The PWM carrier frequency has no effect on the output from the analog (L-AN) terminal: perhaps you’re seeing what you want to see.
Been there, done that! 
Not “idiot”, just a bunch of stuff you’ve never considered before and the vocabulary is unfamiliar.
A quick intro …
The high voltage power supply adjusts the tube current according to a signal from the controller. That signal can be either:
- An analog voltage between 0 V and 5 V
- Digital pulses at 0 V or 5 V, not between
For the analog signal, 0 V = no current = 0% and 5 V = maximum current = 100%. The voltages are approximate, but that’s the general idea.
The digital signal is a repeating pulse at a fixed frequency (“carrier frequency”) with its duty cycle setting the tube current, which is called pulse width modulation = PWM. The duty cycle is the ratio of the time at 5 V to the total cycle time = 1/frequency. A duty cycle of 0 (always low) = 0% and a duty cycle of 1 (always high) = 100%. Again, those are approximate.
The power supply can accept either of those signals on the same terminal because it has filter converting the PWM signal to the equivalent analog signal; an analog input signal passes through the filter unchanged.
The filter has a cutoff frequency around 200 Hz, with signal frequencies far below that defined as “analog” and signals far above it as “digital”. The default Ruida carrier frequency is 20 kHz = far above 200 Hz, so the filter output is a smooth, clean analog signal. That’s the straight green line along the bottom of this scope shot:
You can set the PWM carrier close to the cutoff frequency, but the filter will not produce a smooth analog voltage.
The green wave shows the result of a 5 kHz carrier:
This is a 1 kHz carrier:
Whether those irregular currents produce usable results depends on what you consider usable.
To further muddy the waters, it seems my KT332N controller has an internal PWM demodulating filter converting the digital signal on its PWM terminal to the corresponding analog signal on its L-AN terminal. Other controllers have hardware DACs producing a better-quality signal.
Make any more sense?