[arm-allstar] Audio FOB output

[Scott Zimmerman]

 > The DMK URI has an active low-pass filter of classic design -
Indeed, but if you run the numbers you may be surprised as to it's cutoff point. (The single RC pole 
rolls at 1.88KHz and the two-pole filter rolls at about 3.2KHz) These points *may* be too low for 
some signalling applications where square wave type response is required.

The RB-RIM's 5-pole filter was designed with a roll-off starting at 10KHz. This allows for better 
square wave signalling response, while still yielding protection to adjacent channels on VHF 
repeater pairs @ 15KHz channel spacings.

Yes, five poles of filtering are probably overkill, but since two poles weren't going to be adequate 
(hence another op-amp would be required anyhow) the additional poles were a VERY low cost addition.

http://www.repeater-builder.com/products/usb-rim.html

Scott

Scott Zimmerman
Amateur Radio Call N3XCC
474 Barnett Road
Boswell, PA 15531

On 4/7/2016 3:55 PM, Doug Crompton via arm-allstar wrote:
> 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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