SDR operated with a noise source


Allan Isaacs
 

I notice my SDR-Play when fed with a home brew noise source gives signal strength readings that do not match the display.
Generally the meter indicates some 6 to 10dBm higher than the display. This also is the case with the noise source switched off and just baseline background noise.
With the noise source switched off the baseline noise may be at -125dBm but the meter indicates -119dBm. Typically, attached is a picture of the noise source used with a low pass filter. The noise signal looks like -116dBm at 190KHz and the meter shows -109.7dBm.
Is this explainable?
Allan G3PIY


Willi Passmann
 

Allan, the different readings probably are related to different
bandwidths. 190 kHz for the spectrum and waterfall display, but maybe 3
kHz (or whatever you selected) for the demodulated signal that the S
meter reflects.

vy 73,
Willi, DJ6JZ

Am 14.01.2021 um 13:25 schrieb Allan Isaacs:

The noise signal looks like -116dBm at 190KHz and the meter shows
-109.7dBm.


Allan Isaacs
 

That does seem to make sense Willi.
If I measure the noise source with a wideband power meter it tells me it is
0dBm so the noise within the selected bandwidth of the SDR at 190KHz is
summed over the 3.5KHz setting of the selected AM mode.. Presumably, all
things being equal, if I selected CW 50Hz the meter reading would be closer
to that shown in the display?
Allan G3PIY

-----Original Message-----
From: main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] On Behalf
Of Willi Passmann
Sent: 14 January 2021 12:35
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] SDR operated with a noise source

Allan, the different readings probably are related to different
bandwidths. 190 kHz for the spectrum and waterfall display, but maybe 3
kHz (or whatever you selected) for the demodulated signal that the S
meter reflects.

vy 73,
Willi, DJ6JZ

Am 14.01.2021 um 13:25 schrieb Allan Isaacs:
The noise signal looks like -116dBm at 190KHz and the meter shows
-109.7dBm.


Willi Passmann
 

Allan,
As I understand it, values would be equal only if you could select the
same bandwidth for the spectrum range and the demodulation range. But
you can try and tell us what you find out :-)

vy 73,
Willi, DJ6JZ

Am 14.01.2021 um 13:47 schrieb Allan Isaacs:

That does seem to make sense Willi.
If I measure the noise source with a wideband power meter it tells me it is
0dBm so the noise within the selected bandwidth of the SDR at 190KHz is
summed over the 3.5KHz setting of the selected AM mode.. Presumably, all
things being equal, if I selected CW 50Hz the meter reading would be closer
to that shown in the display?
Allan G3PIY

-----Original Message-----
From: main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] On Behalf
Of Willi Passmann
Sent: 14 January 2021 12:35
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] SDR operated with a noise source

Allan, the different readings probably are related to different
bandwidths. 190 kHz for the spectrum and waterfall display, but maybe 3
kHz (or whatever you selected) for the demodulated signal that the S
meter reflects.

vy 73,
Willi, DJ6JZ

Am 14.01.2021 um 13:25 schrieb Allan Isaacs:
The noise signal looks like -116dBm at 190KHz and the meter shows
-109.7dBm.






Allan Isaacs
 

Hi Willi
I tried lots of settings, visual gain and receiver gain, bandwidth etc but
the meter consistently read 6dB or more too high compared with the trace..
I then set the receiver to 500KHz scan with AM bandwidth to 2Hz and the
meter read correctly. Switching to AM 20KHz and it read 10dB too high.
I guess the meter is only good for comparative readings on a proper signal
but on noise you need to look at the trace and read off the vertical
scales..
If there was a trimmer so that you could tweak the meter to read the same as
the trace it would be nice, but I'm only experimenting with a noise source
to take the place of a tracking generator..
Allan G3PIY

-----Original Message-----
From: main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] On Behalf
Of Willi Passmann
Sent: 14 January 2021 13:38
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] SDR operated with a noise source

Allan,
As I understand it, values would be equal only if you could select the
same bandwidth for the spectrum range and the demodulation range. But
you can try and tell us what you find out :-)

vy 73,
Willi, DJ6JZ

Am 14.01.2021 um 13:47 schrieb Allan Isaacs:
That does seem to make sense Willi.
If I measure the noise source with a wideband power meter it tells me it
is
0dBm so the noise within the selected bandwidth of the SDR at 190KHz is
summed over the 3.5KHz setting of the selected AM mode.. Presumably, all
things being equal, if I selected CW 50Hz the meter reading would be
closer
to that shown in the display?
Allan G3PIY

-----Original Message-----
From: main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] On Behalf
Of Willi Passmann
Sent: 14 January 2021 12:35
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] SDR operated with a noise source

Allan, the different readings probably are related to different
bandwidths. 190 kHz for the spectrum and waterfall display, but maybe 3
kHz (or whatever you selected) for the demodulated signal that the S
meter reflects.

vy 73,
Willi, DJ6JZ

Am 14.01.2021 um 13:25 schrieb Allan Isaacs:
The noise signal looks like -116dBm at 190KHz and the meter shows
-109.7dBm.







Alan G4ZFQ
 

I tried lots of settings, visual gain and receiver gain, bandwidth etc but
the meter consistently read 6dB or more too high Allan
Does this not help?

https://www.sdr-radio.com/s-meter

73 Alan G4ZFQ


Allan Isaacs
 

Not really
Allan G3PIY

-----Original Message-----
From: main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] On Behalf
Of Alan G4ZFQ
Sent: 14 January 2021 16:47
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] SDR operated with a noise source

I tried lots of settings, visual gain and receiver gain, bandwidth etc but
the meter consistently read 6dB or more too high
Allan
Does this not help?

https://www.sdr-radio.com/s-meter

73 Alan G4ZFQ


jdow
 

Until you calibrate it there is no way SDRC can be thought of as being calibrated.

Without a thorough of what Simon is displaying you cannot take the spectrum levels as meaningful.

Without an even more detailed knowledge of the S-Meter function you cannot track it back to anything meaningful, pretty much the same as your average rice box. (I have managed to completely fool Simon's S-Meter tool by 20 dB or more. Unless he changed it the tool is merely a "keep the bloody ignorant hams whining about wanting an S-Meter shut up.)

SDRC is very pointedly not a measurement system giving precise readings of power. And it probably cannot be converted into such a thing without a gawdawful amount of work.

But, you are welcome to try.
{^_^}

On 20210114 07:40:58, Allan Isaacs wrote:
Hi Willi
I tried lots of settings, visual gain and receiver gain, bandwidth etc but
the meter consistently read 6dB or more too high compared with the trace..
I then set the receiver to 500KHz scan with AM bandwidth to 2Hz and the
meter read correctly. Switching to AM 20KHz and it read 10dB too high.
I guess the meter is only good for comparative readings on a proper signal
but on noise you need to look at the trace and read off the vertical
scales..
If there was a trimmer so that you could tweak the meter to read the same as
the trace it would be nice, but I'm only experimenting with a noise source
to take the place of a tracking generator..
Allan G3PIY

-----Original Message-----
From: main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] On Behalf
Of Willi Passmann
Sent: 14 January 2021 13:38
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] SDR operated with a noise source

Allan,
As I understand it, values would be equal only if you could select the
same bandwidth for the spectrum range and the demodulation range. But
you can try and tell us what you find out :-)

vy 73,
Willi, DJ6JZ

Am 14.01.2021 um 13:47 schrieb Allan Isaacs:
That does seem to make sense Willi.
If I measure the noise source with a wideband power meter it tells me it
is
0dBm so the noise within the selected bandwidth of the SDR at 190KHz is
summed over the 3.5KHz setting of the selected AM mode.. Presumably, all
things being equal, if I selected CW 50Hz the meter reading would be
closer
to that shown in the display?
Allan G3PIY

-----Original Message-----
From: main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] On Behalf
Of Willi Passmann
Sent: 14 January 2021 12:35
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] SDR operated with a noise source

Allan, the different readings probably are related to different
bandwidths. 190 kHz for the spectrum and waterfall display, but maybe 3
kHz (or whatever you selected) for the demodulated signal that the S
meter reflects.

vy 73,
Willi, DJ6JZ

Am 14.01.2021 um 13:25 schrieb Allan Isaacs:
The noise signal looks like -116dBm at 190KHz and the meter shows
-109.7dBm.


















Allan Isaacs
 

That’s more or less what I expected. The SDR meter provides a jolly useful measure of change and not anything absolute.

I began to get interested in this topic when discussing the Tiny 2.8 Spectrum Analyser that has no tracking generator unlike my Rigol.

 

I built a noise source that seems to work OK and provides, after some iterations, a reliable output from around 40KHz to a couple of GHz so should enable the Tiny SA (or other things, even SDRs) to check things like filters.

I’ve been using my Rigol as well as my SDR and both give the same subjective results (in other words both provide equal filter test characteristics) but with the Rigol giving accurate power readings or attenuation figures.

One point of interest is the total power from the noise source. Although the power level is not especially high at any given frequency, for example it shows as S9 +30dB on a decent communications receiver from 100KHz to 30MHz, it develops 1mW (0dBm) as measured by my HP power meter (10MHz to 10GHz). If the power meter spec covered 0-10GHz that 0dBm figure would register a higher output, perhaps as high as +10dBm? That means that it would be a bad idea to further amplify the noise output for fear of accidentally damaging a spectrum analyser which typically has a max input rating of 20dBm.

Allan G3PIY

 


From: main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] On Behalf Of jdow
Sent: 15 January 2021 10:25
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] SDR operated with a noise source

 

Until you calibrate it there is no way SDRC can be thought of as being calibrated.

Without a thorough of what Simon is displaying you cannot take the spectrum levels as meaningful.

Without an even more detailed knowledge of the S-Meter function you cannot track it back to anything meaningful, pretty much the same as your average rice box. (I have managed to completely fool Simon's S-Meter tool by 20 dB or more. Unless he changed it the tool is merely a "keep the bloody ignorant hams whining about wanting an S-Meter shut up.)

SDRC is very pointedly not a measurement system giving precise readings of power. And it probably cannot be converted into such a thing without a gawdawful amount of work.

But, you are welcome to try.
{^_^}

 


jdow
 

I don't see here that you understand noise at all well, especially the characterization and measurement of noise. First off, S9+30 dB is in practical terms no more descriptive than, "You have a bodacious signal, OM." My experience with giving calibrated signal level reports and how poorly they were received suggests a properly calibrated S9 point is about as likely as a purple unicorn poaching your breakfast cereal.

However, -43 dBm is a fairly large number. If it really is 10 GHz wide then it's level in a 1 Hz bandwidth is about -143 dB. Yeah, that's a fair number to survive insertion of some attenuators for an accurate measurement process. (Boltzmann noise, which you cannot go below, is about -174 dBm, -204 dBm. Note: That's dBm not S "-7" or something. S-Units are "good buddy" measurements for somebody interested in any degree of precision.) Including or not including the bottom 100 MHz is not easily measured compared to leaving out the bottom 100kHz or bottom 1 Hz. The formula for Boltzmann noise is kTB, k = Boltzmann's constant, T is temperature degrees Kelvin, and B is the bandwidth in question.

Hopefully you see the "oops" above. I left out your S9+30 measurement. I don't know the bandwidth you used for measurement. Let's figure you used 10k kHz for AM measurements. Move that up to a 10 GHz bandwidth and you are talking about 60 dB more noise power over that whole 10 GHz bandwidth - it becomes a bit under 1 watt, actually +23 dBm. That is a ridiculously high power level for a noise generator. To measure a noise figure you want a noise source roughly equal in power to the noise figure you wish to measure. Since it would be generating noise at -83 dBm/Hz it's not good for noise figure measurement.

That thumbnail analysis is why I figure you are groping in the dark. There are a lot of good articles to read on the web. Some will probably be at a good starting level for gaining a quick study level of knowledge. And even the highly technical ones are not all that hard to understand when you extract the fundamental numbers and formulas.

{^_^}

On 20210115 03:53:01, Allan Isaacs wrote:

That’s more or less what I expected. The SDR meter provides a jolly useful measure of change and not anything absolute.

I began to get interested in this topic when discussing the Tiny 2.8 Spectrum Analyser that has no tracking generator unlike my Rigol.

 

I built a noise source that seems to work OK and provides, after some iterations, a reliable output from around 40KHz to a couple of GHz so should enable the Tiny SA (or other things, even SDRs) to check things like filters.

I’ve been using my Rigol as well as my SDR and both give the same subjective results (in other words both provide equal filter test characteristics) but with the Rigol giving accurate power readings or attenuation figures.

One point of interest is the total power from the noise source. Although the power level is not especially high at any given frequency, for example it shows as S9 +30dB on a decent communications receiver from 100KHz to 30MHz, it develops 1mW (0dBm) as measured by my HP power meter (10MHz to 10GHz). If the power meter spec covered 0-10GHz that 0dBm figure would register a higher output, perhaps as high as +10dBm? That means that it would be a bad idea to further amplify the noise output for fear of accidentally damaging a spectrum analyser which typically has a max input rating of 20dBm.

Allan G3PIY

 


From: main@SDR-Radio.groups.io [mailto:main@SDR-Radio.groups.io] On Behalf Of jdow
Sent: 15 January 2021 10:25
To: main@SDR-Radio.groups.io
Subject: Re: [SDR-Radio] SDR operated with a noise source

 

Until you calibrate it there is no way SDRC can be thought of as being calibrated.

Without a thorough of what Simon is displaying you cannot take the spectrum levels as meaningful.

Without an even more detailed knowledge of the S-Meter function you cannot track it back to anything meaningful, pretty much the same as your average rice box. (I have managed to completely fool Simon's S-Meter tool by 20 dB or more. Unless he changed it the tool is merely a "keep the bloody ignorant hams whining about wanting an S-Meter shut up.)

SDRC is very pointedly not a measurement system giving precise readings of power. And it probably cannot be converted into such a thing without a gawdawful amount of work.

But, you are welcome to try.
{^_^}

 



Re Claudio
 

Console is a really fantastic and giantic tool !

Unluckily on the metering part there was a big effort for many interesting features , but it doesn't work because the metering display something related to the maximum peak found into the RF band .
If you want to check this , just measure a clean carrier , then modulate it in FM inside the RF bandwidth
Despite to the fact that the total power will of course remain the same , the metering level will go down by many dB .

If you will use in the same conditions an analog receiver ( ICR7000 for instance ) the metering level will not change with clean carrier or FM modulation ( like it has to be ) .
Among many other , you can try HDSDR and SpectraVue to have correct metering .
If I remember well also SDRuno has correct metering .

73-Claudio I1RFQ 


Roger Need
 

On Thu, Jan 14, 2021 at 04:25 AM, Allan Isaacs wrote:
I notice my SDR-Play when fed with a home brew noise source gives signal strength readings that do not match the display.
Generally the meter indicates some 6 to 10dBm higher than the display. This also is the case with the noise source switched off and just baseline background noise.
With the noise source switched off the baseline noise may be at -125dBm but the meter indicates -119dBm. Typically, attached is a picture of the noise source used with a low pass filter. The noise signal looks like -116dBm at 190KHz and the meter shows -109.7dBm.
Is this explainable?
Allan G3PIY
The S-meter in SDR console is not calibrated for SDRplay devices.  The reason is that Simon chose not to use the gain calibration tables to get accurate dBm levels at the input of the receiver.

From an earlier discussion in this group I seem to recall that he does not measure the total power in the bandwidth to arrive at the signal level.  Rather he looks for the peak signal in the bandwidth.  There was a long discussion about this and he implemented it the way he wanted.

If you want accurate dBm measurements (+/- 1 dB) at the input of the RSP use SDRuno.  It is the only SDR program for the RSP that does this automatically for all IF and RF gain settings. It measures the total power in the bandwidth so it will measure the output of your noise generator correctly.   The S-meter also conforms to the IARU standard of 6 dB per S unit (most Japanese analog radios use 2 to 4 db per S unit).

Roger