My first thought was to try another receiver like a portable Walkman, but you
said you tried another receiver with the same results. So, not a receiver
problem but I'd be sure with a third receiver anyhow. If both transmitters are
doing the same thing, then it doesn't sound like that could be the answer. FM
broadcast transmitters don't create anything weird that could cause this
phenomenon. The only thing left is your PC. Have you tried plugging in a set of
headphones to verify your source is stable? It could be something the PC is
doing to the sound card. Other than that, you have a pretty strange problem. I
use two of the Ramsey kit FM stereo transmitters. One is a tuned LC using the
BA1404 and one is a fully synthesized programmable transmitter that is very
stable. I have excellent results with both.
Rick

Herbert Graf wrote:

> Hello to all, was wondering if someone might be able to helping me figure
> out what wrong with the FM transmitter I have. First a little background...
> :)
>
> I built an MP3 player out of a Pentium PC and installed it in the car. I
> currently have an audio line going to the front which I connect to a tape
> adapter, the car radio has no audio in, and I don't want to modify it. This
> system works, but is a little involved.
>
> So, I had the "brilliant" idea of using an FM band transmitter instead. I
> tried a few retail cheapies (read RadioShack) and none worked well, the
> radio catches the signal but gets louder, and softer, then louder again, at
> a freq of about 0.5Hz.
>
> I decided I'd buy a cheap kit and build my own Stereo FM transmitter, I
> decided on the CK222 by CanaKit (www.canakit.com). It is a simple stereo
> transmitter based on the BA1404 with a single transistor acting as a power
> amp. I assembled it and got it work, however it exibits the EXACT same
> problem, the radio signal comes through but it alternates between really
> quite and loud, and about 0.5Hz. I am truly at a loss as to WHY this is
> occuring, I also tried my home theatre receiver, with the same results!@ :(
>
> I connected my trustly HP 1710A scope to it (a 100MHz analog dual trace
> beauty from the 70's!) and I don't quite understand the waveforms fully
> (both are DC coupled, 0.1V/div). I made links to them here:
>
> http://repatch.dyndns.org:82/06080001.jpg (20ns/div) 84k
> http://repatch.dyndns.org:82/06080002.jpg (50ns/div) 109k
>
> There are TWO signals, one at just below 50MHz (which doesn't seem to
> modulate at all) and one at something less (which DOES seem to modulate),
> the interesting thing is the lower freq signal doesn't change much if I
> change timebase, which tells me there are some harmonics or something
> happening since I'm at the edge of the scopes input bandwidth. I had the
> transmitter set at 88.6 MHz.
>
> If anybody has any ideas as to what might be causing this I'd love to know,
> I have little RF experience (hence the buying of a retail product at first,
> and then a kit) but would love to figure this out. Could it be the AFC
> circuit in the digital tuners "hunting" for the not so perfect stereo
> signal? I say this since if I short out the crystal on the board (thereby
> removing the FM stereo pilot signal at 19kHz (I think) and causing the
> receiver to go into mono mode) the problem magically disappears. Thanks in
> advance for any help/advice. TTYL
>
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