dbOffset values in fftviewer to compute dBFS

mariocannistra · 21 March 2018 13:02

Hello.

I’m looking into LimeSuite source code on github.
In file \src\fftviewer_wxgui\fftviewer_frFFTviewer.cpp
there are some statements that compute the signal power in dBFS to be presented in the GUI plugin’s window.

This is the section of code I’m interested in:

void fftviewer_frFFTviewer::OnUpdatePlots(wxThreadEvent& event)
{
double dbOffset = lmsIndex == 2 ? 93.319 : 69.2369;
if (mStreamRunning.load() == false)
return;

const int fftSize = streamData.fftBins[0].size();

float chPwr[2] = { 0, 0 };
double cFreq[2] = { 0, 0 };
txtCenterOffset1->GetValue().ToDouble(&cFreq[0]);
txtCenterOffset2->GetValue().ToDouble(&cFreq[1]);
double bw[2] = {1, 1};
txtBW1->GetValue().ToDouble(&bw[0]);
txtBW2->GetValue().ToDouble(&bw[1]);

for (int c = 0; c<2; ++c)
{
    float f0 = (cFreq[c] - bw[c]/2) * 1e6;
    float fn = (cFreq[c] + bw[c]/2) * 1e6;
    float sum = 0;
    int bins = 0;
    for (int i = 0; i<fftSize; ++i)
        if (f0 <= fftFreqAxis[i] && fftFreqAxis[i] <= fn)
        {
            sum += streamData.fftBins[0][i];
            ++bins;
        }
    chPwr[c] = sum;
}

float pwr1 = (chPwr[0] != 0 ? (10 * log10(chPwr[0])) - dbOffset : -300);
lblPower1->SetLabel(wxString::Format("%.3f", pwr1));
float pwr2 = (chPwr[1] != 0 ? (10 * log10(chPwr[1])) - dbOffset : -300);
lblPower2->SetLabel(wxString::Format("%.3f", pwr2));
lbldBc->SetLabel(wxString::Format("%.3f", pwr2-pwr1));

…

I see that the power is computed using 10 * log10( channelPower )) - dbOffset
where channelPower is the sum of the fft bins
and dbOffset can be either 93.319 or 69.2369

4 questions:
Could you please clarify what are those 2 decimal values for dbOffset?
Are them calibration offsets? if so are them valid for any purchased board? Why 2 values?

thank you,
mario

andrewback · 22 March 2018 06:50

Tagging @IgnasJ.

IgnasJ · 22 March 2018 08:13

Hi,
The values are to bring the maximum to 0. Two values are for 12-bit and 16-bit as they have different maximum values. Sample format is selectable.

mariocannistra · 22 March 2018 08:33

Thank you @IgnasJ
Is it possible to use this plugin as a measurement tool?
Example: should I place a noise source with fixed attenuator at the LNAL input of LimeSDR, could I measure the noise power over the tuned frequency and bandwidth?
Can I convert the dBFS output to dBm?

thank you,
mario

Karolis · 23 March 2018 08:03

Hello Mario,

Yes it is possible to use FFT viewer to measure signal to noise ratio. The FFT viewer provides bandwidth and offset to integrate noise power over a particular bandwidth - this feature can easily be used to measure noise at ADC input.

Measuring absolute noise power is a little more difficult as it needs some sort of reference to measure absolute gain and added noise from the receiver (e.g. Y factor method with Noise Diode source with known ENR). Once gain and noise figure have been measured over a particular frequency range, input noise power can then be measured and corrected for receiver noise figure.

Please be aware when measuring noise with a strong CW signal present (approx -25dB), there is a risk the noise power measurement accuracy will be reduced by the phase noise profile of the synthesizer.

For a dBm <-> dBFS conversion you would need to know the 0 dBFS peak voltage value, which in this case is equal to 0.8 V.

Regards,
Karolis

tony.bigbee · 23 March 2018 19:16

@IgnasJ Why is the data format dependent on the stream type (SISO,MIMO)? Looking at the Initialize function, for the case of a LimeSDR USB or LimeSDR Mini device, it appears that the number of channels would always be <=2, thus the lmsIndex would always be <=1 and hence the format always be LMS_FMT_I12. Thanks.

github.com

myriadrf/LimeSuite/blob/c02f4358b6eb3a83f333e8951419f34b18667dde/src/fftviewer_wxgui/fftviewer_frFFTviewer.cpp#L38


cmbStreamType->Clear();
const int num_ch = LMS_GetNumChannels(lmsControl, false);
if (num_ch>2)
{
    cmbStreamType->Append(_T("LMS1 SISO"));
    cmbStreamType->Append(_T("LMS1 MIMO"));
    cmbStreamType->Append(_T("LMS2 SISO"));
    cmbStreamType->Append(_T("LMS2 MIMO"));
    cmbStreamType->Append(_T("Ext. ADC/DAC"));
}
else if (num_ch == 2)
{
    cmbStreamType->Append(_T("LMS SISO"));
    cmbStreamType->Append(_T("LMS MIMO"));
}
else
{
    cmbStreamType->Append(_T("LMS SISO"));
}
cmbStreamType->SetSelection(0);
SetNyquistFrequency();

github.com

myriadrf/LimeSuite/blob/c02f4358b6eb3a83f333e8951419f34b18667dde/src/fftviewer_wxgui/fftviewer_frFFTviewer.cpp#L182


    if (dlg.ShowModal() == wxID_CANCEL)
        captureSamples.store(false);
    else
        captureFilename = dlg.GetPath().To8BitData();
}
else
    captureSamples.store(false);
chkCaptureToFile->Disable();
spinCaptureCount->Disable();
chkEnTx->Disable();
lmsIndex = cmbStreamType->GetSelection()/2;
if (mStreamRunning.load() == true)
    return;
switch (cmbStreamType->GetSelection()%2)
{
case 0: //SISO
    if (cmbChannelVisibility->GetSelection() > 1)
        cmbChannelVisibility->SetSelection(0);
    cmbChannelVisibility->Disable();
    threadProcessing = std::thread(StreamingLoop, this, fftSize, 1, 0);
    break;

github.com

myriadrf/LimeSuite/blob/c02f4358b6eb3a83f333e8951419f34b18667dde/src/fftviewer_wxgui/fftviewer_frFFTviewer.cpp#L383


    for(int ch=0; ch<channelsCount; ++ch)
    {
        samplesToCapture[ch] = pthis->spinCaptureCount->GetValue();
        captureBuffer[ch].resize(samplesToCapture[ch]);
        samplesCaptured[ch] = 0;
    }


if (LMS_GetNumChannels(pthis->lmsControl, false)>2)
    ch_offset += 2*pthis->lmsIndex;


auto fmt = pthis->lmsIndex == 2 ? lms_stream_t::LMS_FMT_I16 : lms_stream_t::LMS_FMT_I12;
for(int i=0; i<channelsCount; ++i)
{
    pthis->rxStreams[i].channel = i + ch_offset;
    pthis->rxStreams[i].fifoSize = fifoSize;
    pthis->rxStreams[i].isTx = false;
    pthis->rxStreams[i].dataFmt = fmt;
    pthis->rxStreams[i].throughputVsLatency = 0.8;
    LMS_SetupStream(pthis->lmsControl, &pthis->rxStreams[i]);


    pthis->txStreams[i].handle = 0;

github.com

myriadrf/LimeSuite/blob/c02f4358b6eb3a83f333e8951419f34b18667dde/src/fftviewer_wxgui/fftviewer_frFFTviewer.cpp#L253


}
else
{
    gaugeTxBuffer->SetValue(0);
    lblTxDataRate->SetLabel(printDataRate(0));
}
}


void fftviewer_frFFTviewer::OnUpdatePlots(wxThreadEvent& event)
{
double dbOffset = lmsIndex == 2 ? 93.319 : 69.2369;
if (mStreamRunning.load() == false)
    return;


const int fftSize = streamData.fftBins[0].size();


float chPwr[2] = { 0, 0 };
double cFreq[2] = { 0, 0 };
txtCenterOffset1->GetValue().ToDouble(&cFreq[0]);
txtCenterOffset2->GetValue().ToDouble(&cFreq[1]);
double bw[2] = {1, 1};

IgnasJ · 24 March 2018 15:19

Data format does not depend on SISO/MIMO mode. FFTviewer in LimeSuite uses 12-bit format by default. You can modify it to use 16-bit format if you want, but LMS7 chip gives only 12 bits anyway. The only exception in LimeSuite is made for external 14-bit ADC/DAC.

tony.bigbee · 24 March 2018 23:28

Thanks. The code is difficult to understand and uses indirect and conditional logic to determine the format. I wouldn’t call the use of 12-bit format default, per se, but I see that that is the net result.

lmsIndex = cmbStreamType->GetSelection()/2;

There are no code comments in StreamingLoop nor elsewhere.
Is the “external 14-bit ADC/DAC” device to which you refer an internal testing device, something coming down the pike, or just a concept?

mariocannistra · 9 April 2018 09:10

Hello @Karolis
Thank you for your kind answer and sorry for my late reply.
I would like to measure thermal noise produced by a 47 ohm metal film resistor connected in place of the antenna.
I already soldered to a coax and placed the resistor inside an epoxy layer to isolate from water…

Do you think the LimeSDR is sensitive enough to measure such noise ? Probably setting LNA/TIA/PGA in a proper way?

I also have a diode based noise source but I’m waiting for resistors to build an RF attenuator to arrive in the mail…
In the mean time I would like to measure hot and cold noise placing the resistor at 0 and 100 °C …
I’m expecting the following values:

Kind regards,
Mario