Procaster Frequency Stability
Warbling sound when other frequencies are present in fringe areas.
Difficult issue to resolve, so some basic observations are made to see if anything obvious is wrong. Could not re-create warbling condition. Frequency stability is fine and phase jitter is minimal. PLL seems to be operating as expected. Loop filter values for VCO control have been selected for lowest impedance to prevent any interference to PLL operation near internal loading coil.
Warbling could be heterodyning interaction between the Procaster PLL and an accurate crystal controlled carrier of a nearby broadcast station.
SETUP FOR VIDEO
Oscilloscope probe setup: placed near antenna
Observation: clean AM modulated signal
Frequency counter setup: Input lead placed near antenna
Observation: Frequency deviation varies by a few Hz and is within spec. (Ignore frequency value on oscilloscope - frequency counter is the true reading).
Spectrum Analyser: air pick up via 1m antenna
Observation: normal AM modulation with harmonics well below minimum 20dB permitted.
Some improvement may be gained by adding a gate driver to the final MOSFET output stage. Perhaps under better grounding conditions, the additional power developed is causing some effect to be reflected back into the PLL - but this is not certain. We will, however, add the gate driver improvement on the next run.
The Procaster range is typically 1 mile during the day with a good install and good location - it was never meant for long range reach being that its ERP is basically microwatts.
At night, all bets are off due to the Skywave effect. This is what you are hearing and refer to as 'warbling'. I see nothing wrong with the performance.
If a crystal is used instead of a PLL then stability would be better and the 'warbling' sound might be reduced, but the user would have little choice over which frequency to broadcast on.
Other low power transmitters may have a higher Q in the tuned circuit but as the Q increases, the audio bandwidth decreases and the audio quality suffers. The Procaster design is
a compromise between ease of setup, good audio quality and reasonable range. Those who want better range should consider obtaining a broadcast license and use a more powerful transmitter.
Normal broadcast stations use crystal oscillators for precise frequency control, but the Procaster was fitted with a PLL which gives people the freedom to choose their own frequency.
The aim of the Procaster design was to allow a non-technical person to easily setup a short range AM broadcast transmitter without having knowledge of radio workings.
The original intent of the FCC allowing unlicensed radio broadcasts did not include the ranges we see here but rather broadcasting around ones own property. So we have exceeded that already.
Audio recording - Procaster running at night