The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Tag: SDR

Software Defined Radios and circuitry

  • Monthly Science: 60 kHz Preamp Resonator Bandwidth

    Putting a small capacitor in series with the tuning fork resonator pulls the series resonant frequency upward and reduces the amplitude:

    60 kHz Quartz TF Resonator - CX variations
    60 kHz Quartz TF Resonator – CX variations

    So something around 10 pF, net of stray capacitance and suchlike, should suffice. Plunk a small twiddlecap on the preamp board and tune for best picture:

    LF Crystal Tester - resonator protection
    LF Crystal Tester – resonator protection

    Using the DDS generator as a manual signal source with 1.0 Hz step size shows the resonator tightens up the preamp’s response quite nicely:

    60 kHz Preamp - Bandwidth - 1 Hz steps
    60 kHz Preamp – Bandwidth – 1 Hz steps

    I’m not convinced the preamp will have filter skirts that low farther away from the peak, but it’ll do for a start.

    Zoom in on the peak with 0.1 Hz steps:

    60 kHz Preamp - Bandwidth - 100 mHz steps
    60 kHz Preamp – Bandwidth – 100 mHz steps

    The bandwidth looks like 0.6 Hz, centered just slightly above 60.000 kHz, which should be fine for a first pass.

    I’m tickled: all the hardware & firmware fell neatly into place to make those graphs possible!

    Next step: install it in the attic and see whether the filter cuts back the RF clutter enough to stabilize the SDR’s AGC gain.

  • 60 kHz Preamp: Filtering and Rebiasing

    The LT1920 instrumentation amplifier now sports two silver-mica caps on its inputs as a differential-mode input filter cutting back strong RF signals (clicky for more dots):

    60 kHz Preamp Schematic - DM filter inst amp - BP filter rebias - 2017-09-22
    60 kHz Preamp Schematic – DM filter inst amp – BP filter rebias – 2017-09-22

    In principle, a DM filter should eliminate RF rectification from out-of-band signals, although I think the attic is quiet enough to not need any help. The caps form a simple RC LP filter rolling off at 5.490 kΩ × 150 pF → 193 kHz, high enough above the 60 kHz signal to not make much difference down there.

    The silver-mica caps come from the Big Box o’ Caps, which contained an envelope with a few large 150 pF ±1% caps and a bag stuffed with similar 147 pF ±1% caps. Mixed in with the latter were some smaller 147 pF caps (*) of no particular tolerance (perhaps 5%), from which I neurotically matched a pair to 0.05 pF without too much effort. Doesn’t matter, given the other tolerances and suchlike, but it was amusing.

    I’d inadvertently grounded the cold end of the 330 Ω input resistor in the LM353 bandpass filter, now properly tied at the Vcc/2 virtual ground to take the DC load off the LT1920 output: a 100 nF cap (27 Ω at 60 kHz) stores the bias level without messing up the filter shape.

    A similar cap rebiases the protected resonator at the LT1010 buffer input:

    60 kHz Preamp Schematic - protected resonator - output rebias - 2017-09-22
    60 kHz Preamp Schematic – protected resonator – output rebias – 2017-09-22

    The new caps aren’t all that visible and the resonator vanishes in the clutter:

    60 kHz Preamp - protected resonator filter - overview
    60 kHz Preamp – protected resonator filter – overview

    Next: find out how well it works!

    (*) Yes, there were two envelopes between 150 pF and 147 pF:

    Silver-mica caps
    Silver-mica caps

  • LF DDS Sine Generator With 0.1 Hz Steps

    Gutting the crystal tester program and grafting a simple joystick interface onto the corpse produces an LF sine wave generator with 0.10 Hz steps:

    FG085 vs AD9850 DDS frequencies
    FG085 vs AD9850 DDS frequencies

    The FG085 function generator shows 60000 Hz and the AD9850 shows 60001.58 Hz, but they’re running at exactly the same frequency:

    DDS 1.58 FG085 0.0
    DDS 1.58 FG085 0.0

    I trust the AD9850 readout, because I just finished zero-beating it against the GPS-locked 10 MHz frequency reference: it’s dead on. The scope’s frequency measurement is clearly out of its depth at this resolution.

    The “user interface” doesn’t amount to much. The DDS starts at 60.000 kHz, as defined by a program constant. Push the joystick left-right to step by 0.1 Hz (actually, multiples of 0.0291 Hz, so either 0.087 or 0.116 Hz, whichever makes the answer come out closer to the next multiple of 0.1 Hz). Push it up-down to step by 1.0 Hz (insert similar handwaving here). Push the button inward to reset to 60.000 kHz.

    The OLED displays the frequency (in big digits), the output of the log amplifier (which isn’t hooked up here) in dB (over 4 μV), the DDS clock oscillator temperature, and a few lines of static prompting. The camera shutter blanked the last line, which should read “Button = reset”.

    There’s no amplitude adjustment, other than the DDS current-control twiddlepot and the buffer amp’s gain-setting jumpers, but I (think I can) gimmick up an adequate inductive kicker for the fake preamp antenna circuit.

    Not much to look at, but now I can (manually) probe the frequency response of the 60 kHz preamp with sufficient resolution to figure out if / how the tuning fork resonator filter behaves.

    The Arduino source code as a GitHub Gist:

    // Sine wave generator
    // Ed Nisley – KE4ZNU
    // 2017-09-20
    #include <avr/pgmspace.h>
    #include <U8g2lib.h>
    #include <U8x8lib.h>
    #include <Adafruit_MCP4725.h>
    //———————
    // Pin locations
    #define PIN_SYNC 5
    #define PIN_CX_SHORT 6
    #define PIN_DDS_RESET 7
    #define PIN_DDS_LATCH 8
    #define PIN_HEARTBEAT 9
    #define PIN_LOG_AMP A0
    #define PIN_JOYBUTTTON A1
    #define PIN_JOY_Y A2
    #define PIN_JOY_X A3
    // SPI & I2C use hardware support: these pins are predetermined
    #define PIN_SS 10
    #define PIN_MOSI 11
    #define PIN_MISO 12
    #define PIN_SCK 13
    #define PIN_IIC_SDA A4
    #define PIN_IIC_SCL A5
    // IIC Hardware addresses
    // OLED library uses its default address
    #define LM75_ADDR 0x48
    #define SH1106_ADDR 0x70
    #define MCP4725_ADDR 0x60
    // Useful constants
    #define GIGA 1000000000LL
    #define MEGA 1000000LL
    #define KILO 1000LL
    #define ONE_FX (1LL << 32)
    #define CALFREQ (10LL * MEGA * ONE_FX)
    // Structures for 64-bit fixed point numbers
    // Low word = fractional part
    // High word = integer part
    struct ll_fx {
    uint32_t low; // fractional part
    uint32_t high; // integer part
    };
    union ll_u {
    uint64_t fx_64;
    struct ll_fx fx_32;
    };
    // Define semi-constant values
    union ll_u CenterFreq = {(60000 – 0) * ONE_FX}; // center of scan
    //union ll_u CenterFreq = {(32768 – 2) * ONE_FX}; // center of scan
    #define NOMINAL_OSC ((125 * MEGA) * ONE_FX)
    union ll_u Oscillator = {NOMINAL_OSC}; // oscillator frequency
    int16_t OscOffset = 287; // offset from NOMINAL_OSC at room-ish temperature
    // Coefficients for oscillator offset as function of temperature
    #define TC_SQUARE ((1340 * ONE_FX) / 1000)
    #define TC_LINEAR ((-1474 * ONE_FX) / 10)
    #define TC_INTERCEPT ((3415 * ONE_FX) )
    // Frequency range & step size
    uint16_t TestWidth = 5*2; // width must be an even integer
    union ll_u StepSize = {ONE_FX / 10}; // 0.1 Hz is smallest practical decimal step
    union ll_u NomFreq, ActualFreq; // displayed vs actual DDS frequency
    union ll_u TestFreq;
    // Global variables of interest to everyone
    union ll_u CtPerHz; // will be 2^32 / oscillator
    union ll_u HzPerCt; // will be oscillator / 2^32
    char Buffer[10+1+10+1]; // string buffer for fixed point number conversions
    union ll_u Temperature; // read from LM75A
    // Hardware library variables
    U8X8_SH1106_128X64_NONAME_HW_I2C u8x8(U8X8_PIN_NONE);
    //U8X8_SH1106_128X64_NONAME_4W_HW_SPI u8x8(PIN_DISP_SEL, PIN_DISP_DC , PIN_DISP_RST);
    //U8X8_SH1106_128X64_NONAME_4W_SW_SPI u8x8(PIN_SCK, PIN_MOSI, PIN_DISP_SEL, PIN_DISP_DC , PIN_DISP_RST);
    #define DAC_WR false
    #define DAC_WR_EEP true
    #define DAC_BITS 12
    #define DAC_MAX 0x0fff
    Adafruit_MCP4725 XAxisDAC; // I²C DAC for X axis output
    uint32_t XAxisValue; // DAC parameter uses 32 bits
    union ll_u LogAmpdB; // computed dB value
    // Timekeeping
    #define HEARTBEAT_MS 3000
    unsigned long MillisNow,MillisThen;
    //———–
    // Useful functions
    // Pin twiddling
    void TogglePin(char bitpin) {
    digitalWrite(bitpin,!digitalRead(bitpin)); // toggle the bit based on previous output
    }
    void PulsePin(char bitpin) {
    TogglePin(bitpin);
    TogglePin(bitpin);
    }
    // These may need debouncing in some circuits
    void WaitButtonDown() {
    word ai;
    do {
    ai = analogRead(PIN_JOYBUTTTON);
    } while (ai > 600);
    }
    void WaitButtonUp() {
    word ai;
    do {
    ai = analogRead(PIN_JOYBUTTTON);
    } while (ai < 400);
    }
    void WaitButton() {
    Serial.print(F("Waiting for button:"));
    WaitButtonDown();
    delay(10);
    WaitButtonUp();
    delay(100);
    Serial.println(F(" done"));
    }
    // Hardware-assisted SPI I/O
    void EnableSPI(void) {
    digitalWrite(PIN_SS,HIGH); // set SPI into Master mode
    SPCR |= 1 << SPE;
    }
    void DisableSPI(void) {
    SPCR &= ~(1 << SPE);
    }
    void WaitSPIF(void) {
    while (! (SPSR & (1 << SPIF))) {
    // TogglePin(PIN_HEARTBEAT);
    // TogglePin(PIN_HEARTBEAT);
    continue;
    }
    }
    byte SendRecSPI(byte Dbyte) { // send one byte, get another in exchange
    SPDR = Dbyte;
    WaitSPIF();
    return SPDR; // SPIF will be cleared
    }
    //————–
    // DDS module
    void EnableDDS(void) {
    digitalWrite(PIN_DDS_LATCH,LOW); // ensure proper startup
    digitalWrite(PIN_DDS_RESET,HIGH); // minimum reset pulse 40 ns, not a problem
    digitalWrite(PIN_DDS_RESET,LOW);
    delayMicroseconds(1); // max latency 100 ns, not a problem
    DisableSPI(); // allow manual control of outputs
    digitalWrite(PIN_SCK,LOW); // ensure clean SCK pulse
    PulsePin(PIN_SCK); // … to latch hardwired config bits
    PulsePin(PIN_DDS_LATCH); // load hardwired config bits = begin serial mode
    EnableSPI(); // turn on hardware SPI controls
    SendRecSPI(0x00); // shift in serial config bits
    PulsePin(PIN_DDS_LATCH); // load serial config bits
    }
    // Write delta phase count to DDS
    // This comes from the integer part of a 64-bit scaled value
    void WriteDDS(uint32_t DeltaPhase) {
    SendRecSPI((byte)DeltaPhase); // low-order byte first
    SendRecSPI((byte)(DeltaPhase >> 8));
    SendRecSPI((byte)(DeltaPhase >> 16));
    SendRecSPI((byte)(DeltaPhase >> 24));
    SendRecSPI(0x00); // 5 MSBs = phase = 0, 3 LSBs must be zero
    PulsePin(PIN_DDS_LATCH); // write data to DDS
    }
    //————–
    // Log amp module
    #define LOG_AMP_SAMPLES 10
    #define LOG_AMP_DELAYMS 10
    uint64_t ReadLogAmp() {
    union ll_u LogAmpRaw;
    LogAmpRaw.fx_64 = 0;
    for (byte i=0; i<LOG_AMP_SAMPLES; i++) {
    LogAmpRaw.fx_32.high += analogRead(PIN_LOG_AMP);
    delay(LOG_AMP_DELAYMS);
    }
    LogAmpRaw.fx_64 /= LOG_AMP_SAMPLES; // figure average from totally ad-hoc number of samples
    LogAmpRaw.fx_64 *= 5; // convert from ADC counts to voltage at 5 V/1024 counts
    LogAmpRaw.fx_64 /= 1024;
    LogAmpRaw.fx_64 /= 24; // convert from voltage to dBV at 24 mV/dBV
    LogAmpRaw.fx_64 *= 1000;
    return LogAmpRaw.fx_64;
    }
    //———–
    // Read LM75A and convert to signed fixed point
    // Returns signed value in something otherwise used as unsigned
    // Blithely ignores most IIC error conditions
    int64_t GetTemperature() {
    union ll_u Temp;
    Wire.requestFrom(LM75_ADDR,2);
    if (Wire.available() == 2) {
    Temp.fx_32.high = Wire.read();
    Temp.fx_32.low = (uint32_t)Wire.read() << 24;
    if (Temp.fx_32.high & 0x00000080L) { // propagate – sign
    Temp.fx_32.high |= 0xffffff00L;
    }
    }
    else {
    Temp.fx_64 = 256 * ONE_FX; // in-band error flagging: 256 C
    }
    return Temp.fx_64;
    }
    //———–
    // Compute frequency offset from oscillator temperature
    // This is an ordinary signed integer
    // Because 1 Hz resolution at 125 MHz is Good Enough
    int16_t ComputeOffset() {
    union ll_u Temperature;
    union ll_u T1;
    Temperature.fx_64 = GetTemperature();
    T1.fx_64 = TC_SQUARE;
    if (TC_SQUARE) // skip multiply for linear fit
    T1.fx_64 = MultiplyFixedPt(T1,Temperature);
    T1.fx_64 += TC_LINEAR;
    T1.fx_64 = MultiplyFixedPt(T1,Temperature);
    T1.fx_64 += TC_INTERCEPT;
    PrintFixedPtRounded(Buffer,Temperature,3);
    printf("Offset: %d at %s C\n",(int16_t)T1.fx_32.high,Buffer);
    return (int16_t)(T1.fx_32.high); // extract integer part
    }
    //———–
    // Zero-beat oscillator to 10 MHz GPS-locked reference
    void ZeroBeat() {
    union ll_u TempFreq,TempCount;
    printf("Zero beat DDS oscillator against GPS\n");
    TempFreq.fx_64 = CALFREQ;
    u8x8.clearDisplay();
    byte ln = 0;
    u8x8.drawString(0,ln++,"10 MHz Zero Beat");
    u8x8.drawString(0,ln++," <- Jog -> ");
    u8x8.drawString(0,ln++," ^ recalc ");
    u8x8.drawString(0,ln++," Button = set ");
    int32_t OldOffset = -OscOffset; // ensure first update
    while (analogRead(PIN_JOYBUTTTON) > 500) {
    TogglePin(PIN_HEARTBEAT); // show we got here
    int ai = analogRead(PIN_JOY_Y) – 512; // totally ad-hoc axes
    if (ai < -100) {
    OscOffset += 1;
    }
    else if (ai > 100) {
    OscOffset -= 1;
    }
    ai = analogRead(PIN_JOY_X) – 512;
    if (ai < -100) {
    OscOffset = ComputeOffset();
    }
    if (OscOffset != OldOffset) {
    ln = 5;
    sprintf(Buffer,"Offset %9d",OscOffset);
    u8x8.drawString(0,ln,Buffer);
    CalcOscillator(OscOffset); // recalculate constants
    TempCount.fx_64 = MultiplyFixedPt(TempFreq,CtPerHz); // recalculate delta phase count
    WriteDDS(TempCount.fx_32.high); // DDS output should be exactly 10 MHz
    OldOffset = OscOffset;
    }
    Temperature.fx_64 = GetTemperature();
    PrintFixedPtRounded(Buffer,Temperature,3);
    ln = 7;
    u8x8.drawString(0,ln,"DDS Temp");
    u8x8.drawString(16-strlen(Buffer),ln,Buffer);
    delay(100);
    }
    printf("Oscillator offset: %d at %s C\n",OscOffset,Buffer);
    WaitButtonUp();
    u8x8.clearDisplay();
    }
    //———–
    // Round scaled fixed point to specific number of decimal places: 0 through 8
    // You should display the value with only Decimals characters beyond the point
    // Must calculate rounding value as separate variable to avoid mystery error
    uint64_t RoundFixedPt(union ll_u TheNumber,unsigned Decimals) {
    union ll_u Rnd;
    Rnd.fx_64 = (ONE_FX >> 1) / (pow(10LL,Decimals)); // that's 0.5 / number of places
    TheNumber.fx_64 = TheNumber.fx_64 + Rnd.fx_64;
    return TheNumber.fx_64;
    }
    //———–
    // Multiply two unsigned scaled fixed point numbers without overflowing a 64 bit value
    // Perforce, the product of the two integer parts mut be < 2^32
    uint64_t MultiplyFixedPt(union ll_u Mcand, union ll_u Mplier) {
    union ll_u Result;
    Result.fx_64 = ((uint64_t)Mcand.fx_32.high * (uint64_t)Mplier.fx_32.high) << 32; // integer parts (clear fract)
    Result.fx_64 += ((uint64_t)Mcand.fx_32.low * (uint64_t)Mplier.fx_32.low) >> 32; // fraction parts (always < 1)
    Result.fx_64 += (uint64_t)Mcand.fx_32.high * (uint64_t)Mplier.fx_32.low; // cross products
    Result.fx_64 += (uint64_t)Mcand.fx_32.low * (uint64_t)Mplier.fx_32.high;
    return Result.fx_64;
    }
    //———–
    // Long long print-to-buffer helpers
    // Assumes little-Endian layout
    void PrintHexLL(char *pBuffer,union ll_u FixedPt) {
    sprintf(pBuffer,"%08lx %08lx",FixedPt.fx_32.high,FixedPt.fx_32.low);
    }
    // converts all 9 decimal digits of fraction, which should suffice
    void PrintFractionLL(char *pBuffer,union ll_u FixedPt) {
    union ll_u Fraction;
    Fraction.fx_64 = FixedPt.fx_32.low; // copy 32 fraction bits, high order = 0
    Fraction.fx_64 *= GIGA; // times 10^9 for conversion
    Fraction.fx_64 >>= 32; // align integer part in low long
    sprintf(pBuffer,"%09lu",Fraction.fx_32.low); // convert low long to decimal
    }
    void PrintIntegerLL(char *pBuffer,union ll_u FixedPt) {
    sprintf(pBuffer,"%lu",FixedPt.fx_32.high);
    }
    void PrintFixedPt(char *pBuffer,union ll_u FixedPt) {
    PrintIntegerLL(pBuffer,FixedPt); // do the integer part
    pBuffer += strlen(pBuffer); // aim pointer beyond integer
    *pBuffer++ = '.'; // drop in the decimal point, tick pointer
    PrintFractionLL(pBuffer,FixedPt);
    }
    void PrintFixedPtRounded(char *pBuffer,union ll_u FixedPt,unsigned Decimals) {
    char *pDecPt;
    FixedPt.fx_64 = RoundFixedPt(FixedPt,Decimals);
    PrintIntegerLL(pBuffer,FixedPt); // do the integer part
    pBuffer += strlen(pBuffer); // aim pointer beyond integer
    pDecPt = pBuffer; // save the point location
    *pBuffer++ = '.'; // drop in the decimal point, tick pointer
    PrintFractionLL(pBuffer,FixedPt); // do the fraction
    if (Decimals == 0)
    *pDecPt = 0; // 0 places means discard the decimal point
    else
    *(pDecPt + Decimals + 1) = 0; // truncate string to leave . and Decimals chars
    }
    //———–
    // Calculate useful "constants" from oscillator info
    void CalcOscillator(int16_t Offset) {
    Oscillator.fx_64 = NOMINAL_OSC + (Offset * ONE_FX); // offset may be negative, It Just Works
    HzPerCt.fx_32.low = Oscillator.fx_32.high; // divide oscillator by 2^32 with simple shifting
    HzPerCt.fx_32.high = 0;
    CtPerHz.fx_64 = -1; // Compute (2^32 – 1) / oscillator
    CtPerHz.fx_64 /= (uint64_t)Oscillator.fx_32.high; // remove 2^32 scale factor from divisor
    }
    //———–
    //– Helper routine for printf()
    int s_putc(char c, FILE *t) {
    Serial.write(c);
    }
    //———–
    void setup () {
    union ll_u TempFreq,TempCount;
    pinMode(PIN_HEARTBEAT,OUTPUT);
    digitalWrite(PIN_HEARTBEAT,LOW); // show we got here
    pinMode(PIN_SYNC,OUTPUT);
    digitalWrite(PIN_SYNC,LOW);
    Serial.begin (115200);
    fdevopen(&s_putc,0); // set up serial output for printf()
    Serial.println (F("60 kHz Sine Generator"));
    Serial.println (F("Ed Nisley – KE4ZNU – September 2017\n"));
    // DDS module controls
    pinMode(PIN_DDS_LATCH,OUTPUT);
    digitalWrite(PIN_DDS_LATCH,LOW);
    pinMode(PIN_DDS_RESET,OUTPUT);
    digitalWrite(PIN_DDS_RESET,HIGH);
    // Light up the display
    Serial.println("Initialize OLED");
    u8x8.begin();
    u8x8.setFont(u8x8_font_artossans8_r);
    // u8x8.setPowerSave(0);
    u8x8.setFont(u8x8_font_pxplusibmcga_f);
    u8x8.draw2x2String(0,0,"Sine Gen");
    u8x8.drawString(0,3,"Ed Nisley");
    u8x8.drawString(0,4," KE4ZNU");
    u8x8.drawString(0,5,"2017-09-20");
    u8x8.drawString(0,6,"Press Button …");
    // configure SPI hardware
    pinMode(PIN_SS,OUTPUT); // set up manual controls
    digitalWrite(PIN_SS,HIGH);
    pinMode(PIN_SCK,OUTPUT);
    digitalWrite(PIN_SCK,LOW);
    pinMode(PIN_MOSI,OUTPUT);
    digitalWrite(PIN_MOSI,LOW);
    pinMode(PIN_MISO,INPUT_PULLUP);
    SPCR = B00110000; // Auto SPI: no int, disabled, LSB first, master, + edge, leading, f/4
    SPSR = B00000000; // not double data rate
    TogglePin(PIN_HEARTBEAT); // show we got here
    // Set up X axis DAC output
    XAxisDAC.begin(MCP4725_ADDR); // start up MCP4725 DAC at Sparkfun address
    // XAxisDAC.setVoltage(0,DAC_WR_EEP); // do this once per DAC to set power-on at 0 V
    XAxisDAC.setVoltage(0,DAC_WR); // force 0 V after a reset without a power cycle
    // LM75A temperature sensor requires no setup!
    // External capacitor in test fixture
    // Turn relay off to keep the heat down
    pinMode(PIN_CX_SHORT,OUTPUT);
    digitalWrite(PIN_CX_SHORT,LOW);
    // Frequencies
    PrintFixedPtRounded(Buffer,CenterFreq,1);
    printf("Center freq: %s Hz\n",Buffer);
    NomFreq = CenterFreq;
    // Wake up and load the DDS
    OscOffset = ComputeOffset();
    CalcOscillator(OscOffset);
    Serial.print("\nStarting DDS: ");
    TempFreq.fx_64 = CALFREQ;
    TempCount.fx_64 = MultiplyFixedPt(TempFreq,CtPerHz);
    EnableDDS();
    WriteDDS(TempCount.fx_32.high);
    Serial.println("running\n");
    WaitButton(); // pause until button release
    u8x8.setPowerSave(0);
    u8x8.clearDisplay();
    Serial.println("\nStartup done\n");
    MillisThen = millis();
    ZeroBeat(); // compensate for oscillator clock offset
    TempCount.fx_64 = MultiplyFixedPt(NomFreq,CtPerHz); // set up initial frequency
    WriteDDS(TempCount.fx_32.high);
    u8x8.drawString(0,5," <- Jog -> ");
    u8x8.drawString(0,6," ^ 1 Hz v ");
    u8x8.drawString(0,7," Button = reset ");
    }
    //———–
    void loop () {
    byte ln;
    union ll_u DDSCount;
    TestFreq = NomFreq; // assume no change
    if (analogRead(PIN_JOYBUTTTON) > 500) { // button unpushed?
    int ai = analogRead(PIN_JOY_Y) – 512; // X axis = left-right
    if (ai < -100)
    TestFreq.fx_64 = NomFreq.fx_64 + StepSize.fx_64;
    else if (ai > 100)
    TestFreq.fx_64 = NomFreq.fx_64 – StepSize.fx_64;
    else {
    ai = analogRead(PIN_JOY_X) – 512; // Y axis = up-down
    if (ai < -100)
    TestFreq.fx_64 = NomFreq.fx_64 + ONE_FX;
    else if (ai > 100)
    TestFreq.fx_64 = NomFreq.fx_64 – ONE_FX;
    }
    }
    else
    TestFreq = CenterFreq; // reset on button push
    DDSCount.fx_64 = MultiplyFixedPt(TestFreq,CtPerHz); // compute DDS delta phase
    DDSCount.fx_32.low = 0; // truncate count to integer
    ActualFreq.fx_64 = MultiplyFixedPt(DDSCount,HzPerCt);
    if (TestFreq.fx_64 != NomFreq.fx_64) { // avoid writing same value
    WriteDDS(DDSCount.fx_32.high);
    NomFreq = TestFreq; // set up new value
    }
    ln = 0;
    PrintFixedPtRounded(Buffer,ActualFreq,2); // display actual frequency
    u8x8.draw2x2String(0,ln,Buffer);
    ln = 3;
    LogAmpdB.fx_64 = ReadLogAmp(); // show current response
    PrintFixedPtRounded(Buffer,LogAmpdB,1);
    u8x8.drawString(0,ln,"Response");
    u8x8.drawString(16-strlen(Buffer),ln++,Buffer);
    Temperature.fx_64 = GetTemperature(); // and temperature
    PrintFixedPtRounded(Buffer,Temperature,3);
    u8x8.drawString(0,ln,"DDS Temp");
    u8x8.drawString(16-strlen(Buffer),ln++,Buffer);
    delay(100);
    }
    view raw SineGenerator.ino hosted with ❤ by GitHub

  • 60 kHz Preamp: Power Supply Noise

    This took entirely too long to figure out:

    Ground noise - 24 VDC wall wart - probe on gnd lug
    Ground noise – 24 VDC wall wart – probe on gnd lug

    That’s with the scope probe ground clip connected to the wall wart coax connector barrel and the scope probe tip on the ground clip. It’s not the noise on the 24 VDC supply, it’s the noise injected into the ground connection!

    Huh. Makes it tough to sort out low-level signals, it does indeed.

    Consider one of my bench power supplies at 24 V:

    Ground noise - bench supply 24 V - probe on gnd lug
    Ground noise – bench supply 24 V – probe on gnd lug

    Nice & quiet, the way power should be. One might quibble about the residual noise, but at least it’s not blasting out horrific bursts at 120 Hz.

    For completeness, the PCB inside the offending SMAKN 24 V wall wart:

    SMAKN 24 VDC wart - PCB
    SMAKN 24 VDC wart – PCB

    “High Quality Commercial Grade” my aching eyeballs.

    [Update: Edits based on eagle-eyed observations in the comments. ]

    Not as many missing components as I expected, though, if the truth be told. The missing transformer common-mode choke seems odd and, AFAICT, the resistor inductor angling out from the R1 callout doesn’t connect to anything, connects directly to the AC line because  C5 is missing and the pad joining them doesn’t go anywhere else it replaces the jumper (?) to the bottom-left pad and the missing parts. The red LED in the upper right isn’t visible through the black case, although it might serve as a voltage regulator.

    Over on the far right, beyond the transformer and between the two capacitor cans, is a component marked C9 with an oddly angled part. Seen from the other end, it’s a ferrite bead:

    SMAKN 24 VDC wart - output ferrite
    SMAKN 24 VDC wart – output ferrite

    I don’t know why that spot has an inductor symbol with a capacitor part callout.

    The other side of the PCB looks clean:

    SMAKN 24 VDC wart - PCB solder side
    SMAKN 24 VDC wart – PCB solder side

    It’ll probably serve well in a noise-tolerant application, maybe an LED power supply.

    As pointed out in the comments, there’s a UL mark:

    SMAKN 24 VDC wart - label
    SMAKN 24 VDC wart – label

    Not sure what I’ll replace it with, although a small 24 V power supply brick may suffice.

  • 60 kHz Preamp: Tuning Fork Resonator Protection

    Limiting the resonator drive to about 1 μW in the face of wildly varying RF from the antenna (or the occasional finger fumble) requires brute force. A nose-to-tail pair of Schottky diodes seems to do the trick:

    Tuning Fork Resonator Filter - protection and biasing
    Tuning Fork Resonator Filter – protection and biasing

    The 100 Ω resistor blunts the drive from the LM353 op amp (implementing a bandpass filter) when the signal peaks exceed 200-ish mV in either direction from the Vcc/2 bias stored in the 10 μF cap.

    The 11.5 kΩ resistor downstream of the resonator isolates it from the Vcc/2 bias, with the 100 nF cap sinkholing the signal and the 4.7 kΩ resistor preventing feedback into the bias supply. The cap looks like 26 Ω at 60 kHz, so the feedback runs -52 dB from the output and the bias supply knocks it down a bit more. The preceding amps apply 40-ish dB of gain from the antenna terminals, so the loop gain looks OK.

    It’s another few components on the board:

    LF Crystal Tester - resonator protection
    LF Crystal Tester – resonator protection

    The blue twiddlecap should allow pulling the tuning fork’s series resonance upward to exactly 60 kHz.

    Applying way too much signal to the antenna terminals in order to get 1 Vpp from the LM353 shows the limiter in action:

    BP and Xtal filter out - 10.0 v sine 10 Meg xfmr
    BP and Xtal filter out – 10.0 v sine 10 Meg xfmr

    The resonator sees no more than 200 mV in either direction from the bias level, so it’s all good.

    On the low end, the diodes have no effect:

    BP and Xtal filter out - 1.1 v sine 10 Meg xfmr
    BP and Xtal filter out – 1.1 v sine 10 Meg xfmr

    Pay no attention to all that noise.

    My first thought was to put the diodes across the resonator, a Bad Idea: straight up, doesn’t work. The 1N5819 datasheet shows they have about 300 pF of junction capacitance at zero bias and a pair of ’em will swamp the resonator’s internal 0.8 pF parallel capacitance and punch it out of the circuit.

  • 60 kHz Tuning Fork Resonator: Maximum Overdrive

    Datasheets loosely associated with the tuning fork resonators in hand suggest 1 μW maximum drive power, which works out to maybe 100 mVrms = 150 mVpk at about 10 kΩ ESR. If you inadvertently apply 500 mVpk = 375 mVrms, the resulting 14 μW does this:

    Broken 60 kHz Tuning Fork Resonator - overview
    Broken 60 kHz Tuning Fork Resonator – overview

    I was applying a precisely tuned 60 kHz sine wave to the first pass at a crystal filter grafted onto the loop antenna preamp and wasn’t paying attention to the amplitude. For all I know, though, the poor thing died from a power-on transient. I’m pretty sure I didn’t break it during extraction, because it stopped being a resonator while in the circuit.

    The missing tine fell out of the can:

    Broken 60 kHz Tuning Fork Resonator - tine detail
    Broken 60 kHz Tuning Fork Resonator – tine detail

    Laser trim scars form a triangle near the tip, a T a bit further down, a slot just above the nicely etched gap.

    A closer look at the fractured base:

    Broken 60 kHz Tuning Fork Resonator - detail
    Broken 60 kHz Tuning Fork Resonator – detail

    The metalization appears black here and gold in person.

    So, yeah, one down and 49 to go …

  • LF Crystal Tester: 60 kHz Resonator Frequency Distribution

    Histogramming all 50-ish resonator frequencies shows reasonably good distributions:

    Notably, there’s no obvious suckout in the middle, as with those eBay Hall-effect sensors.

    60 kHz Resonant Frequencies - CX 24 pF - histogram
    60 kHz Resonant Frequencies – CX 24 pF – histogram

    I don’t know what to make of the difference between the parallel series-capacitor and basic serial resonant frequencies for each tuning fork:

    60 kHz Resonant Frequencies - CX 24 pF - delta histogram
    60 kHz Resonant Frequencies – CX 24 pF – delta histogram

    Perhaps each resonator’s frequency depends on its (laser-trimmed) tine mass and follows a more-or-less normal distribution, but the parallel-serial difference series capacitor changes the frequency based on (well-controlled) etched dimensions producing quantized results from three different masks / wafers / lots, with the motional inductance and capacitance incompletely modeling the physics?

    For reference, the resonators look like this:

    Quartz resonator - detail
    Quartz resonator – detail

    Producing the histograms uses the LibreOffice frequency() array function, which requires remembering to whack Ctrl-Shift Enter to activate the function’s array-ness.

    [Update: Faceplant about “parallel” resonance, which is actually the shifted resonant peak due to the 24 pF series cap. Apparently I typo-ed the second histogram subheading and ran with the error; the figures are now correct.]