"Richard Harrison" wrote
The FM amplifier does not need linearity. Amplitude distortion is
irrevelant. Severe clipping to remove amplitude variations
is common practice.

Not so. You confuse receivers with transmitters. Limiting is supplied by
the IF strips of FM receivers to reduce/remove AM components on the incoming
wave, but FM broadcast transmitters are operated well below any
limiting/clipping level, and that is probably true of ham FM txs also.

Synchronous and asynchcronous AM are low in broadcast FM tx RF stages (the
FCC spec is -50dBc), but not because the FM amplifiers are "clipping."
Broadcast FM txs easily can be adjusted over an output power range of ~25%
to 105% or more simply by adjusting drive to the PA (keeping constant PA
volts). This technique often is used for output power control/VSWR
foldback, actually.

Clipping generates harmonics and FCC rules limit harmonic
transmission in all services. .. I would be surprised if some
final filter were not used to guarantee compliance with the rules.

You are confused again. I wrote that no "tank circuit or in-band filter(s)"
were necessary to achieve the high efficiency I described. Your post I was
responding to states that a "tank circuit and other filters" are necessary
for high efficiency -- that is not true.

Harmonics are present at the PA output of an FM transmitter, but "clipping"
is not the process whereby they are generated, as I state above. They are
reduced to legal values using a lowpass/harmonic filter. The FCC
attenuation spec for harmonics and spurs more than 600kHz from Fc is 80dB
below the unmodulated carrier.

The lowpass/harmonic filter does not improve efficiency--it has a small
amount of insertion loss in the FM band.

RF

Richard Fry wrote:
"Not so. You confuse receivers and transmitters."

FM transmitters often use Class C amplifiers and frequency multipliers
on the modulated signal. An AM signal can not be amplified by a Class C
amplifier because of severe distortion of the modulated signal. In FM,
amplitude distortion is irrelevant no matter where it occurs, receiver
or transmitter. The recovered audio will sound just fine. It`s one of
the many advantages of FM.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:

Richard Fry wrote:
"Not so. You confuse receivers and transmitters."

FM transmitters often use Class C amplifiers and frequency multipliers
on the modulated signal. An AM signal can not be amplified by a Class C
amplifier because of severe distortion of the modulated signal. In FM,
amplitude distortion is irrelevant no matter where it occurs, receiver
or transmitter. The recovered audio will sound just fine. It`s one of
the many advantages of FM.

The question seems to be: If an FM transmitter's output signal
is not a reasonably pure sine wave, is a low-pass filter used
between the transmitter and antenna to reduce the harmonics?
--
73, Cecil http://www.qsl.net/w5dxp


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Cecil Moore wrote:
Richard Harrison wrote:

Richard Fry wrote:
"Not so. You confuse receivers and transmitters."

FM transmitters often use Class C amplifiers and frequency multipliers
on the modulated signal. An AM signal can not be amplified by a Class C
amplifier because of severe distortion of the modulated signal. In FM,
amplitude distortion is irrelevant no matter where it occurs, receiver
or transmitter. The recovered audio will sound just fine. It`s one of
the many advantages of FM.


The question seems to be: If an FM transmitter's output signal
is not a reasonably pure sine wave, is a low-pass filter used
between the transmitter and antenna to reduce the harmonics?

How can a filter filter correctly when its input is terminated in
an indeterminate impedance?
73,
Tom Donaly, KA6RUH

How can a filter filter correctly when its input is terminated in
an indeterminate impedance?

============================

No problem! The frequency response of a filter, input to output terminals,
is independent of the source resistance.

Reg Edwards wrote:
How can a filter filter correctly when its input is terminated in
an indeterminate impedance?


============================

No problem! The frequency response of a filter, input to output terminals,
is independent of the source resistance.



That may be true in the Danelaw where you live, Reg, but not in the
rest of the universe. Imagine a circuit consisting of a voltage source
in series with a source resistance followed by a shunt capacitance in
parallel with a load resistor. Write the output voltage equation for this
circuit. Now, replace the source resistance with a short. Write the
output equation for _this_ circuit. Compare the two equations.
Are the frequency responses the same? Now use an indeterminate
impedance for the source resistance. Can you tell what the voltage output
of this circuit is?
73,
Tom Donaly, KA6RUH

"Cecil Moore" wrote:
The question seems to be: If an FM transmitter's output signal
is not a reasonably pure sine wave, is a low-pass filter used
between the transmitter and antenna to reduce the harmonics?
___________________

Yes. This question was answered in my post in this thread of 00:24UTC
today, which I will paste below:

"Harmonics are present at the PA output of an FM transmitter, but "clipping"
is not the process whereby they are generated, as I state above. They are
reduced to legal values using a lowpass/harmonic filter. The FCC
attenuation spec for harmonics and spurs more than 600kHz from Fc is 80dB
below the unmodulated carrier.

The lowpass/harmonic filter does not improve efficiency--it has a small
amount of insertion loss in the FM band."

RF

"Richard Harrison" wrote

In FM, amplitude distortion is irrelevant no matter where
it occurs, receiver or transmitter. The recovered audio
will sound just fine. It`s one of the many advantages of FM.
___________________

It may sound just fine to you, but carefully made performance measurements
of a received FM signal having high AM show otherwise. AM on FM is far from
irrelevant.

RF