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Posted: Wed Mar 27, 2019 2:02 pm
by glovisol
Yes indeed, the RDSplay RSP "S" meter takes into account the specified variation in readings between frequencies BELOW 30 MHz and frequencies ABOVE 30 MHz. This is clearly explained, with useful tables, in the cited reference, which I post again below:


I just wish to add that nobody has ever stated or implied that the "S" meter reading should also take into account antenna gain, propagation conditions or the price of one pound of coffee on the Nairobi stock exchange. The "S" meter indication is just a convenient way for one radio operator to tell another that his signal is coming in, AT HIS RECEIVER'S INPUT, with a specific level, read by a CALIBRATED instrument: this is far easier an more practical than stating a dBm value and this is the reason why it has been with us since the beginning of Radio Communications.

The entire point here is to realise that with RSP class receivers the "S" reading is not an arbitrary number, but a facility which tells EXACTLY what the level is at the receiver's input.


Posted: Fri Mar 29, 2019 4:58 pm
by ON5HB
There you make a mistake.

The S-meter only gives the sum of signals at the input, there is no way it will tell the real strength unless your noise floor is 0 volt and this is never the case.

I have seen many cases where the s-meter read 9+20db and the other station was barely audible, the reading was qrm/qrn.

Another thing is USB/LSB can hitch-hike on a carrier without being strong but still have a "strong" level at the input.

As such the meter and it's scale is useless, calibrated or not.


Posted: Fri Mar 29, 2019 6:10 pm
by Tech_Support
The S-meter gives a measurement of the total signal power within the demodulation bandwidth. It is incapable of distinguishing between wanted signal and interference within the same bandwidth and so it is perfectly possible to have a reading of s9+20 and have a barely audible signal. S-meters give a measurement of power and not signal quality and 'twas ever thus'.

If you are suggesting that the S-meter is useless as an accurate indicator of signal quality, then you are quite correct. If you are questioning whether it provides a reasonable accurate measurement of total RF power within the demodulation bandwidth (as defined by the SP2 filter) then you are incorrect.

The S-meter and associated power meter in SDRuno are calibrated to the correct IARU standard which is that s9=-73 dBm for signals at 30 MHz or below and s9=-93 dBm for frequencies above 30 MHz. The accuracy of the power measurement is typically 1 dB across all gain settings, which is a fraction of an s-unit. This is unusual to have this level of accuracy for any radio allows the RSPs to be used as a perfectly respectable RF power meter and many people have already found.




Posted: Fri Mar 29, 2019 6:51 pm
by sdrom33
Hi ON5HB seems to me you are confusing signal intensity with signal s/n. You can have a strong signal in a strong noise, so signal looks noisy, but still it is S9 all the same if s meter o.k.

S meter for s/n someone must still invent.


Posted: Fri Mar 29, 2019 7:28 pm
by glovisol
Bas wrote:
As such the meter and it's scale is useless, calibrated or not.
It takes old timers like me, who daily used in the past Power Meters (then called "bolometers") in design laboratories, to understand and appreciate the fantastic value of the accurate and reliable power measurement so effortlessly and inexpensively provided by any SDRplay Spectrum Processor and associated software.

Among other applications, an RSP-1A in the lab can be used to measure power with a fantastic accuracy, even high power, provided suitable high dissipation, calibrated power attenuators are available. Previously, especially when testing solid state Power Amplifiers, which were (and are) prone to the phenomenon of subharmonic generation, all capacitor coupled wattmeters (Bird, for instance) had to be pensioned and replaced by power attenuators and Bolometers: quite an expensive proposition.

If one became distracted, a very expensive bolometer head would blow up and have to be replaced and with that money you could buy a big lot of RSP-1A's....I could have never suspected then that in the future I would have on my desk a receiver with the precision of a Bolometer at a fraction of its cost!

This is the reason why, Bas, I do try to make all involved aware of the fact that the small black box they have in their hands is far more valuable than they might think.

The same is true for the "S" meter function, which also shares the measurement stability, reliability and precision of the RSP power meter.


Posted: Sat Mar 30, 2019 10:08 am
by ON5HB
I was talking about the S-meter as instrument to measure signals on HF that are not lab-conditions.
As an instrument measuring e.g. power in a lab it's perfect as the conditions are always the same and all variables are known.
But to give signal reports on areal radio stations it's not very useful.

I never said anything else. :D