[arm-allstar] arm-allstar Digest, Vol 23, Issue 7

[Ken]

Regarding this:
"On the difference between a DMK URI and modified FOB I have never noticed a
difference and I am fairly critical of audio. There is slightly more
filtering in the DMK product but they both use the same chip. The DMK URI
schematic is readily available at their web site and you can compare the
differences. The simple FOB has a wider frequency response as one of its
uses is for headphone listening of stereo music. This is usually not an
issue though as most radios roll off above the communication quality upper
limit."

I have to chime in that, if you put a scope on output of most of the simple
fobs, you will see an astounding amount of 48 kHz signal.  This frequency is
above what your dog can hear (44 kHz) and most anything you attach to it
will naturally tend to filter (integrate) the signal. It's probably OK to
apply it to the microphone input of most radios, because there is filtering
on that.  Where you could get in trouble is if you apply it directly to the
modulator stage of some transmitter (perhaps by using the "tone" input of
some commercial rigs, or the 9600-packet input of some ham rigs).  In that
case, you might generate some spurs at, for example, +/- 48 and +/- 96 kHz.
The spectrum scope on most service monitors has a default BW setting that
might not go out this far, so you might miss seeing these.

73
Ken