Let's try this again, but wait to push the send button until done...

"Lloyd" wrote in message ...
On 8 Nov 2004 01:07:53 -0800, PhalanxX wrote:
Hi I wonder if any one can help. My friend says that it is possible to
produce a frequency modulation from a phase modulation modulator.
Surelly this cant be true. How is that possible????

Phase and frequency modulation are very closely related, and it is
possible to convert one into the other by properly processing the
audio supplied to the modulator.


Hi LLoyd, and YEP!

To further explain. One (frequency) is the integral of the other (phase)
(derivative visa-versa). You can't change one without changing the other.
Change the phase, and the freq jumps momentarily. Change the frequency and
the phase changes at a different rate.


For example, if the audio is phase shifted

While present, phase shifting is a secondary effect of the frequency
shaping and unneeded for understanding.

and given a 1/f amplitude response by a so-called audio
integrator (a low-pass filter), it is theoretically possible

Not only theory here. This is how all two way transmitters were
modulated before synthesizers came into vogue.

to apply
the resulting audio to a frequency modulator and to obtain the same
result that a phase modulator would produce. However, as "f"
approaches zero, 1/f becomes so large that the method runs into
practical problems. Conversely, a phase modulator can be supplied
with preemphasized audio whose amplitude is proportional to its
frequency, and fm output will theoretically result.

OOPS! I believe you have this backwards. I always have to look at the
intrgral/derivative carefully. Phase modulator and a "low pass" (constant,
in-band, 6 db per octave de-emphasis) gives FM. The Two-way FM transmitters
used a single pole (-6 dB per octave) low pass with a corner below 300 Hz.
for this required phase modulator shaping.

Ahead of that in the audio lineup was an amplitude limiter (Motorola
called it IDC Instantaneous Deviation Limiter) to limit the peak FM
deviation, so the two represented an FM modulator.

The practical problem mentioned above was that of the subaudible tone
modulation (Motorola=PL, GE =? Channel Guard). With the Phase
modulator/integrator approach, the low freq tones stretched its capability.
The phase modulator needed to be very good to handle this amount of phase
shift.

Point to ponder: (limited to narrow band, communication FM, not
broadcast)
Because the normal FM scheme also uses (+6 dB per octave) preemphasis,
there effectively is a preemphasis which cancels the FM/PM de-emphasis.
They both could be eliminated IF a suitable RATE limiting circuit could be
substituted ahead of the phase modulator to act as the peak deviation
limiter. This would be a circuit which limits the rate of change of audio
voltage going into the phase modulator...
Then... So, why have preemphasis ?? Do you know?
73,
--
Steve N, K,9;d, c. i My email has no u's.