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switch PLL reference to avoid integer spurs
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@jankae
jankae committed Mar 3, 2025
1 parent 224b2ab commit b133728
Showing 1 changed file with 72 additions and 16 deletions.
88 changes: 72 additions & 16 deletions Software/VNA_embedded/Application/VNA.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -50,6 +50,10 @@ static constexpr uint16_t maxPointsBetweenHalts = 40;
static constexpr uint16_t full2portDatapointSize = 66;// See Protocol::VNADatapoint::
static constexpr uint32_t reservedUSBbuffer = (maxPointsBetweenHalts + 2 /*additional buffer*/) * (full2portDatapointSize + 8 /*USB packet overhead*/);

static uint32_t PLLRefFreqs[] = {HW::PLLRef, HW::PLLRef - 1000000};
static constexpr uint8_t PLLRefFreqsNum = sizeof(PLLRefFreqs)/sizeof(PLLRefFreqs[0]);
static uint8_t sourceRefIndex, LO1RefIndex;

using namespace HWHAL;

static uint64_t getPointFrequency(uint16_t pointNum) {
Expand All @@ -71,16 +75,43 @@ static uint64_t getPointFrequency(uint16_t pointNum) {
}
}

static void setPLLFrequencies(uint64_t f) {
static bool setPLLFrequencies(uint64_t f) {
uint64_t sourceFreq = 0;
uint64_t LOFreq = 0;
if(f > HW::Info.limits_maxFreq) {
Source.SetFrequency(f / sourceHarmonic);
LO1.SetFrequency((f + HW::getIF1()) / LOHarmonic);
sourceFreq = f / sourceHarmonic;
LOFreq = (f + HW::getIF1()) / LOHarmonic;
} else {
if(f >= HW::BandSwitchFrequency) {
Source.SetFrequency(f);
sourceFreq = f;
}
LO1.SetFrequency(f + HW::getIF1());
LOFreq = f + HW::getIF1();
}
if(sourceFreq > 0) {
Source.SetFrequency(sourceFreq);
}
LO1.SetFrequency(LOFreq);
bool needsRefSwitch = false;
if(settings.suppressPeaks) {
// Integer spurs can cause a small peak.
uint32_t sourceDist = Source.DistanceToIntegerSpur();
uint32_t LODist = LO1.DistanceToIntegerSpur();
if((sourceDist > 0) && (sourceDist < 3 * HW::getIF2())) {
LOG_INFO("Source spur at %lu: %lu", (uint32_t) f, sourceDist);
sourceRefIndex = !sourceRefIndex;
Source.SetReference(PLLRefFreqs[sourceRefIndex], false, 1, false);
Source.SetFrequency(sourceFreq);
needsRefSwitch = true;
}
if((LODist > 0) && (LODist < 3 * HW::getIF2())) {
LOG_INFO("LO spur at %lu", (uint32_t) f);
LO1RefIndex = !LO1RefIndex;
LO1.SetReference(PLLRefFreqs[LO1RefIndex], false, 1, false);
LO1.SetFrequency(LOFreq);
needsRefSwitch = true;
}
}
return needsRefSwitch;
}

static bool needs2LOshift(uint64_t f, uint32_t current2LO, uint32_t IFBW, uint32_t *new2LO) {
Expand Down Expand Up @@ -108,11 +139,17 @@ static bool needs2LOshift(uint64_t f, uint32_t current2LO, uint32_t IFBW, uint32
}

bool VNA::Setup(Protocol::SweepSettings s) {
// Abort possible active sweep first
VNA::Stop();
vTaskDelay(5);
data.clear();
HW::SetMode(HW::Mode::VNA);
// Abort possible active sweep first

sourceRefIndex = 0;
LO1RefIndex = 0;
Source.SetReference(PLLRefFreqs[sourceRefIndex], false, 1, false);
LO1.SetReference(PLLRefFreqs[LO1RefIndex], false, 1, false);

FPGA::SetMode(FPGA::Mode::FPGA);
FPGA::WriteRegister(FPGA::Reg::ADCPrescaler, HW::getADCPrescaler());
FPGA::WriteRegister(FPGA::Reg::PhaseIncrement, HW::getDFTPhaseInc());
Expand Down Expand Up @@ -199,7 +236,9 @@ bool VNA::Setup(Protocol::SweepSettings s) {
}
// SetFrequency only manipulates the register content in RAM, no SPI communication is done.
// No mode-switch of FPGA necessary here.
setPLLFrequencies(freq);
if(setPLLFrequencies(freq)) {
needs_halt = true;
}
uint32_t new_LO2;
auto needs_shift = needs2LOshift(freq, last_LO2, actualBandwidth, &new_LO2);
if(needs_shift) {
Expand Down Expand Up @@ -249,6 +288,11 @@ bool VNA::Setup(Protocol::SweepSettings s) {
FPGA::Samples::SPPRegister, needs_halt);
last_lowband = lowband;
}
// reset a possibly changed PLL reference index
sourceRefIndex = 0;
LO1RefIndex = 0;
Source.SetReference(PLLRefFreqs[sourceRefIndex], false, 1, false);
LO1.SetReference(PLLRefFreqs[LO1RefIndex], false, 1, false);
// revert clk configuration to previous value (might have been changed in sweep calculation)
Si5351.SetCLK(SiChannel::RefLO2, HW::getIF1() - HW::getIF2(), Si5351C::PLL::B, Si5351C::DriveStrength::mA2);
Si5351.ResetPLL(Si5351C::PLL::B);
Expand Down Expand Up @@ -438,10 +482,30 @@ void VNA::SweepHalted() {
Si5351.Disable(SiChannel::LowbandSource);
FPGA::Enable(FPGA::Periphery::SourceRF);
}

if (pointCnt == 0 && settings.suppressPeaks) {
sourceRefIndex = 0;
LO1RefIndex = 0;
Source.SetReference(PLLRefFreqs[sourceRefIndex], false, 1, false);
LO1.SetReference(PLLRefFreqs[LO1RefIndex], false, 1, false);
// update PLL reference frequencies
Si5351.SetCLK(SiChannel::Source, PLLRefFreqs[sourceRefIndex], Si5351C::PLL::A, Si5351C::DriveStrength::mA8);
Si5351.SetCLK(SiChannel::LO1, PLLRefFreqs[LO1RefIndex], Si5351C::PLL::A, Si5351C::DriveStrength::mA8);
last_LO2 = HW::getIF1() - HW::getIF2();
Si5351.SetPLL(Si5351C::PLL::B, last_LO2*HW::LO2Multiplier, HW::Ref::getSource());
Si5351.ResetPLL(Si5351C::PLL::B);
Si5351.WaitForLock(Si5351C::PLL::B, 10);
HAL_Delay(2);
}

if(settings.suppressPeaks) {
// does not actually change PLL settings, just calculates the register values and
// is required to determine the need for a 2.LO shift
setPLLFrequencies(frequency);
if(setPLLFrequencies(frequency)) {
// update PLL reference frequencies
Si5351.SetCLK(SiChannel::Source, PLLRefFreqs[sourceRefIndex], Si5351C::PLL::A, Si5351C::DriveStrength::mA8);
Si5351.SetCLK(SiChannel::LO1, PLLRefFreqs[LO1RefIndex], Si5351C::PLL::A, Si5351C::DriveStrength::mA8);
}
if(needs2LOshift(frequency, last_LO2, actualBandwidth, &last_LO2)) {
Si5351.SetPLL(Si5351C::PLL::B, last_LO2*HW::LO2Multiplier, HW::Ref::getSource());
Si5351.ResetPLL(Si5351C::PLL::B);
Expand All @@ -451,14 +515,6 @@ void VNA::SweepHalted() {
}
}

if (pointCnt == 0) {
last_LO2 = HW::getIF1() - HW::getIF2();
Si5351.SetPLL(Si5351C::PLL::B, last_LO2*HW::LO2Multiplier, HW::Ref::getSource());
Si5351.ResetPLL(Si5351C::PLL::B);
Si5351.WaitForLock(Si5351C::PLL::B, 10);
HAL_Delay(2);
}

if(adcShiftRequired) {
FPGA::WriteRegister(FPGA::Reg::ADCPrescaler, alternativePrescaler);
FPGA::WriteRegister(FPGA::Reg::PhaseIncrement, alternativePhaseInc);
Expand Down

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