[Elecraft] Mildly confused - Assumed filter width and what Isee in waterfall do not match

[Joe Subich, W4TV]

> I guess my basic answer, is that should be my choice, not the 
> radio firmware. If I want to run 4 (or say, 10khz) into my 
> computer (and out) that should be my choice.

The K3 does not support an audio bandwidth higher than 4.2 KHz 
in ANY MODE.  If you do some checking you will find the DSP's 
digital to analog converter effectively includes a 4.2 KHz 
"brickwall" filter ... even in AM mode with the "bandwidth" 
set to 5 KHz (10 KHz IF).  

If you expect audio response above 4.2 KHz, you will need to 
convince Wayne and Lyle to relax the upper frequency limit. 
HOWEVER, for digital modes I'm not so sure that is a good 
idea.  With sound cards sampling at 11025 Hz, the input audio 
needs to be band limited to 5.5 KHz maximum and 4.5 KHz offers 
a fair margin for safety to handle the occasional application 
that runs at 8 or 9 KHz.  

For AM - since the digital modes are not a consideration - it 
would be nice to have 5.5 or 6.0 KHz response. 

73, 

   ... Joe, W4TV 
 


> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Thomas 
> Bingenheimer
> Sent: Wednesday, January 07, 2009 9:07 PM
> To: elecraft at mailman.qth.net; Bob Cunnings
> Subject: Re: [Elecraft] Mildly confused - Assumed filter 
> width and what Isee in waterfall do not match
> 
> 
> I guess my basic answer, is that should be my choice, not the 
> radio firmware. If I want to run 4 (or say, 10khz) into my 
> computer (and out) that should be my choice. After all, how 
> is an SDR all that different. Again, I suspect this is a 
> oversight in the firmware, not a design choice. Why should 
> such a flexible radio be hobbled in this manner? (assuming, 
> of course, that it is actually limited as such, as opposed to 
> me setting it up wrong :) ).
> 
> 
> --- On Wed, 1/7/09, Bob Cunnings <bob.cunnings at gmail.com> wrote:
> 
> > From: Bob Cunnings <bob.cunnings at gmail.com>
> > Subject: Re: [Elecraft] Mildly confused - Assumed filter width and 
> > what I see in waterfall do not match
> > To: elecraft at mailman.qth.net
> > Date: Wednesday, January 7, 2009, 8:59 PM
> > Disabling the AGC in the K3 certainly does "do the
> > trick" in terms of
> > preventing strong signals in the passband from reducing
> > receiver gain
> > and thus affecting the weaker signal I'm decoding -
> > that's exactly why
> > I do it.
> > 
> > As for ADC overload -that's a risk I'm well aware
> > of but most of the
> > time it's a non-issue, for me at least. If it happened
> > I would react
> > to it, but I rarely encounter signals in the passband at 
> anywhere near
> > the level to cause trouble of that sort.
> > 
> > Bob NW8L
> > 
> > On Wed, Jan 7, 2009 at 6:23 PM, Don Wilhelm <w3fpr at embarqmail.com> 
> > wrote:
> > > Bob,
> > >
> > > That will do the trick in an analog only receiver, but
> > a receiver like the
> > > K3 has an analog front end followed by an ADC and DSP
> > processor.  The trick
> > > is to keep from overloading the ADC - should that
> > happen, the copy will be
> > > garbage.
> > >
> > > That situation is not limited to the K3 - overload of
> > the soundcard ADC is
> > > also a possibility with equally bad results.
> > >
> > > I have to admit that in many cases, one can operate
> > with a wide bandwidth
> > > with no problem, but when that strong signal enters
> > the passband, the wide
> > > bandwidth possibilities are "all over".  I
> > chose to take preventive measures
> > > before that happens.
> > >
> > > 73,
> > > Don W3FPR
> > >
> > > Bob Cunnings wrote:
> > >>
> > >> I simply disable AGC when I wish to run a wide
> > receive bandwith with
> > >> PSK31 for "point and click" tuning -
> > precisely to avoid such a
> > >> problem.
> > >>
> > >> Bob NW8L
> > >>
> > >> On Wed, Jan 7, 2009 at 5:44 PM, Don Wilhelm
> > <w3fpr at embarqmail.com> wrote:
> > >>
> > >>>
> > >>> Thomas,
> > >>>
> > >>> I will not answer your questions directly,
> > >>> BUT
> > >>>  From an operational standpoint, using a wide
> > bandwidth for data modes
> > >>> is *not* the best way to do it.
> > >>> The reason is AGC in the radio.  Any signal in
> > the receiver passband can
> > >>> activate the AGC - and that is fine *if and
> > only if* the strongest
> > >>> station in the receiver passband is the one
> > you are working - usually
> > >>> that is not the case.   The strong signal will
> > reduce the receiver gain
> > >>> due to its AGC action and the station you are
> > trying to QSO with will be
> > >>> reduced along with it.  Overload of the DAC by
> > the strong signals is
> > >>> another similar consideration - fortunately,
> > the K3 employs a hardware
> > >>> AGC ahead of the DAC to avoid just that
> > possibility.  The DSP ADC can
> > >>> handle an S9+20 signal without overload, but
> > there are signals stronger
> > >>> than that even in the sub-bands commonly used
> > for digital.
> > >>>
> > >>> Using a narrow passband for data modes allows
> > one to avoid that
> > >>> situation.  Yes, one must tune with the VFO to
> > place the desired station
> > >>> inside the receiver passband, but the
> > possibility of a QRM free QSO is
> > >>> much greater with the narrow passband.
> > >>>
> > >>> Just because the software application can
> > display a 4 kHz slice of the
> > >>> spectrum is not sufficient reason to use a
> > wide receiver passband IMHO.
> > >>>
> > >>> Elecraft may well consider it just because
> > some folks want to operate
> > >>> that way, but it certainly does not make much
> > sense to me.
> > >>>