fl2k output voltage level changes every few seconds

Daniel Correa daniel74f at gmail.com
Fri May 3 13:38:00 UTC 2019

Hi. When running the command line
fl2k_fm -c 5000 /dev/zero
to generate a continuous wave, the output voltage of the fl2k dongle
regularly switches between two different amplitude levels  (peak to
peak voltage) every 2 or 3 seconds, by about %30 to %70 depending on
the load. There's no visible gradual transition, it just switches
instantly. The output peak-to-peak voltage is about 350 milivolts with
no load. I've calculated the output impedance to be about 750 ohm at
that frequency for the larger output level.
I'm using the HDMI version of the dongle with the wire soldered after
a capacitor (or resistor maybe) which was already there, to the red
VGA pixel output of the chip.
Could the issue be because some signal conditioning circuitry is
missing that is present in the actual VGA version of the dongle, or
that the pin is actually pulled to ground in this dongle as it is
unused, and the chip doesn't like it and for some reason it then
decides to regularly switch voltages? Any other ideas?

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