S Meter

 

 

 


 

Introduction

August 24th, 2019: An Analog S-meterh as been added to SDR Console, available in 3.0.13. There is a new entry in the ribbon bar where yopu select the S Meter style and other options.

Standard

The standard comes from "IARU Region 1 Technical Recommendation R.1":

STANDARDISATION OF S-METER READINGS

  1. One S-unit corresponds to a signal level difference of 6 dB,
  2. On the bands below 30 MHz a meter deviation of S-9 corresponds to an available power of -73 dBm from a continuous wave signal generator connected to the receiver input terminals,
  3. On the bands above 144 MHz this available power shall be -93 dBm (Note: not currently implemented),
  4. The metering system shall be based on quasi-peak detection with an attack time of 10 msec ± 2 msec and a decay time constant of at least 500 msec.

Noise Floor

A few weeks previously a reasonable logic was implemented for measuring the noise floor. Purists will not be happy, but they rarely are, but it works for me.

Take the output from the SDR radio, ignore 15% of the bandwidth at the high and low end of the output to avoid the ant-alias filtering, and we're left with a healthy 70% of the signal. Now sort the FFT bins by value, take the mean of the lowest 10% and that's the noise floor.

So the S Meters can display not just the signal level, but also the Signal to Noise Ratio (SNR).

When no signals are present with the SNR display it is quite normal to see a SNR reading. This is because Noise is the mean (average) whereas Signal is the peak value, so even in a quiet part of the band there will be a difference between the peak and mean. To see this yourself change the Smoothing algorithm to None (Ribbon Bar, View, Spectrum, Smoothing). The default smoothing is very good at eliminating noise.

You see the same effect with a 'normal' receiver which has a S meter - even on a quiet band it will be moving and following peaks in the noise.

Digital

This is the original V3 design, a standard digital meter/

Analog

Here are a few screenshots of the new Analog S Meter. Each of the three 'needles' or 'pointers' can be individually enabled.

S Units

First a tradition meter in S units.

  • Red = peak signal
  • Yellow = current signal
  • Grey = noise floor

 

dBm

Here's a dBm meter, same colours as above.

 

SNR

And here's a Signal to Noise (SNR) meter. When observing what may appear to be just background noise you will typically see a level of ~8dB SNR. This is because the noise floor value is the mean (average) of the noise, but the noise itself will not be constant. If you don't belive this, just watch the S Meter of a traditional radio on what appears to be a quiet band.

Note: when displaying SNR the Grey noise floor needle is not shown.

Transparency (version 3.0.14).

Responsive-V

 

Responsive-S