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mitigation for peaks caused by limited fractional divider in PLLs

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This commit is contained in:
Jan Käberich 2020-09-20 10:13:06 +02:00
parent fc3ce7a828
commit 57b4ebfb26
23 changed files with 654 additions and 274 deletions
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53
Software/VNA_embedded/Application/SpectrumAnalyzer.cpp
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@ -18,6 +18,7 @@ static uint32_t sampleNum;
static Protocol::PacketInfo p;
static bool active = false;
static uint32_t lastLO2;
static uint32_t actualRBW;
static float port1Measurement, port2Measurement;
@ -40,20 +41,42 @@ static void StartNextSample() {
FPGA::WriteRegister(FPGA::Reg::PhaseIncrement, 1120);
break;
case 1:
// Shift first LO to other side
LO1freq = freq - HW::IF1;
LO2freq = HW::IF1 - HW::IF2;
break;
// Shift first LO to other side
// depending on the measurement frequency this is not possible or additive mixing has to be used
if(freq >= HW::IF1 + HW::LO1_minFreq) {
// frequency is high enough to shift 1.LO below measurement frequency
LO1freq = freq - HW::IF1;
break;
} else if(freq <= HW::IF1 - HW::LO1_minFreq) {
// frequency is low enough to add 1.LO to measurement frequency
LO1freq = HW::IF1 - freq;
break;
}
// unable to reach required frequency with 1.LO, skip this signal ID step
signalIDstep++;
/* no break */
case 2:
// Shift both LOs to other side
LO1freq = freq + HW::IF1;
LO2freq = HW::IF1 + HW::IF2;
break;
case 3:
// Shift second LO to other side
LO1freq = freq - HW::IF1;
LO2freq = HW::IF1 + HW::IF2;
break;
// Shift second LO to other side
// depending on the measurement frequency this is not possible or additive mixing has to be used
if(freq >= HW::IF1 + HW::LO1_minFreq) {
// frequency is high enough to shift 1.LO below measurement frequency
LO1freq = freq - HW::IF1;
break;
} else if(freq <= HW::IF1 - HW::LO1_minFreq) {
// frequency is low enough to add 1.LO to measurement frequency
LO1freq = HW::IF1 - freq;
break;
}
// unable to reach required frequency with 1.LO, skip this signal ID step
signalIDstep++;
/* no break */
case 4:
// Use default frequencies with different ADC samplerate to remove images in final IF
LO1freq = freq + HW::IF1;
@ -66,7 +89,7 @@ static void StartNextSample() {
int32_t LO1deviation = (int64_t) LO1.GetActualFrequency() - LO1freq;
LO2freq += LO1deviation;
// only adjust LO2 PLL if necessary (if the deviation is significantly less than the RBW it does not matter)
if((uint32_t) abs(LO2freq - lastLO2) > s.RBW / 2) {
if((uint32_t) abs(LO2freq - lastLO2) > actualRBW / 2) {
Si5351.SetCLK(SiChannel::Port1LO2, LO2freq, Si5351C::PLL::B, Si5351C::DriveStrength::mA2);
Si5351.SetCLK(SiChannel::Port2LO2, LO2freq, Si5351C::PLL::B, Si5351C::DriveStrength::mA2);
lastLO2 = LO2freq;
@ -88,19 +111,23 @@ void SA::Setup(Protocol::SpectrumAnalyzerSettings settings) {
// in almost all cases a full sweep requires more points than the FPGA can handle at a time
// individually start each point and do the sweep in the uC
FPGA::SetNumberOfPoints(1);
// calculate amount of required points
points = 2 * (s.f_stop - s.f_start) / s.RBW;
// adjust to integer multiple of requested result points (in order to have the same amount of measurements in each bin)
points += s.pointNum - points % s.pointNum;
binSize = points / s.pointNum;
LOG_DEBUG("%u displayed points, resulting in %lu points and bins of size %u", s.pointNum, points, binSize);
// calculate required samples per measurement for requested RBW
// see https://www.tek.com/blog/window-functions-spectrum-analyzers for window factors
constexpr float window_factors[4] = {0.89f, 2.23f, 1.44f, 3.77f};
sampleNum = HW::ADCSamplerate * window_factors[s.WindowType] / s.RBW;
// round up to next multiple of 128
sampleNum += 128 - sampleNum%128;
if(sampleNum >= HW::MaxSamples) {
sampleNum = HW::MaxSamples;
}
actualRBW = HW::ADCSamplerate * window_factors[s.WindowType] / sampleNum;
FPGA::SetSamplesPerPoint(sampleNum);
// calculate amount of required points
points = 2 * (s.f_stop - s.f_start) / actualRBW;
// adjust to integer multiple of requested result points (in order to have the same amount of measurements in each bin)
points += s.pointNum - points % s.pointNum;
binSize = points / s.pointNum;
LOG_DEBUG("%u displayed points, resulting in %lu points and bins of size %u", s.pointNum, points, binSize);
// set initial state
pointCnt = 0;
// enable the required hardware resources