depending on the gain values.
=> no change to get the real input power of a unknown signal.

Now the questions. Did I used a wrong setting in sdrc?

If not, how can I perform with rspdx and sdrc a useful measurement of the real input signal power level?

Or how can I calculate the real input power depending on the different gain settings in sdr-console settings?
The only positive thing is that the relative change of input power is shown in the right way. This mean a change of 10db of input power level is shown as a change of 10 db in the sdrc display.

Any good ideas?

jdow
Posts: 315
Joined: Mon Aug 10, 2020 8:17 pm

Re: Rx Amplitude do not show the real values

#2

Unread post by jdow »

A general rule to follow that will not get you in trouble with SDRs, or for that matter ANY radio, is that it is uncalibrated until you apply a calibration cluebat to the poor thing. Your setup is guaranteed to NOT be calibrated to any any material degree. You will have to insert some accurate signal levels and use the various settings to calibrate everything. With a recently identified problem involving "Visual Gain" all I can say is "Good luck. You WILL need it."

{^_^}

User avatar
PD3LK
Posts: 319
Joined: Thu Aug 06, 2020 10:36 am

Re: Rx Amplitude do not show the real values

#3

Unread post by PD3LK »

JD is correct, you cannot use a SDR Rx and SDRC - nor SDRuno - as a reliable meassuring tool (if that is what you want).
Unless you have a calibrated signal generator and make a comparisation chart for every bandwidth, frequency, agc & gain settings and so on.

Tom1000
Posts: 2
Joined: Fri Feb 26, 2021 3:54 pm

Re: Rx Amplitude do not show the real values

#4

Unread post by Tom1000 »

Hey,
I partly agree with you, If I want a measurement device I woiuld take may analyzer. BUT....
Can you explain why SDRUno show approximately the real value (good enought for what I can expect from such a sdr receiver) and SDR console show something which is fare away from reality?
In general I like SDRC and I'm happy that OM's like Simon share this product with our community, but why do SDRUno includes the Gain settings in the calculation and SDRC obviously not?
I'don't like to start a never ending threat about a simple question. Therefore let me say thank's for your comment and let me close this topic.

jdow
Posts: 315
Joined: Mon Aug 10, 2020 8:17 pm

Re: Rx Amplitude do not show the real values

#5

Unread post by jdow »

Different design decisions.
{^_^}

BengtM
Posts: 12
Joined: Fri Nov 13, 2020 6:45 pm

Re: Rx Amplitude do not show the real values

#6

Unread post by BengtM »

I also wonder how the power measurement is done in SDRc. It's not the input power to the receiver at least. It's more the output power level since it's vary with gain settings. As Tom1000 points out SDR Uno shows a very exact power level. And there is rather much written in this topic and even a video. No calibration needed. But the accuracy is probably within +/- 2-5 dB but even this is rather good if you need to estimate the noise level or rather interference level on different bands. So it would be nice to know what SDRc is really showing when it is set to "dBm". Now it's just a value and can't be used for anything...


jdow
Posts: 315
Joined: Mon Aug 10, 2020 8:17 pm

Re: Rx Amplitude do not show the real values

#7

Unread post by jdow »

SDRC is not designed as a precision measurement system. As a matter of fact I have "fooled" its method of calculating signal level to think a fairly healthy signal was not really there at all. It required a contrived signal on my part. Over all I figure the SDRC accuracy is no worse than the average rice box and most other ham gear over the years and probably a good deal better than those "things".

{^_^}

BengtM
Posts: 12
Joined: Fri Nov 13, 2020 6:45 pm

Re: Rx Amplitude do not show the real values

#8

Unread post by BengtM »

I understand that SDRc is not precision measurement system but the power values are probably not within +/- 10 dB. Serious DX:ers care about noise levels, signal levels and compare different antennas and equipment. We also compare with each other to find possible improvements. It would be nice to do such measurements without investing a lot in such systems from Keysight, R&S etc. Rice boxes only shows constant "S9", at least when I listen to stations during weekends. But that's another story...
If there is a wish list for SDRc, put better accuracy of input power on it. Another related thing is the power level in the spectrogram. It should also reflect the input, not the output. The Noise Figure is fairly constant (at least with moderate RF gain). So when you adjust the IF gain, the noise level should not move at all or very little.

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