[arm-allstar] FOB 48KHz Aliases and spurs
sam.nabkey at gmail.com
Fri Apr 8 14:33:21 EST 2016
No serious repeater user uses ham transceivers with data Jacks and mic
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
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:
> Many other general purpose tools:
> 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
> > 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
> > 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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