[HBR] Look Ma, no I.F. cans!

[Peter Bertini]

I agree with Tim... most filters have at least one or two tuned circuits to
deal with  spurious responses. Also, that 73 Magazine article was extremely
generic in nature.

Pete

On Thu, Mar 26, 2015 at 2:43 PM, Shoppa, Tim <tshoppa at wmata.com> wrote:

> I would be a little dubious that no IF LC filtering is necessary... real
> world crystal filters do have a bunch of spurious resonances well outside
> the passband. These are easily cleaned up with very simple LC bandpass
> filtering (doesn't have to be IF cans).
>
> What makes many crystal filters sound bad, is that their designers often
> go for brickwall steepness above everything else, and this results in
> substantial phase shift in the passband, which results in ringing.
> Staggered LC filters do not generally have this problem. Crystal filters
> can also be designed to move the phase shift outside the passband - some
> good crystal filter designs that do this will be called "Gaussian" (smooth
> phase shift being the goal with no steep walls) or "Gaussian to 12 dB" (a
> compromise between steep walls and phase shift, that moves the phase shift
> well outside the passband). I have a homebrew Gaussian to 12 dB crystal
> filter that is just a joy to listen through.
>
> Tim N3QE
>
> -----Original Message-----
> From: HBR [mailto:hbr-bounces at mailman.qth.net] On Behalf Of Peter Bertini
> Sent: Thursday, March 26, 2015 2:34 PM
> To: HBR Receiver List
> Subject: Re: [HBR] Look Ma, no I.F. cans!
>
> I've managed to acquire a few crystals that were used in the Heathkit HR-1
> IF filter; my plans are to use them in a lattice roofing filter at the 1st
> IF in a HBR project. The original Heathkit design used two rocks in a half
> lattice filter, which gave pretty mediocre performance for a Novice CW rig.
>
> I think I would still stay with trying to achieve the sharpest selectivity
> at the lower IF. Not the best for dynamic range, but more in keeping with
> the original design.  I'd also be tempted to emulate the variable
> selectivity system used in the Hallicrafters SX-101, where the coupling can
> be varied to control the IF bandwidth.
>
> Pete
>
> On Thu, Mar 26, 2015 at 1:25 PM, Brian Burns <brian at lessonsinlutherie.com>
> wrote:
>
> > Hello All,
> >
> >
> >
> > In browsing back issues of  73 magazine I came across an article on
> > selectivity in the April 1961 issue entitled "Take Your Pick". It's
> > about filters in all their various permutations. Here is the link to
> > the on-line back issues:
> >
> >
> >
> > https://archive.org/details/73-magazine
> >
> >
> >
> > It's an excellent article, and about halfway through is the
> > description and schematic for a "crystal coupled I.F. amplifier"---no
> > I.F. transformers, just crystals. The author says just add RF stage,
> > mixer, detector, and audio stage, and you have a "good receiver". He
> > says the selectivity is equal to that of a mechanical filter.
> >
> >
> >
> > My only reservation is that Carl, KM1H, says that selectivity gotten
> > with coils and capacitors sounds better. All those super high Q
> > crystals look likely to ring a good deal.
> >
> >
> >
> > Cheers,
> >
> >
> >
> > Brian
> >
> >
> >
> > ---
> > This email has been checked for viruses by Avast antivirus software.
> > http://www.avast.com
> > ______________________________________________________________
> > HBR mailing list
> > Home: http://mailman.qth.net/mailman/listinfo/hbr
> > Help: http://mailman.qth.net/mmfaq.htm
> > Post: mailto:HBR at mailman.qth.net
> >
> > This list hosted by: http://www.qsl.net Please help support this email
> > list: http://www.qsl.net/donate.html
> >
> ______________________________________________________________
> HBR mailing list
> Home: http://mailman.qth.net/mailman/listinfo/hbr
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:HBR at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> ______________________________________________________________
> HBR mailing list
> Home: http://mailman.qth.net/mailman/listinfo/hbr
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:HBR at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>