Improve spectrum analyzer

- revert LO2 shift mechanism (restores previous SA speed)
- allow tracking generator to reach all(?) frequencies with sufficient accuracy

Software/PC_Application/LibreVNA-GUI/Device/LibreVNA/librevnadriver.cpp

@@ -501,32 +501,6 @@ bool LibreVNADriver::setSA(const DeviceDriver::SASettings &s, std::function<void
     p.spectrumSettings.syncMode = (int) sync;
     p.spectrumSettings.syncMaster = syncMaster ? 1 : 0;
 
-    if(p.spectrumSettings.trackingGenerator && p.spectrumSettings.f_stop >= 25000000) {
-        // Check point spacing.
-        // The highband PLL used as the tracking generator is not able to reach every frequency exactly. This
-        // could lead to sharp drops in the spectrum at certain frequencies. If the span is wide enough with
-        // respect to the point number, it is ensured that every displayed point has at least one sample with
-        // a reachable PLL frequency in it. Display a warning message if this is not the case with the current
-        // settings.
-        auto pointSpacing = (p.spectrumSettings.f_stop - p.spectrumSettings.f_start) / (p.spectrumSettings.pointNum - 1);
-        // The frequency resolution of the PLL is frequency dependent (due to PLL divider).
-        // This code assumes some knowledge of the actual hardware and probably should be moved
-        // onto the device at some point
-        double minSpacing = 25000;
-        auto stop = p.spectrumSettings.f_stop;
-        while(stop <= 3000000000) {
-            minSpacing /= 2;
-            stop *= 2;
-        }
-        if(pointSpacing < minSpacing) {
-            auto requiredMinSpan = minSpacing * (p.spectrumSettings.pointNum - 1);
-            auto message = QString() + "Due to PLL limitations, the tracking generator can not reach every frequency exactly. "
-                            "With your current span, this could result in the signal not being detected at some bands. A minimum"
-                            " span of " + Unit::ToString(requiredMinSpan, "Hz", " kMG") + " is recommended at this stop frequency.";
-            InformationBox::ShowMessage("Warning", message, "TrackingGeneratorSpanTooSmallWarning");
-        }
-    }
-
     return SendPacket(p, [=](TransmissionResult r){
         if(cb) {
             cb(r == TransmissionResult::Ack);

Software/VNA_embedded/Application/Drivers/Si5351C.cpp

@@ -219,17 +219,6 @@ bool Si5351C::WritePLLConfig(PLLConfig config, PLL pll) {
 	} else {
 		success &=SetBits(Reg::PLLInputSource, mask);
 	}
-
-	// Reset the PLL
-	mask = pll == PLL::A ? 0x20 : 0x80;
-	//success &=SetBits(Reg::PLLReset, mask);
-	reg = pll == PLL::A ? Reg::MSNA_CONFIG : Reg::MSNB_CONFIG;
-	for(uint8_t i=0;i<8;i++) {
-		uint8_t readback;
-		ReadRegister((Reg)((int)reg + i), &readback);
-		LOG_DEBUG("PLL readback %d: 0x%02x", i, readback);
-	}
-
 	return success;
 }
 

Software/VNA_embedded/Application/SpectrumAnalyzer.cpp

@@ -44,6 +44,17 @@ static uint64_t firstPointTime;
 static bool firstPoint; // indicates the first point to be transmitted to the GUI, sets the firstPointTime
 static bool zerospan;
 
+static uint32_t PLLRefFreqs[] = {HW::PLLRef, HW::PLLRef - 1000000};
+static constexpr uint8_t PLLRefFreqsNum = sizeof(PLLRefFreqs)/sizeof(PLLRefFreqs[0]);
+static uint8_t PLLRefIndex;
+
+static void setPLLReference() {
+	Si5351.SetCLK(SiChannel::Source, PLLRefFreqs[PLLRefIndex], Si5351C::PLL::A, Si5351C::DriveStrength::mA8);
+	Si5351.SetCLK(SiChannel::LO1, PLLRefFreqs[PLLRefIndex], Si5351C::PLL::A, Si5351C::DriveStrength::mA8);
+	Source.SetReference(PLLRefFreqs[PLLRefIndex], false, 1, false);
+	LO1.SetReference(PLLRefFreqs[PLLRefIndex], false, 1, false);
+}
+
 static void StartNextSample() {
 	uint64_t freq = s.f_start + (s.f_stop - s.f_start) * pointCnt / (points - 1);
 	freq = Cal::FrequencyCorrectionToDevice(freq);
@@ -65,12 +76,15 @@ static void StartNextSample() {
 		negativeDFT = true;
 
 		// set tracking generator to default values
-		trackingFreq = 0;
 		trackingLowband = false;
 		attenuator = 0;
 		// this is the first step in each point, update tracking generator if enabled
 		if (s.trackingGenerator) {
+			auto oldTrackingFreq = trackingFreq;
 			trackingFreq = freq + s.trackingGeneratorOffset;
+			if(trackingFreq != oldTrackingFreq) {
+				// need to change the source frequency
+				oldTrackingFreq = trackingFreq;
 				if(trackingFreq > 0 && trackingFreq <= (int64_t) HW::Info.limits_maxFreq) {
 					// tracking frequency is valid, calculate required settings and select band
 					auto amplitude = HW::GetAmplitudeSettings(s.trackingPower, trackingFreq, s.applySourceCorrection, s.trackingGeneratorPort);
@@ -85,6 +99,30 @@ static void StartNextSample() {
 						FPGA::Disable(FPGA::Periphery::SourceRF);
 						trackingLowband = true;
 					} else {
+						// Source PLL can not hit all frequencies with high enough accuracy with a single reference frequency.
+						// Check available reference frequencies for best match
+						uint8_t bestIndex = 0;
+						uint32_t smallestDeviation = UINT32_MAX;
+						for(uint8_t i=0;i<PLLRefFreqsNum;i++) {
+							Source.SetReference(PLLRefFreqs[i], false, 1, false);
+							Source.SetFrequency(trackingFreq);
+							auto actualSource = Source.GetActualFrequency();
+							uint32_t deviation = (uint32_t) abs(actualSource - trackingFreq);
+							if(deviation < smallestDeviation) {
+								smallestDeviation = deviation;
+								bestIndex = i;
+								if(deviation < actualRBW / 20) {
+									// good enough, no need to check any further
+									break;
+								}
+							}
+						}
+						if(bestIndex != PLLRefIndex) {
+							LOG_INFO("Switching ref index (current %d, requested f=%lu%06lu)", PLLRefIndex
+									,(uint32_t) (trackingFreq / 1000000), (uint32_t)(trackingFreq%1000000));
+							PLLRefIndex = bestIndex;
+							setPLLReference();
+						}
 						Source.SetFrequency(trackingFreq);
 						Source.SetPowerOutA(amplitude.highBandPower, true);
 						FPGA::Enable(FPGA::Periphery::SourceChip);
@@ -95,6 +133,7 @@ static void StartNextSample() {
 					}
 				}
 			}
+		}
 		break;
 	case 1:
 		LO2freq = HW::getIF1() - HW::getIF2();
@@ -158,7 +197,9 @@ static void StartNextSample() {
 
 	// 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) > actualRBW / 100) {
-		Si5351.SetPLL(Si5351C::PLL::B, LO2freq*HW::LO2Multiplier, HW::Ref::getSource());
+//		Si5351.SetPLL(Si5351C::PLL::B, LO2freq*HW::LO2Multiplier, HW::Ref::getSource());
+		Si5351.SetCLK(SiChannel::Port1LO2, LO2freq, Si5351C::PLL::B, Si5351C::DriveStrength::mA2);
+		Si5351.SetCLK(SiChannel::Port2LO2, LO2freq, Si5351C::PLL::B, Si5351C::DriveStrength::mA2);
 		lastLO2 = LO2freq;
 	}
 	if (s.UseDFT) {
@@ -228,6 +269,10 @@ void SA::Setup(Protocol::SpectrumAnalyzerSettings settings) {
 	// set initial state
 	pointCnt = 0;
 	signalIDstep = 0;
+	trackingFreq = 0;
+	PLLRefIndex = 0;
+	setPLLReference();
+
 	// enable the required hardware resources
 	Si5351.Enable(SiChannel::Port1LO2);
 	Si5351.Enable(SiChannel::Port2LO2);