FM DDS: SPI Mock 3

Running some serial I/O in the background adds jitter to the timer interrupt pacing the ADC samples and as-yet-unwired DDS updates. For reference, an overview of the process showing the procession from the IRQ on the left to the SPI outputs near the middle and another IRQ on the far right:

Now, speed up the sweep and delay the trace by 25 μs to put the triggering pulse off-screen to the left and the second pulse at the center division:

The orange smear in the middle should be a tidy pulse, but it isn’t.

The  25 μs timer interrupt now has the highest priority on the front burner:

IntervalTimer AudioSampler;

... snippage ...

  AudioSampler.priority(0);
  if (!AudioSampler.begin(AudioSamplerIRQ, SamplePeriod)) {
    Serial.printf("Timer start failed\n");
    while (true) {
      FlipPin(BUILTIN_LED);
      delay(75);
    }
  }

Although nothing can interrupt it, other code / handlers may disable interrupts around their own critical sections and delay the tick. If the triggering tick (the off-screen one starting the trace) is delayed, then the on-screen pulse will appear “too soon”, to the left of center. If the triggering tick is on time, but the on-screen pulse is delayed, it’ll appear “too late” on the right.

The blur is (roughly) symmetric around the center graticule line, so the handwaving seems about right.

In round numbers, the jitter moves the interrupt ±325 ns on either side of its nominal position, with most of the pulses within ±100 ns. I doubt the jitter distribution is Gaussian, but vigorous handwaving says the RMS jitter might amount to 75 ns.

At the 4 kHz audio band limit, a 75 ns sampling error a phase error of 0.1°, so the maximum amplitude jitter would be sin(0.1°) = 0.002 = -55 dB, which might suffice for amateur-radio audio.

I think, anyhow.