The Avalanche Photodiode Receiver

// Receiver bandwidth sweep. The ESP32 drives an LED at a swept frequency through the
// optical path and records the peak-to-peak amplitude at the transimpedance amplifier's
// output, so you can find the -3 dB point for each feedback resistor.
//
// Wiring:
//   GPIO26 (DAC2) -> LED driver (LED + series resistor to ground)
//   GPIO34 (ADC1) -> TIA output, biased to mid-rail by the divider on the board
//
// Run it once per Rf value (10k, 100k, 1M) and paste the three CSV blocks into the
// notebook. The -3 dB frequency should move as 1/sqrt(Rf), NOT as 1/Rf. Confirming that
// is the whole point of the exercise.

const int PIN_LED = 26;
const int PIN_TIA = 34;

// Geometric sweep: a linear one wastes nine tenths of its points at the top end.
const float F_MIN = 100.0;      // Hz
const float F_MAX = 300000.0;   // Hz
const int   N_FREQ = 30;
const int   CYCLES_PER_POINT = 60;

// Generate a square wave in software and sample the response synchronously.
float measurePkPk(float f_hz) {
  const unsigned long half_us = (unsigned long)(500000.0 / f_hz);
  int vmin = 4095, vmax = 0;
  for (int c = 0; c < CYCLES_PER_POINT; c++) {
    dacWrite(PIN_LED, 200);
    delayMicroseconds(half_us);
    int a = analogRead(PIN_TIA);          // sample at the end of the ON half
    dacWrite(PIN_LED, 0);
    delayMicroseconds(half_us);
    int b = analogRead(PIN_TIA);          // and at the end of the OFF half
    if (c > 4) {                          // discard the first few, let it settle
      if (a > vmax) vmax = a;
      if (b > vmax) vmax = b;
      if (a < vmin) vmin = a;
      if (b < vmin) vmin = b;
    }
  }
  return (float)(vmax - vmin);
}

void setup() {
  Serial.begin(115200);
  delay(300);
  analogReadResolution(12);
  analogSetPinAttenuation(PIN_TIA, ADC_11db);

  Serial.println("# TIA bandwidth sweep");
  Serial.println("# set Rf on the socket, note it here, then run");
  Serial.println("frequency_Hz,pkpk_counts,relative_dB");

  float ref = 0.0;
  for (int k = 0; k < N_FREQ; k++) {
    float f = F_MIN * pow(F_MAX / F_MIN, (float)k / (N_FREQ - 1));
    float a = measurePkPk(f);
    if (k == 0) ref = a;                  // the low-frequency value is 0 dB
    float dB = 20.0 * log10(a / ref);
    Serial.printf("%.1f,%.1f,%.2f\n", f, a, dB);
  }

  dacWrite(PIN_LED, 0);
  Serial.println("# done - the -3 dB point is where relative_dB crosses -3");
}

void loop() {}