S-Meter without signals covered by TNF

KD0RC

I have some pretty bad carriers that I can easily notch out with the TNF. The problem is that they show on the S-Meter, even with the TNF engaged. I would like to see the S-Meter cover only signals not covered by any TNFs in the passband.

John KB4DU

Well, the basic answer is that the TNF works in the audio portion of the radio, and the S meter reading is based on the signal strength at the antenna connection before filtering. This is different than a traditional superhet where the S meter is typically in the IF stage after filtering. But the flex has no IF, being direct sampling of the rf to an ADC for processing by software. Steve Hicks from flex has a recent YouTube video on this. I had the same question, but got the above answer at the flex booth at a ham fest.

KD0RC

The S-Meter only takes into account what is in the passband. Signals 10 Hz outside the filter have no influence at all (gotta love DSP!!). It is a matter of programming to remove the TNF slice(s) from the data that comprises the filter passband. Not a trivial task I would imagine, but certainly possible. It may not be worth doing if it would increase latency appreciably (maybe a Flex engineer could comment on this).

The S-Meter does not "see" the antenna connection, it is software that processes the data once the RF signal has been digitized in the ADC. It is coded such that it only uses the data within the filter passband. If this were not so, then the largest signal on the panadapter would generate the S-Meter reading.