SDRplay Community Forum

The S-Meter shows S-5 with no antenna. Also, -94dbm, SNR11.6, RBW 40.8

I understand that this is the aggregate of the total sample times. But S-5 seems odd with the antenna RSP1A 8 inch antenna lead terminated into a dummy load.
Dick N3HKN

Hello Dick,
Can post a picture of your Main Panel settings and your spectrum display? If you are not at maximum gain (minimum attenuation), you will see a higher noise level as SDRuno measures the total equivalent signal at the antenna, which includes the noise generated by the receiver itself. You also don't mention what frequency you are operating at or what your SP2 bandwidth is set to. The results that you will see will vary depending upon whether you are operating at HF or VHF. The S-Meter and power meter measure the total power within the SP2 (demodulation) bandwidth. Finally, please let us know what version of SDRuno you are using.

Sincerely

Tech_Support

See attachment for picture:
Version -> 1.22.0.0 in Properties. I had downloaded version 1.23.1 I thought I had run the installer but looks like 1.22.0.0 is still operational. So, does that make a difference in what I am seeing?
As far as the S-Meter and dbm meter they seem to be close in the values they show (5.5). I never had a dbm meter before so unsure of the correlation.

The slider in Attenuation mode meters show 5.5 In Max gain mode they show roughly 2.5. From what you are implying the Max Gain setting offers an accurate reading. It does correspond to the S-meter on my FT-450D using the same antenna. If that is the recommended operational mode then no problem with the meter reading relative to background noise. My noise is high here since I am in a condo (shared adjacent unit walls). It is part of the President's Love Thy Neighbor program to promote social cohesion!

Dick N3HKN

With RSP1 and SDRuno 1.22 In my case with the same 50 ohm terminated antenna connector my s meter reading is fine until I get to 29.914 mhz and LO at 29.00mhz. At that point the s-meter will jump up a few s units. How much it jumps up depends on if the LNA is on or off and where the IF gain slider is set. But the constant is the jump at 29.914 mhz. As I progress up into VHF and UHF the meter will move up or down an s unit but still remains at the higher reading.

Hi Dick,

Just to clear confusion I am enclosing the table showing the relationship between "S" units and received signal strength (power) in dBm. As far as my experience goes, with the RSP2pro and with the RSPduo, the "S" units shown on the meters correspond to the received dBm with faultless precision even changing the RF gain, provided you do not lower the RF gain to such a point as to bottom the IF automatic gain control.

Cheers,

glovisol

Hi Mark,

Herewith uploaded same table as in previous post, but for frequencies ABOVE 30 MHz....

Cheers,

glovisol

Odd. I wanted to duplicate my above observation. Without changing any settings I shut down SDRuno and started it back up. This time the jump in s units from 0-4 happens right at 30 mhz. IIRC that is where a new front end filter gets switched in. Could the s meter jump be related to that?

My noise floor only changes 1-2 dbm between 29.9 and 30 mhz. Thats why I am surprised at the large s meter jump.

Hello Mark,
If you look at the above posts from glovisol, he shows that the correct definition of S-meter levels changes as you transition from below 30 MHz to above 30 MHz, with S1 being -121 dBm below 30 MHz and S4 being -123 dBm above 30 MHz. The way SDRuno determines which values to use is by checking the LO frequency (NOT the VFO frequency), so if the LO is set below 30 MHz, it uses one set of tables and if it is above 30 MHz, it uses a different set of tables and that is completely correct to be conformant to IARU region 1 standards.

Sincerely

Tech_Support

n3hkn wrote:See attachment for picture:
Version -> 1.22.0.0 in Properties. I had downloaded version 1.23.1 I thought I had run the installer but looks like 1.22.0.0 is still operational. So, does that make a difference in what I am seeing?
As far as the S-Meter and dbm meter they seem to be close in the values they show (5.5). I never had a dbm meter before so unsure of the correlation.

The slider in Attenuation mode meters show 5.5 In Max gain mode they show roughly 2.5. From what you are implying the Max Gain setting offers an accurate reading. It does correspond to the S-meter on my FT-450D using the same antenna. If that is the recommended operational mode then no problem with the meter reading relative to background noise. My noise is high here since I am in a condo (shared adjacent unit walls). It is part of the President's Love Thy Neighbor program to promote social cohesion!

Dick N3HKN

Hello Dick,
All power measurements are made at the ADC input and referred back to the antenna by dividing the power at the ADC input by the total gain. So as well as measuring noise from the antenna, you are also measuring the noise generating by the receiver. As you reduce the RF gain, the receiver noise figure will increase and so the equivalent input noise generated by the receiver will increase. This is why your noise power will increase as you reduce the RF gain. This does not prevent the S-meter from being accurate as the RF gain is reduced, it is simply reflecting that the receiver noise floor is increasing. If you had a signal present, you would see that the signal level would not change either on the S-meter, power meter or the spectrum display. It is simply a case of the receiver noise floor increasing. This is entirely correct and normal behaviour. The reason why you might need to reduce the gain (increase the RF attenuation) is simply if you are having overload problems from very strong signals. If you are not, then the higher the gain, the lower the noise floor and the better the sensitivity.

Sincerely

Tech_Support

Ok, I understand.. Back to having fun.......
THANKS