[arm-allstar] Yet another USB FOB Modification

David McGough kb4fxc at inttek.net
Sat Nov 15 23:16:09 EST 2014 

Hi Everyone,

I have to agree 100% with Scott and Kevin---each device has a proper
application. I would never consider a cheap homebrew FOB when a URI is
available for $100 or less for a production grade repeater--all
their points made are completely valid. And, URI's are REALLY cheap when
trying to fix an oddball problem way up on a tower, in a 40+ knot breeze!

However, for an application like setting up a simplex APRS "hot-spot"  
receiver, with vox, mic and speaker audio, I've had fantastic success with
$3 FOBs! For the home user, looking for a simple way to "get on-line"  
with AllStar (again with mic and speaker audio), the FOB's are very
appealing, too.

...Just my $0.02!


73, David KB4FXC




On Sat, 15 Nov 2014, Scott Zimmerman wrote:

>  > 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
> >  > >
> >  > >
> >  > > _______________________________________________
> >  > >
> >  > > arm-allstar mailing list
> >  > > arm-allstar at hamvoip.org
> >  > > http://lists.hamvoip.org/cgi-bin/mailman/listinfo/arm-allstar
> >  > >
> >  > > Visit the BBB web page - http://www.crompton.com/hamradio/BeagleBoneBlackAllstar/
> >  > >
> >  > _______________________________________________
> >  >
> >  > arm-allstar mailing list
> >  > arm-allstar at hamvoip.org
> >  > http://lists.hamvoip.org/cgi-bin/mailman/listinfo/arm-allstar
> >  >
> >  > Visit the BBB web page - http://www.crompton.com/hamradio/BeagleBoneBlackAllstar/
> >  >
> >
> >
> > _______________________________________________
> >
> > arm-allstar mailing list
> > arm-allstar at hamvoip.org
> > 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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