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

[Shoppa, Tim]

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
>
>
>
> ---
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