S-Meter and received power on the panadapter

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  • Updated 5 years ago
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So i'm trying to understand what I'm seeing on the panadapter display with the dB along the right hand side.

First question is that I found this link here that relates received power to an S-meter reading...are the charts shown correct?

http://www.giangrandi.ch/electronics/...

Looking at the panadapter now on 10m, my noise floor looks to be running an average of -145. Comparing that number to the chart, it seems the noise floor is quite a bit below S1. This is great if true, but seems a bit hard to believe.

Now looking at the display of the S-Meter, in the same area where the panadapter is showing -145, the meter is displaying just a touch above S3. Hovering over the meter shows -105.7 dBm, which agrees (pretty close) with the chart. However, this does not make sense with the numbering on the right side of the panadapter.

From a "dumb look" perspective, it appears that something's not quite in sync here, potentially it's me. Any ideas?
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Michael - N5TGL

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  • feeling kinda dumb

Posted 5 years ago

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Graham - KE9H, Employee

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Michael:

Yes, the SmartSDR S-Meter uses the same criteria as the charts you referred to, where S9 = 50 microVolts into 50 Ohms = -73 dBm. And the measurement of signals that fit inside the receive filters will conform to this.

When you are dealing with noise, which is fairly evenly distributed across many frequencies, you can not talk about the level with out also talking about the bandwidth of the receiver that is measuring the noise.

The panadaptor in SmartSDR is actually made up of thousands of narrow band receivers, side by side, with bandwidths down in the 5 Hertz range. (It changes with panadaptor width.) The output of each of these receivers is plotted on the panadaptor for each frequency, to form the panadaptor display.

Since the bandwidth is much less, the noise each captures is much less. This also means that you can see signals in the panadaptor that are not strong enough to hear with your ears.

If you want to see what the noise floor in a SSB or CW bandwidth is, tune to the frequency with a slice receiver, set your receiver filter bandwidth, and look at the
S-meter. The noise floor as reported on the panadaptor is less by the ratio of the slice receiver bandwidth to the panadaptor bin bandwidth.

--- Graham / KE9H
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Michael - N5TGL

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Aha, that makes sense. I can see now that as I pick a narrow filter, it gets progressively closer to the noise floor of the panadapter. So where a conventional radio with a 2.7k filter will be subject to a S3 noise level (and we are too, in terms of what we can hear) we can see "below the noise"...which makes me REALLY want that panafall now!

This also makes me wish for a better zooming capability for the panadapter. For weak signal PSK and JT9, this is serious, earthshattering RX capability. If you had a really weak JT9 signal (which is very narrow bandwidth) you could squeeze the filter down to match the signal and really get down into the noise.

Good stuff, and thanks for explaining!

73,
Michael
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Sergey, R5AU

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excactly, you are right, you can see signal on PAN what is natively you unable to hear, it is easy to check in CW right now

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