Herbert --

> 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, ...

I'm not sure what's wrong with your transmitter, but I think the "two
signals" you're seeing are simply an artifact of having the scope intensity
turned up too high. You're seeing the "50 MHz" signal during the retrace,
giving the appearance of a lower-frequency signal.

Even with a wideband (+/- 75 kHz deviation) FM transmitter, at VHF
frequencies, you're not going to "see" the modulation on a scope.

> ... I'm at the edge of the scopes input bandwidth. I had the
> transmitter set at 88.6 MHz.

The scope should give you useful qualitative results at that frequency.
Just don't trust the vertical calibration for specific amplitude values
too much. Look for relative changes.

> ... 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.

Interesting. BA1404-based designs tend to be notorious for unwanted
coupling of RF through the power supply and to the audio inputs. I would
suspect that you're seeing some sort of effect of RF getting somewhere it
shouldn't (perhaps the MPX BALANCE inputs?). Search the web for BA1404 and
check out the notes that various people have accumulated on this chip.

-- Dave Tweed

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