Re: SNR meter, bandwidth and gain adjustment for transverters


jdow
 

I fail utterly to understand that statement. It is coherency effects that can take a 20 MHz wide GPS P/Y-Code signal and coherently reduce it to its baseband 50 Hz bandwidth and decode the ephemeris against a jammer or other noise source that is on the order of 50 dB larger. It is coherency that allows a 200 kHz FM signal to show more than a 10 dB improvement in a 10 kHz bandwidth. The signal components add coherently - as voltage. The noise components add randomly - as power. Doing this in the frequency domain requires you know what is signal and what is noise and appropriately add power or voltage. And that varies with the mode. The energy in the selected passband is a constant. And it is the term that goes into BER calculations for digital modulation schemes.

If I take the spectral components of an FM signal and add them all up treating them the same as noise then there is no gain that comes out of the coherency of the signal, there is no trade-off between bandwidth and SNR. So I don't see how you can simply add up power in a collection of FFT bins to get an accurate power reading. Please elucidate.

{^_^}

On 20210706 00:12:17, Simon Brown wrote:

Hi,

 

I don’t care about the demodulator, signal strength is just frequency domain stuff. Doesn’t matter what you do with the IQ.

 

Simon Brown, G4ELI

https://www.sdr-radio.com

 

From: main@SDR-Radio.groups.io <main@SDR-Radio.groups.io> On Behalf Of jdow
Sent: 06 July 2021 07:49
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] SNR meter, bandwidth and gain adjustment for transverters

 

I'd need to see math that proves this is the way to handle coherent sidebands as opposed to summing a large number of complex numbers in pairs of matching sidebands. Are you doing something such that no filtered I and Q value is ever present before the demodulator? That is where it's automatic you get the right value.

{o.o}

On 20210705 23:27:05, Simon Brown wrote:

Hi,

 

Frequency, so using the FFT output. The FFT output is in dBm (log10), so has to be converted back from dB using pow(), averaged and then converted back to dB using log10. This is trivial, doesn’t use much CPU at all.

 

Simon Brown, G4ELI

https://www.sdr-radio.com

 

From: main@SDR-Radio.groups.io <main@SDR-Radio.groups.io> On Behalf Of jdow
Sent: 06 July 2021 07:03
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] SNR meter, bandwidth and gain adjustment for transverters

 

Averaging across frequency or time? Across frequency summation is perhaps closer, particularly if you presume it's all noise and simply add power per bin. But with coherent sidebands on a signal you can get surprised simply adding power I suspect. Alas, I do not KNOW that. I figure it's easier to do the I^2 + Q^2 operation on the IF before the demodulator. With that signal and something a bit like your noise averaging the signal strength reading and noise reading could actually help you know if there is enough signal to expect your demodulator to provide something understandable, especially for digital modulation schemes. (I'd take your minimum 10% data, sum the powers (I^2+Q^2) in each bin, divide by number of bins, and multiply back by number of bins in the filter bandwidth to get the noise power estimate. That last could probably be reduced to something close to a constant multiply operation.) This makes WGN environments quite predictable. Other environments suffer in comparison. So if the calculation above gives too low an SNR, or value converted to Eb/N0 is too low, then you know a digital mode is not even worth trying. Of it is high enough you know you have a sporting chance.

{^_^}


On 20210705 22:46:33, Simon Brown wrote:

FM power is always constant, this *could* be an area for an averaging signal meter.

 

I’m slowly losing the will to live 😊 .

 

Simon Brown, G4ELI

https://www.sdr-radio.com

 

From: main@SDR-Radio.groups.io <main@SDR-Radio.groups.io> On Behalf Of oldjackbob@...
Sent: 06 July 2021 02:47
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] SNR meter, bandwidth and gain adjustment for transverters

 

[Edited Message Follows]
[Reason: Initial send occurred for seemingly no reason.]

Joanne,

I probably misspoke at some point if/when I said BCFM carrier is always the greatest power anywhee in teh passband, so I retract any such statement.

However, that statement does hold true for BCAM, so I've captured a video of SDRC's S-meter reporting on a BCAM station:

https://drive.google.com/file/d/1Egb4DyIxXtZNP6rNNKg5PLfQzFTH_7zI/view?usp=sharing

Several key observations:

1) The carrier is very steady.
2) The modulation varies greatly.
3) the passband is intentionally varied between 2.5k and 12k.
4) The S-meter never varies at all (at least not more than .1dB in any direction).
5) The S-meter is reporting ONLY the peak signal detected anywhere in the passband, nothing more and nothing less.

So tell me where Simon gets it wrong.

Nevermind, that's a loaded question...Simon gets it right.

I'm done here,
Mark


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