[arm-allstar] FOB 48KHz Aliases and spurs

[David McGough]

Sam,

I agree with your comments--for any serious repeater, bite the bullet and 
get a URI or perhaps even better, RIM. After one occasion of mussing and 
fussing with a cantankerous repeater way up in the air in a 30knot breeze 
at 40 degrees F, you'll be glad you did!!

73, David KB4FXC



On Fri, 8 Apr 2016, Sam Nabkey via arm-allstar wrote:

> No serious repeater user uses ham transceivers with data Jacks and mic
> inputs.
> 
> A good quality purpose built Uri type interface is all I would use.
> 
> Unless you like to putz with filters and mess with it?  Or make the
> neighbors mad?
> 
> My 2cents.
> On Apr 8, 2016 15:28, "David McGough via arm-allstar" <
> arm-allstar at hamvoip.org> wrote:
> 
> >
> >
> >
> > A simple RC 2nd order passive filter is probably adequate and should
> > provide 12db/octave rolloff. The main drawback with a passive filter is
> > insertion loss, which is why active designs that provide gain to
> > compensate, combined with higher order filters, are popular.
> >
> > Here is a website with some on-line design tools:
> >
> > 2nd order filter design:
> > http://sim.okawa-denshi.jp/en/CRCRtool.php
> >
> > Many other general purpose tools:
> > http://sim.okawa-denshi.jp/en/Fkeisan.htm
> >
> >
> > If anyone is interested, a 192KHz/24bit sound card (something like:
> > http://www.ebay.com/itm/131767155488 ) could be used to build a
> > high-resolution audio spectrum analyzer to "test" the lower sample rate
> > (48KHz) USB FOB. Some typical software for this type of application is
> > found here: http://www.qsl.net/dl4yhf/spectra1.html
> >
> > Anyhow, if you're using a mic-input on your transmitter, and even most
> > high-speed MODEM inputs, the transmitter frequency response is well below
> > 20KHz, and DAC Aliases are not of concern.  HOWEVER, if you were to place
> > an unfiltered FOB directly behind an old GE Mastr II exciter, you
> > might have spurs thru the entire ham band and into the business band!!!
> > So, the key here is to know your radio hardware--or, add a filter and
> > eliminate all concerns.
> >
> >
> > 73, David KB4FXC
> >
> >
> >
> > On Fri, 8 Apr 2016, Ken via arm-allstar wrote:
> >
> > > The response cutoff issue on the FOBS is probably a non-issue for most.
> > > Check it with a scope if you think it is a problem and put a non active
> > RC
> > > filter whihc should be fine for 48Khz especially with the rolloff of the
> > > radio, etc.
> > > 73 Doug
> > > WA3DSP
> > >
> > > I agree that the 48 kHz would only be a problem in specific cases, where
> > the
> > > modulation was introduced directly into the frequency control circuit.
> > > For the vast majority - using standard audio inputs - it's no problem.
> > > I would point out that a simple RC filter has 20 dB per decade roll-off.
> > > With (for example) a 4.8 kHz corner frequency, the response would be down
> > > only 20 dB at 48 kHz.  That is nowhere near enough for the case where you
> > > are injecting directly.
> > >
> > > Regards
> > > Ken
> > >
> > >
> > >
> > >
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> > >
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> > >
> >
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>