PWM Frequency Library cause judder on other PWM pin

Hello,

I am attempting to run both a normal PWM output and a PWM output on a different pin at a different frequency. To do this I installed the PWM Frequency Library (available here), and uploaded the following code onto my Arduino Uno:

#include <PWM.h>
int32_t frequency;

void setup() {
  frequency = 1;
  Timer1_Initialize();
  pinMode(9, OUTPUT);
  pinMode(3, OUTPUT);
  SetPinFrequencySafe(9, frequency);
  pwmWrite(9, 127);
  analogWrite(3, 127);
}

void loop() {
  delay(100);
}

Upon uploading the PWM at a custom frequency on pin 9 works as expected. The “normal” PWM on pin 3 also works, apart from a rather annoying issue. When the output from pin 9 is HIGH, the PWM on pin 3 drops slightly. Once the output from 9 goes LOW, the PWM on pin 3 returns to it’s original level.

Does anyone know how I can fix this?

Thank you,
Andrew

How much is "drops slightly" in volts? How much current does Pin 9 have to source? Have you checked the power rail to see if it is drooping?