Understanding Signal Jitter in PWM Outputs
Signal jitter refers to small timing variations in the expected waveform, which can be troublesome when capturing PWM outputs using a Rigol Oscilloscope. Firmware developers may face challenges in distinguishing actual signal anomalies from measurement noise caused by the oscilloscope itself.
Initial Hardware and Oscilloscope Check
Verify all hardware connections are secure to minimize external electrical noise. Ensure cables and connectors are clean and undamaged.
Isolate the power source. Use a dedicated power supply to reduce noise and fluctuations from other devices on the same circuit.
Check the oscilloscope's probe compensation. A mismatched probe can contribute to measurement inaccuracies. Follow the manufacturer's instructions for probe compensation.
Optimizing Oscilloscope Settings
Set the oscilloscope bandwidth limiting option to reduce high-frequency noise. You can typically find this setting in the bandwidth or input menu. Bandwidth limiting filters out unnecessary high-frequency noise while maintaining the signal's integrity.
Adjust trigger settings for a stable waveform. Use edge-trigger mode and set it to trigger on the rising or falling edge of your PWM signal. Fine-tune the trigger level just above the waveform's noise floor for consistent captures.
Enable noise rejection features such as digital filtering. Utilize Rigol's built-in features to filter out intermittent noise. This may vary by model, so consult your oscilloscope's user manual.
Software Solutions and Code Optimizations
If noise persists, implement averaging in firmware. Capture multiple samples and compute the average to smooth out erratic values. Integrate this within your firmware to improve stability:
#define NUM_SAMPLES 10
uint16_t samples[NUM_SAMPLES];
uint16_t average_pwm_output() {
uint32_t sum = 0;
for (int i = 0; i < NUM_SAMPLES; i++) {
sum += samples[i];
}
return sum / NUM_SAMPLES;
}
Optimize PWM frequency and duty cycle settings to minimize jitter. Carefully choose PWM parameters based on the application requirements and the PWM module resolution.
Protocol-Specific Considerations
Assess the system's clock configuration for consistency. Ensure that the MCU clock speed is stable and not subject to irregularities that might affect PWM output.
Consider using a phase-locked loop (PLL) in your clock setup to reduce irregularities in the system clock, which indirectly affects PWM stability. Firmware developers should review the MCU documentation to implement and configure PLL correctly.
Advanced Oscilloscope Measurements
Use persistence mode to visualize jitter over a period. Persistence mode allows you to see how often and where the signal varies, giving you insights into how jitter behaves.
Capture and analyze data using deep memory. Access features to correlate irregular behavior over long durations and examine more subtle instances of jitter.
Utilize external synchronization if available. Employ a synchronized clock source if your oscilloscope model supports it, aligning the test equipment with your device under test.
By addressing the above factors systematically, you can effectively pinpoint and mitigate jitter issues in your PWM signals when using a Rigol Oscilloscope.
