🚀 go-pugleaf

In article <1992Oct05.172142.5600@ssc.com>, markz@ssc.com (Mark Zenier) writes:

|> Sorry, the question should have been: Do commerical FM stereo transmitters
|> work this way?  (I've been trying to come up with a cheap stable FM stereo
|> exciter for a couple of years.)

The BE line of exciters, known to have the best track records (my
opinion), still use balanced modulators. Any gain/phase trims are
easily accomplished and are stable. Although I haven't tried the
sampling technique, I would think that carrier feedthrough (the measure
of balance of a balanced modulator) would be hard to make anything
else but poor on a sampling system.

|> My speculation is that, within the broadcast community, this technique
|> is well known.  But outside that, most people are working off the
|> description from the FCC regulations, which is the frequency domain
|> description.  (Since they are more interested in bandwidth and 
|> interference than simple design.)

While it is known, it is not used often because of phase and filtering
contraints (which would not a problem with the I/Q chroma circuit
operating at 3.58MHz due to the much less demanding nature of chroma).
It is hard to maintain stereo separation (the first parameter to degrade)
unless the L-R is extremely accurate.

|> So you end up with things like the North Country Radio/Graf and Sheets
|> transmitter (Radio-Electronics March and April 1988) which used a
|> balanced modulator for the 38 kHz subcarrier and was overly complex.
|> And didn't work well, according to a friend who built one.
|> (Which makes sense, keeping your phase responses right through all
|> that would be hard.)

Actually, the balanced modulator approach is not difficult at all,
even with the MC1495 chip, if properly biased. The 1495 is not
state of the art for distortion, though. As far as separation, which
is the best measure of how well the differencing/modulator combo works,
I've gotten a respectable 60 dB from the 1495 on the bench.

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