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

[Brian Burns]

Hello Pete,

~ 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.

The 73 article is just credited to "staff" or I would have referenced the
author and his call. He discusses single crystal, crystal lattice, crystal
coupled, ceramic, mechanical, etc. etc. 

One of the best things about the article is that he lists the advantages and
disadvantages of each of the possible selectivity solutions, and all from
the homebrewer's point of view. He says that the crystal coupled circuit has
out-of-passband response that is nearly 90 db down, and is equaled only by
the mechanical filter. It uses three crystals.

~ 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.

I've got the 85 kHz IF cans, but one of my concerns about double conversion
is all the birdies that get produced with three oscillators running. With
the "crystal coupled" system, you can go with single conversion, and a high
first IF for good image rejection, and still get your good selectivity.
Crystals have amazingly high Q's. Some sort of switchable selectivity would
certainly be handy.

Cheers,

Brian


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