It is my understanding that the displayed BW is that of the audio. At least
with my single 2.8KHz filter, there is a caveat: the "operational" bandwidth
can't exceed the i-f filter bandwidth, even though the display will lull you
into thinking so, if you're not careful.
With my radio if I "normalize" the BW, the Lo Cut defaults to 100 Hz and the Hi
Cut to 2.9KHz. If for some reason I adjust the Hi cut to a higher frequency,
the display will still show the Lo Cut at 100 Hz. If for example I decide to
widen the BW to 3.5 KHz (Hi = 3.6) to "improve fidelity" the Lo Cut readout
will still display 100 Hz, but the low frequency response isn't what it was
before.
In doing this with my radio, a 300 Hz tone is down 12 dB. This was because
changing the DSP limit moved the i-f filter pass-band with respect to the
signal. In other words, the DSP BW is *not* independent of the i-f filter BW.
Of course it can be argued that the readouts are displaying the audio pass-band
and this is to be expected. But I think this is wrong. Perhaps some of the
reports of lousy audio result from this mistake.
Wes Stewart, N7WS
--- On Wed, 4/8/09, Don Wilhelm wrote:
> From: Don Wilhelm
> Subject: Re: [Elecraft] AM RX bandwith
> To: "Jan Erik Holm"
> Cc: "Elecraft Reflector"
> Date: Wednesday, April 8, 2009, 8:37 AM
> Jim,
>
> The change is only in the displayed width. Before it was
> the IF
> bandwidth, and now it is the audio bandwidth - for AM, that
> will be 1/2
> of the IF bandwidth, so 10 kHz before equals 5 kHz
> displayed now. It
> is consistent with the other bandwidth displays that
> indicate the audio
> channel width.
>
> 73,
> Don W3FPR
>
> Jan Erik Holm wrote:
> > This might be a stupid question since I think this
> > has been on the list before.
> >
> > Just upgraded from 2.76 to 3.06. I´m using 13 KHz
> > FM filter for AM RX´ing.
> > Before I could see 10 KHz DSP bandwith but now 5.00
> KHz.
> > Something has changed. What bandwith do I get? Did
> > I get 10 KHz before and now it is changed to 5 KHz or
> > is it just "one sideband" I´m seeing? I´m
> just puzzled.
> >
> > This is how I like to have it. Would like to be able
> to
> > widen the DSP all the way out to the filter (13 KHz)
> > but it seems to me it´s not possible, maybe it never
> > was.
> >
> > Sorry if this has been on the agenda before.
> >
> > 73 Jim SM2EKM
> >
> > PS: When I think about it maybe I saw 9 KHz before and
> > not 10, well one or the other.
> >
> >
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