Re: Best Filter for the K3 when operating PSK31

Posted by Joe Subich, W4TV-4 on Dec 30, 2009; 3:58pm
URL: http://elecraft.85.s1.nabble.com/Best-Filter-for-the-K3-when-operating-PSK31-tp4230438p4232746.html



> As for gain pumping and inter-signal IMD: the likelihood that
> you are actually going to encounter a situation that requires a
> narrow roofing filter for PSK seems quite remote. Extremely
> strong PSK signals are not that common, PSK signals turn on and
> off only on time scales of several seconds, and PSK detection is
> not highly amplitude-sensitive, so both the likelihood and the
> effects of gain pumping and inter-signal IMD would appear to be
> much reduced compared to other modes.

That's easy to say in theory.  However, when your neighbor fires
up with his amplifier in a pile-up and activates the HACG you
need the 200 Hz filter "bandpass" decoding simply is not going
to do the job.

73,

   ... Joe, W4TV
 

> -----Original Message-----
> From: [hidden email]
> [mailto:[hidden email]] On Behalf Of Richard Ferch
> Sent: Wednesday, December 30, 2009 9:19 AM
> To: [hidden email]
> Subject: Re: [Elecraft] [K3] Best Filter for the K3 when
> operating PSK31
>
>
> G4ILO wrote:
> > Brandon673 wrote:
> >>
> >> I would like to hear opinions and reasons for the best
> filter to use
> >> for operating PSK31 with the K3.
> >>
> >
> > You don't need anything more than the SSB filter.
>
> I agree with Julian. Remember, it is the DSP that is responsible for
> signal filtering. You do not need narrow roofing filters to enable
> narrow DSP filtering - you can narrow the DSP filter down to
> 50 Hz even
> if the only roofing filter you have is 2.7 kHz wide. The job of the
> roofing filters is just to keep unwanted strong signals out of the IF
> bandpass to prevent gain pumping (desense) and IMD (splatter
> generated
> in the receiver) caused by those unwanted signals.
>
> Regarding DSP filter settings: Wide filter settings are
> useful to scan
> the band, for S&Ping, and for multiple signal decoding. Once you have
> picked a signal to work, or if you are calling CQ, narrow DSP filter
> settings can be helpful. You can put your DSP filter controls in
> SHIFT-WIDTH mode and use the two controls to focus on the desired
> frequency, or you can use an Align or Optimize feature in
> your software
> to center the desired signal at a chosen frequency so you
> only need to
> adjust the WIDTH control.
>
> As for gain pumping and inter-signal IMD: the likelihood that you are
> actually going to encounter a situation that requires a
> narrow roofing
> filter for PSK seems quite remote. Extremely strong PSK
> signals are not
> that common, PSK signals turn on and off only on time scales
> of several
> seconds, and PSK detection is not highly amplitude-sensitive, so both
> the likelihood and the effects of gain pumping and inter-signal IMD
> would appear to be much reduced compared to other modes.
>
> Bottom line: I'd suggest you base your decisions re roofing
> filters on
> your needs for other modes, not on PSK31.
>
> 73,
> Rich VE3KI
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