In making FM transmitter distortion measurements, does anyone inject the
1 kHz audio signal into an FM transmitter by holding the microphone next
to a speaker? It would seem that neither speaker nor microphone would
produce enough distortion of a single frequency to affect the results,
and in any case, the microphone is part of the system being tested.

The idea would be to avoid having to fiddle with the variety of
microphone connectors out there.

I suppose I can and should compare the results of this technique with
applying the audio signal directly to the microphone input terminal.


Appreciate any thoughts. TIA.

Chuck
NT3G

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chuck wrote:

In making FM transmitter distortion measurements, does anyone inject the
1 kHz audio signal into an FM transmitter by holding the microphone next
to a speaker? It would seem that neither speaker nor microphone would
produce enough distortion of a single frequency to affect the results,
and in any case, the microphone is part of the system being tested.

The idea would be to avoid having to fiddle with the variety of
microphone connectors out there.

I suppose I can and should compare the results of this technique with
applying the audio signal directly to the microphone input terminal.

Applying the signal to the microphone input terminals addresses just
part of a system, allowing you to point to the transmitter itself and
either say "that is a problem" or "that is not a problem". Applying the
signal to the system as a whole only lets you wave a hand at your bench
and say "somewhere over there, there is a problem".

I'd check transmitter and microphone separately. Then if they passed
and I'm feeling conscientious I'd check them together.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

You have to control the levels so that the transmitter doesn't get
driven into deviation limiting. Usually testing is done at 60% of system
deviation. I am assuming this is a land mobile transmitter? Connecting a
tone generator to the microphone terminals is best way to control the test.

chuck wrote:

In making FM transmitter distortion measurements, does anyone inject
the 1 kHz audio signal into an FM transmitter by holding the
microphone next to a speaker? It would seem that neither speaker nor
microphone would produce enough distortion of a single frequency to
affect the results, and in any case, the microphone is part of the
system being tested.

The idea would be to avoid having to fiddle with the variety of
microphone connectors out there.

I suppose I can and should compare the results of this technique with
applying the audio signal directly to the microphone input terminal.


Appreciate any thoughts. TIA.

Chuck
NT3G

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