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Old March 4th 05, 08:53 PM
Tom Donaly
 
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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
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Old March 5th 05, 11:27 AM
Reg Edwards
 
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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.


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Old March 5th 05, 09:09 PM
Tom Donaly
 
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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
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