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<body class='hmmessage'><div dir='ltr'><font style="" face="Tahoma,sans-serif" color="#000000">The DMK URI has an active low-pass filter of classic design -<br><br><a href="http://dmkeng.com/images/URIx%20Schematic.pdf" target="_blank">http://dmkeng.com/images/URIx%20Schematic.pdf</a><br><br>and described here -<br><br><a href="http://www.electronics-tutorials.ws/filter/filter_5.html" target="_blank">http://www.electronics-tutorials.ws/filter/filter_5.html</a><br><br>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.<br id="FontBreak"></font><br><b><font style="font-size:16pt;" size="4">73 Doug</font><font style="font-size:16pt;" size="4"><br></font><font style="font-size:16pt;" size="4">WA3DSP</font><font style="font-size:16pt;" size="4"><br></font><font style="font-size:16pt;" size="4">http://www.crompton.com/hamradio</font></b><font style="font-size:16pt;" size="4"><br></font><br><br><div>> To: arm-allstar@hamvoip.org<br>> Date: Thu, 7 Apr 2016 15:15:55 -0400<br>> Subject: Re: [arm-allstar] arm-allstar Digest, Vol 23, Issue 7<br>> From: arm-allstar@hamvoip.org<br>> CC: ke2n@cs.com<br>> <br>> Regarding this:<br>> "On the difference between a DMK URI and modified FOB I have never noticed a<br>> difference and I am fairly critical of audio. There is slightly more<br>> filtering in the DMK product but they both use the same chip. The DMK URI<br>> schematic is readily available at their web site and you can compare the<br>> differences. The simple FOB has a wider frequency response as one of its<br>> uses is for headphone listening of stereo music. This is usually not an<br>> issue though as most radios roll off above the communication quality upper<br>> limit."<br>> <br>> I have to chime in that, if you put a scope on output of most of the simple<br>> fobs, you will see an astounding amount of 48 kHz signal. This frequency is<br>> above what your dog can hear (44 kHz) and most anything you attach to it<br>> will naturally tend to filter (integrate) the signal. It's probably OK to<br>> apply it to the microphone input of most radios, because there is filtering<br>> on that. Where you could get in trouble is if you apply it directly to the<br>> modulator stage of some transmitter (perhaps by using the "tone" input of<br>> some commercial rigs, or the 9600-packet input of some ham rigs). In that<br>> case, you might generate some spurs at, for example, +/- 48 and +/- 96 kHz.<br>> The spectrum scope on most service monitors has a default BW setting that<br>> might not go out this far, so you might miss seeing these.<br>> <br>> 73<br>> Ken<br>> <br>> <br>> _______________________________________________<br>> <br>> arm-allstar mailing list<br>> arm-allstar@hamvoip.org<br>> http://lists.hamvoip.org/cgi-bin/mailman/listinfo/arm-allstar<br>> <br>> Visit the BBB and RPi2 web page - http://hamvoip.org<br>> <br></div> </div></body>
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