A problem
 

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????


phil

"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. For example, if the audio is phase
shifted and given a 1/f amplitude response by a so-called audio
integrator (a low-pass filter), it is theoretically possible 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.

Consult some texts on the subject for fuller explanations of practical
methods and circuits. The only web discussion I could find after a
quick Google search was at http://www.imec.be/esscirc/papers-97/47.pdf,
but I'm sure that more careful searching will result in even better
explanations.

There's a pretty good explanation of the relationship of the two in the ARRL
General license class manual (the full manual not the question and answer
book).

Dee D. Flint, N8UZE

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.

It is because frequency is given by "f"....

You will have seen "2*PI*f" which is the conversion
from frequency is cycles per second (Hertz) to frequency
in radians per second - a measure of by how much the
phase angle changes every second.

Therefore phase angle is the integral of frequency; frequency
is the differential of phase angle. Change one and you change
the other.

Interestingly, when it comes to measuring frequency stability
in a GSM phone it is done by differentiating the phase.

"PhalanxX" wrote in message
om...
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????

"Lloyd" wrote in message ...
...


Oh that's nice. My last post doesn't appear to be here.


Lioyd,
I am sorry. I didn't mean it strongly, and I can see that it might
appear to be directed at you specificly, but I meant it as a question to the
list. You are correct. My question was improber and a bit arrogant. Just
showing off, I guess. Tail between legs...back to corner..
--
Steve N, K,9;d, c. i My email has no u's.

Lesee if this one sticks...

On 10 Nov 2004 23:28:11 GMT, "Lloyd" wrote:

The last Lloyd seems to disappear whenever LRod posts, leading me to
have suspicions about a possible connection between those two.

The only connection between me and any lloyd is that I am periodically
compelled to wipe the newsgroup floor with the one from Athens, AL.

Also, that lloyd could not possibly have played in the thread about
frequency/phase modulation, or any other highly technical discussion..


- -
LRod

Master Woodbutcher and seasoned termite

Shamelessly whoring my website since 1999

http://www.woodbutcher.net