8
Testing FM Systems with FONIX 6500-CX Analyzers
Figure 1.1.3A
FM setup
Isolating the environmental microphone
When the FM system is usually used in FM+ENV mode, the testing procedures call for the
environmental microphone to be turned on as well at the FM microphone. In most quiet test-
ing environments, this will call for the FM setup described above. However, if you are test-
ing in a noisy environment, you may want to isolate the environmental microphone from the
room noise.
To check if you need to provide sound isolation for the environmental microphone when
testing the FM microphone in FM+ENV mode, do this procedure:
1.
Set the FM microphone up for testing as described above.
2.
Set the FM receiver to receive FM only.
3.
Observe the RMS OUT for this FM-only signal.
4.
Set the FM receiver to receive FM+ENV.
5.
Observe the RMS OUT for the FM+ENV setting. If the RMS OUT is more than 3 dB
higher for FM+ENV than for FM-only, then you will need to isolate the environmen-
tal microphone when testing the FM microphone in FM+ENV mode. If the RMS
OUT for both signals is within 3 dB, no additional setup is necessary.
6.
To isolate the environmental microphone, it is preferable to use a spare sound cham-
ber if one is available to you. Otherwise, take a shoebox, line it with packing foam,
and place the environmental microphone inside, covering it with some more foam.
See Figure 1.1.3B. This should isolate the room noise from the environmental micro-
phone.
Transmitter
Receiver
Foam Pad
6500 Main
Electronics Module
Test Chamber
2 ft.
(60 cm)
FM Mic
(or Environmental Mic)
