Re: Console Signal meter reading


Allan Isaacs
 

Hi Michel

Yes, I agree.

I just watched the W8BYA video and you can see the SDR Play can be set up to indicate precise signal levels by trial and error and of course within realistic limits.

My receivers are an early SDR Play, an old Lime and a repaired Andrus. The available console settings are completely different for each of these and obviously I need to follow the calibration procedure suggested by Gedas to get correlation between true and indicated RF levels if indeed this is possible.

  

For example the Andrus has only a basic gain setting 0, -12, -24 and -30dBm with no IF gain or video gain settings.

 

As an example.

My Andrus sees a broadcast station on 198KHz, with no antenna connection, at -102dBm with gain 0dB and base noise visually at less than -145dBm but reading -136dBm and b/w +/-6KHz. If I change the gain to minus 30dB I see the base noise rise visually to minus 127dBm with a reading of minus 112dBm or visually minus 122dBm at +/-1KHz b/w. The broadcast station reads minus 108dBm but I can hear no audio. Changing the gain to -24dB provides a reading of minus 103dBm with very noisy audio; minus 12dB gives me the same reading of minus 102dBm with noisy audio. Finally reverting to 0dB gain I see the same minus 102dBm with clear audio.

What is changing with gain setting is the signal to noise ratio.

For weak signals the console setting is easy.. set the gain to 0dB. For strong signals I need to use a gain setting between -12 and -30dB. These tests had AGC off and because the signal level was so weak turning AGC on had no effect.

73 Allan G3PIY


From: main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] On Behalf Of michel soldevila
Sent: 25 January 2022 19:53
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] Console Signal meter reading

 

Hi Allan,
Test setup
RF generator on 15 MHz, output level set to 13 dBm or 1Vrms :
- ~ 6 Vcc measured (2.1 V rms) on my Tek 454 scope in case of direct connexion
- ~ 3 Vcc measured (1.05 V rms) on the scope when the generator is loaded with a 50 ohms resistive load 
The displayed output level of the generator is thus correct if the generator is loaded with its internal impedance, which is the case of the SDR antenna port A.
Do you agree ?
73, Michel (F1GOC)

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