Pi Zero W and low khz AM frequency not accurate

[tbird635]

I was trying to use the Pi Zero W to transmit a tone on a carrier frequency of 470khz . I tested it at 1500 khz, and it worked pretty well. But at lower frequencies, the accuracy goes away. Through trial and error, I found the frequency has to be set at 307khz in rpitx to get my 470khz signal. I have found the frequency of AM transmissions is spot on until I get below around 1300khz. At that point it starts diverging . Commanded frequency is no longer the transmitted frequency below this. I have not tried this on anything but a Pi Zero W. I am using an Icom comm receiver to verify this, so it should be accurate. Am I doing something wrong?

It's usable, but would be unusable without a way to verify the frequency. Thanks for the good work !

See the attached files, both showing command line and the the receiver's display.

[libmarleu]

same problem on pi 3b+