Bandwidth of the s-meter in CW-U mode

Yvan Delaserge
Posts: 3
Joined: Sun Dec 04, 2022 11:21 am

Bandwidth of the s-meter in CW-U mode

#1

Unread post by Yvan Delaserge »

Hi all,
I am measuring the noise factor of some antenna preamplifiers by sending a calibrated signal into the preamp followed by my RTL SDR and SDR console.

I am using the so-called "gain method".
To use this method, I must measure the gain of the system (preamp + receiver). This is easy thanks to the excellent SDR Console s-meter which gives very stable readings in dB.

The "gain method" requires to measure the noise at the receiver when the antenna input is terminated with a 50 Ohm load.
Again, this is veary easily done with the SDR Console s-meter.

However, the gain method then requires to calculate the noise density per Hz. To do this, I need to know the passband of the receiver .
When I change the passband of the filter in the left panel of SDR Console, the reading on the s-meter does not change. With a wider filter, the s-meter should show an increase. And a decrease with a narrower filter.

So I assume thas SDR console uses a fixed filter to calculate the reading shown on the s-meter.

What is the width of this filter?

I guess it must be around 100 Hz, but I would like to be sure.

Thanks to the developer. Keep up the good job!

jdow
Posts: 800
Joined: Mon Aug 10, 2020 8:17 pm

Re: Bandwidth of the s-meter in CW-U mode

#2

Unread post by jdow »

Don't try to rely on the S-Meter to measure power in the receiver passband. That's not what Simon has done. (I have spoofed it to give very inaccurate readings.) It probably can be used for differential measurements for antenna testing. I'd be concerned about figures to the right of the decimal point.

Specific to your question, click on the "..." button in the "Filter" panel and follow your nose. Graphs included.

{^_^}

Yvan Delaserge
Posts: 3
Joined: Sun Dec 04, 2022 11:21 am

Re: Bandwidth of the s-meter in CW-U mode

#3

Unread post by Yvan Delaserge »

The SDR console s-meter IS accurate.
I am lucky to own a good signal generator with a calibrated power output and I tried feeding a signal from-80 to -120 dBm to my RTL SDR and got a very linear response from the SDR console s-meter readings. The difference between th s-meter reading and the known power lever from my generator was only a few dBm and very constant over the whole power range that I tested. This figure can easily be accounted for in the calculations as "gain " within the RTL SDR.

Actually I found a way around my problem of a fluctuating s-meter reading for very low level signals, close to the noise floor of the RTL SDR.

I am now using SDR#. The scrolling of the waterfall can be adjusted to be very slow and there is a s-meter with peak signal, noise floor and SNR that appears when hovering the analyzer display with the mouse pointer.
The reading integrates the signal over several seconds and is thus very much more stable.

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