Quote:
Originally Posted by Phil G4SPZ
Such a low component count is bound to come with compromises! Noting what changes you’ve made, I wonder if feeding the modulator from a high impedance supply (i.e. through the 2.7k dropper resistor) may be contributing to the kink? I’m guessing that the kink represents the point where the transistor turns on, and in Class C there will be a short but significant pulse of current drawn from the supply.
As an experiment, would it be worth trying powering the modulator section from its own separate DC supply, say two AA cells in series, as the original probably did?
|
I'd overlooked responding to your post Phil, so apologies for that.
Just to clarify, while under test, the modulator only, without the FM digital module was powered up from 2 AA Cells as per the original (but downstream of R4, which is there to reduce the voltage to 3V when powered from a 'wall wart'). Hence, the scope traces in my previous post are based on that.
I've been sidetracked on other projects, (a home-brew guitar pick-up coil winder of all things!), so haven't yet got around to trying the FM module with the modulator board. When I initially tried it I could get no sense out of it, which was rather disappointing, given that on the original FM/AM converter using the RDA5807A chip and the Guy Fernando PIC, I get ten stations at good strength - same as on a normal FM radio at this location.
Well done to Nigelr on getting the modulator to work and discovering that the kink is still there. (I never thought to check the unmodulated output of the original design to see if it was present with that). Have you connected a digital FM module to the modulator Nigel, or at this stage, just built and tested the modulator?