FM35 11
FM35 audio inputs to speaker outputs of a high power stereo system; such a
connection will destroy the IC chip. Trust us when we say that a true line level
audio source will give you the best results with your FM35.
The default volume setting is roughly ½ of full, which is a gain of 1. The means
that 500mV peak audio coming into the FM35 is 100% modulation, typically,
depending on frequency. The volume display also includes the clipping detec-
tor status and you should adjust the volume until the indicator toggles from ‘-‘
to ‘g’ occasionally, roughly a 50/50 timing between the two, with an occasional
‘c’ mixed in. If the character is always at ‘-‘, the modulation is too low, and if ‘C’
ever shows up (capitol) it means you are overmodulating. It’s best to set your
level up or down until you see the lower case ‘c’, which means that you're on
the edge of clipping. If you see the lowercase ‘c’ it means that you’re distorting
a bit so if you want to avoid that completely, make sure you set the level so
that you only see the ‘g’. Of course you can simply adjust the volume until it
sounds right to you.
The order of characters is approximately on average:
‘-‘ <= +/-60kHz
‘g’ >+/-60kHz and <+/-75kHz
‘c’ >+/-75kHz and <+/-90kHz
‘C’ >+/-90kHz
Remember from the circuit description that the indicator doesn’t specify the
exact level, but the frequency with which the audio signal surpasses approxi-
mately 60kHz of deviation. The indicator is based on music signals, not test
tones.
Again pressing the setup button will get us to the balance setting. The balance
simply attenuates one side or the other depending on the adjustment. Set in
the center there is no attenuation. Use the [UP]/[DOWN] buttons to swing the
balance to the left or the right.
Press setup again to access the power adjustment. The best way to adjust
this is to listen on a receiver into the transmitter input. (front living room, ga-
rage, deck) and have someone else turn up the power until you receive the
signal clearly. Stop adjustment right at this point; this is as far as you will need
to go! You may be surprised by how little power you really need. If you have to
run it at full power you may need to play with the antenna a bit because that
may indicate that you don’t have a good match.
The last setup screen allows you to save all of your changes. Pressing the
[UP] button will save them to the internal FLASH memory; [DOWN] will con-
tinue to use the current settings, but will not save them to FLASH in case you
need to make some more changes. Cycling through all of the displays again
will get you back to this screen to save all of your changes.
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