I got some cheap low power RF communication modules today, and not having anything else to check them with, i connected the output directly to the scope input using the stock probe. It is a 315MHz ASK TX module. I wanted to see the spectrum of it, so i set up the FFT for it, center at 315MHz and a span of 20MHz. To try and get better resolution on the FFT i zoomed out to 10µs/ from 5µs/, but then the FFT seemed to disappear. I tried the FFT from the math menu, and same seemed to happened, so i just moved the center of the FFT to higher and lower frequencies to see if anything was showing up, and i find this. When at 10µs/, the FFT centered at around 185MHz showed the expected spectrum, but, at an incorrect frequency. I was inputing a 1MHz signal to the RF module, and that was properly showing up in both cases, at 5µs/ with the correct center frequency, and at 10µs/ with the incorrect center frequency.
Anyone has any idea what is going on here? why changing the timebase to something a bit slower completely confuses the FFT and "moves" it down to much lower frequencies, which are not really present at the displayed frequencies?
Thank you!