S meter calibration

[IronArcher]

How can I accurately calibrate the S Meter?
Especially without a signal generator.
Thanks

[PD3LK]

I can't speak for all supported SDR radio's but with at least with a ANAN or SDRplay there's no way to calibrate the S-Meter yourself.

[IronArcher]

Thanks for the reply!
I'm 98% sure you could calibrate them with a signal generator, but I don't have one of those.
I saw something somewhere where a guy calibrated to a dummy load and it was extremely close.

[PD3LK]

What i mean is, you can't adjust the S-Meter within SDRC.
A dummyload is used for transmitting, i don't see what this has to to with (serious) S-Meter adjustments.

[jdow]

Calibrating with a dummy load presumes a whole lot of unknowns are in fact known. If you mean it is within 10 to 20 dB of accurate, maybe you can do it. But, with the nature of SDRs what you end up with as your configuration is not going to be optimum. And that presumes you have an SDR that is programmed by somebody a little anal retentive about calibration. Simon is anything but anal retentive about S-Meter calibration. He has found a way he likes. I disagree with him. But this is his project and I am merely a kibitzer. I do note that I can generate signals that completely mess up his S-meter. For something in which S-meter's matter, yes I know that is an oxymoron as stated, that is not "ideal."

So that is a show stopping problem before you even get into any calibration process with assumptions about room temperature resistor noise in a dummy load and receiver gains.

For your edification, Leon, the math for setting up calibration with a resistor noise source it's fairly simple, but the S-Meter part of it is severely challenged. Let me see here, S9 is -73 dBm. 1 S-Unit is 6 dB. So S1 is 48 dB lower or about -121 dBm. Keep that important number in mind.

Noise power is kTB. "k" is Boltzmann's constant. T is temperature in degrees K. B is the measurement bandwidth. At "standard" room temperature this power in a 1 Hz bandwidth is quite close to -174 dBm. (At least close enough for discussion at HF.) 3kHz is the nominal SSB bandwidth and is about 35 dB more noise power. Hm, that's only --139 dBm. Well, gee, let's presume the radio has a 10 dB noise figure. Enh, that's still only -129 dBm. Dang, that's still 8 dB low. Well, we could invent an S0 marker. Then you could set the calibration on the noise with a 3 kHz bandwidth. Or note that 10 kHz bandwidth gets you 5 dB and set the meter to waggling a little below S1. Then a relatively honest S-Meter reading would so something more or less accurate.

This should tell you two things, at least. The first thing is that S-Meters are trash for CW when the receiver can pull out a signal from negative S-Units. The second thing is that S-Units are a stupid measurement when honest dBm is much more meaningful. With dBm you can calibrate on the noise in 100 Hz if you wanted, -154 dBm.

Now that I've pounded the concept into the ground with a number two fire hardened oaken stake at a cross-roads at midnight....

{^_^}
By the way, how bad is the noise from that excellent rail network your country supports?

[PD3LK]

Thanks JD, i know about gross calibrating with a suitable resistor but didn't thought about using a dummy load for that.

There's no interference from high speed trains where i live (mid-east). They are mainly in the west part of our country. There are plans for a track to the north part in 2023 but they will not pass my home town Apeldoorn.
https://dutchreview.com/uncategorized/a ... from-2023/
I don't know if they produce much radio interference, i never heared about this from other radio amateurs.

[Simon G4ELI]

At the moment there's no calibration, but the relative signal levels will be as close to perfect as possible.

I may write an advanced RX window where everything can be fine tuned, I was considering this ~5 years ago.

[jdow]

Hm, you must actually be a ways out of town. Apeldoorn is a significant interchange point. It has lines into it from the West, South, and two from the East. That's why I figured they'd be a pain for you. That they aren't is a surprise and gratification. (A train driver named Vincent posts videos of the view from the cab as he drives the train. It shows pretty Dutch countryside and relatively often goes through Apeldoorn.) I guess the trains that do go through town are few enough that they are not a serious problem for hams.

{^_^}

[DK8KW]

I may write an advanced RX window where everything can be fine tuned, I was considering this ~5 years ago.

Hi Simon,

that would be great. Could you perhaps then also add a similar TX window so that the drive-slider shows the output power in dBm? That would be very helpful.

Vy 73

Holger ‚Geri‘, DK8KW

[Simon G4ELI]

I may write an advanced RX window where everything can be fine tuned, I was considering this ~5 years ago.

Hi Simon,

that would be great. Could you perhaps then also add a similar TX window so that the drive-slider shows the output power in dBm? That would be very helpful.

Vy 73

Holger ‚Geri‘, DK8KW

Noted. It's available in mW, so for now just a simple mental conversion .