[arm-allstar] Yet another USB FOB Modification

Scott Zimmerman n3xcc at zewireless.com
Sat Nov 15 22:43:45 EST 2014 

 > The effort to make a $2.50 FOB work is well worth it at that cost.
That depends on how valuable your time is. If your time is worth nothing, then YES. If you can make 
more money doing something else, or you have three grade-school kids like me, then NO.

There are some audio/RF engineering reasons to NOT use an unfiltered FOB as well. All is well if you 
are feeding a microphone input.

 > The DMK URI is only $70.
Correct. However the URI is missing additional filtering and the PC comms. failure detection. A 
through investigation of the URI circuitry will uncover a few other poor engineering practices 
implemented in its design.

I'm not trying to toot my own horn, but rather simply pointing out that there are reasons why the 
RIM is more money. A comparison of the circuitry clearly shows the differences.

If you are running your TX audio into the microphone input of your radio/repeater, you are fine 
using the audio output from a "raw" FOB. However, if you are driving a modulator directly (into a 
synthesizer, or direct FM via a varactor diode) the lack of a low-pass filter after the DAC *WILL* 
produce off frequency signals.

I hate to delve into a deep audio/RF engineering discussion on a primarily software based list, but...

If you happen to have an oscilloscope, and you take a close look at the audio coming out of the DAC 
you'll see "stair steps" on the leading and trailing edges of the sine wave, this is DAC clock 
noise. This noise is a direct result of the number of bits of resolution of the DAC and the rate at 
which the audio is being played back. By its very nature, *ANY* DAC *WILL* produce harmonics since 
it is a fundamentally digital device and is only capable of natively producing pseudo sine waves 
comprised of short duration square waves. (discrete voltages for a specific period of time)

If you run the raw DAC audio into a mic input, the additional filtering of the audio chain cleans up 
the wave, removes the digital noise, and leaves you with a clean waveform headed for the modulator.

If you drive a modulator directly with that same "raw" waveform, sidebands will appear in your FM 
transmitter. (Think FM stereo pilot) Since the DAC is running at 44KHz, this "pilot" falls 44KHz on 
each side of your repeater's center frequency. THIS IS BAD. Sure, one could argue that the frequency 
response of a typical amateur grade modulator is pretty low at 44KHz, and I agree. However, why 
tempt fate? I don't want to be the one to get a pink slip from the FCC for splattering RF across 90+ 
KHz of 2M spectrum. (You need to take into account the harmonics of the 44KHz sampling frequency as 
well.) It is for this reason that a low-pass filter is necessary after the DAC. The human ear can't 
hear the 44KHz clock noise when a FOB is used for its intended purpose, but I can assure you that 
the 44KHz will make it into the modulator and cause issues.

Please, for the sake of protecting neighboring repeater channels, place additional filtering after 
the DAC if you are not running your audio into the mic input of your repeater. The URI has two-poles 
of LP filtering to clean up the DAC's output. I went with 5 poles for my RIM design which allows me 
to run the knee of the filter higher in frequency (allowing for more greater sibilance on wider UHF 
channels and for future wide bandwidth digital modes) while still providing a sharp skirt that 
knocks the 44KHz down to nothing.

Scott

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

On 11/15/2014 9:30 PM, Doug Crompton wrote:
> WOW, when you are using a $50 BBB are you going to spend $100 for a USB interface??? I think not.
> The effort to make a $2.50 FOB work is well worth it at that cost. The DMK URI is only $70.
>
> *73 Doug
> WA3DSP
> http://www.crompton.com/hamradio*
>
>
>  > Date: Sat, 15 Nov 2014 12:40:36 -0500
>  > From: n3xcc at zewireless.com
>  > To: arm-allstar at hamvoip.org
>  > Subject: Re: [arm-allstar] Yet another USB FOB Modification
>  >
>  > > One thing I always like to see is some LED's to indicate what is going on.
>  > I likewise. This along with other reasons is why I brewed by own interface. It's nothing fancy, but
>  > it does have indicator LEDs, additional audio output filtering, (for driving a modulator directly)
>  > and USB failure detection.
>  >
>  > Frankly I was tired of trying to locate modifiable FOBs, then go through the time and effort
>  > required to add all of the additional circuitry I thought necessary.
>  >
>  > You can check out the USB-RIM here: http://www.repeater-builder.com/products/usb-rim.html
>  >
>  > Scott
>  >
>  > Scott Zimmerman
>  > Amateur Radio Call N3XCC
>  > 474 Barnett Road
>  > Boswell, PA 15531
>  >
>  > On 11/15/2014 11:25 AM, Mark Herson, N2MH wrote:
>  > > Yet, yet another comment on FOB mods...
>  > >
>  > > I've done a bunch here on pc prototype board. One thing I always like to
>  > > see is some LED's to indicate what is going on.
>  > >
>  > > In addition to the green heartbead LED, I also include a red LED to
>  > > indicate PTT and a yellow LED to indicate COR coming into the fob.
>  > >
>  > > The red LED needs another transistor in parallel to the usual PTT LED.
>  > > This keys the red LED which is then connected to +5 VDC through a current
>  > > limiting resistor.
>  > >
>  > > The yellow LED is connected from the incoming COR line through a current
>  > > limiting resistor to +5 VDC. This limits the incoming COR to an active
>  > > ground from the radio equipment. Fortunately, this protects the sound fob
>  > > from an incorrect COR condition if the radio is turned off or otherwise
>  > > disconnected from the FOB.
>  > >
>  > > +5 VDC is picked up from the sound fob.
>  > >
>  > > This whole topic reminds me of YAPP from the early days of packet radio.
>  > > YAPP was a file transfer protcol customized for packet radio. YAPP stands
>  > > for Yet Another Packet Protocol :-)
>  > >
>  > > 73, Mark, N2MH
>  > > Allstar Node 40831
>  > > ACID plus 2 BBB's and counting...
>  > >
>  > >> Date: Sun, 9 Nov 2014 02:11:25 -0500
>  > >> From: Doug Crompton <doug at crompton.com>
>  > >> To: "arm-allstar at hamvoip.org" <arm-allstar at hamvoip.org>
>  > >> Subject: [arm-allstar] Yet another USB FOB Modification
>  > >> Message-ID: <BLU172-W299469D7812B52ECE86A28BA830 at phx.gbl>
>  > >> Content-Type: text/plain; charset="iso-8859-1"
>  > >>
>  > >>
>  > >> Here is a link to my modification of the USB FOBS recently noted on this
>  > >> list and available from ebay at $24 for a lot of 10. I will be adding
>  > >> this link to the BBB web Page.
>  > >>
>  > >> http://crompton.com/hamradio/usb_fob_modification/
>  > >>
>  > >>
>  > >> 73 Doug
>  > >> WA3DSP
>  > >> http://www.crompton.com/hamradio
>  > >
>  > >
>  > > _______________________________________________
>  > >
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>  > > http://lists.hamvoip.org/cgi-bin/mailman/listinfo/arm-allstar
>  > >
>  > > Visit the BBB web page - http://www.crompton.com/hamradio/BeagleBoneBlackAllstar/
>  > >
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>
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