Hi,

In your DSO you should find a menu called acquire. In that menu there should be different options like :

1. Sample
2. Average
3. Peak-Detect
....
......

or so.

Select the average mode --> if it supports number of samples to be programmed, select it to 128 or higher now watch the waveform with a timebase between 1ms and 5ms.

Your PWM runs at a high frequency and is continuously changing. If your scope is running in automatic trigger mode, it is repeating the trigger as per the timebase. If you try "Autoset" on the scope, it will perhaps try to lock onto the PWM frequency. But what you are interested is the PWMed envelope.

Using a simple RC may help.

If I find some time I would try to duplicate your setup here. But I still doubt Your scope is not set up properly to visualize the modulation