[arm-allstar] Audio FOB output

[Doug Crompton]

The DMK URI has an active low-pass filter of classic design -

http://dmkeng.com/images/URIx%20Schematic.pdf

and described here -

http://www.electronics-tutorials.ws/filter/filter_5.html

I suspect this is a little overkill as 48khz could be filtered fairly easily using just capacitive filtering without bothering the baseband. I guess some experiments are required. A scope on the audio out of the FOB would be all you would need to see it.
73 Doug
WA3DSP
http://www.crompton.com/hamradio


> To: arm-allstar at hamvoip.org
> Date: Thu, 7 Apr 2016 15:15:55 -0400
> Subject: Re: [arm-allstar] arm-allstar Digest, Vol 23, Issue 7
> From: arm-allstar at hamvoip.org
> CC: ke2n at cs.com
> 
> 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
> 
> 
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