View Single Post
Old 31st Dec 2022, 8:07 am   #31
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
Default Re: Full-Range IF Selectivity; The Hammarlund Variable Crystal Filter

If you stick with a single crystal filter, you can only have a single-pole response until you get far enough out that the IFT-created selectivity starts to be felt.

This single pole response will be just that, you can only change it's Q and that stretches the whole response in the frequency dimension. Your 3dB points get wider but the skirts get proportionately wider too, so you loose the ability to reject adjacent channels. You are tied to a skirt slope of only 20dB per decade change in offset from centre frequency.

So the shape factor of the resulting filter is rather poor. The Hammarlund and AR88 circuits achieve variable bandwidth by varying the Q and stretching the whole frequency response.

If you want better selectivity in terms of better audio bandwidth of wanted stations and no worse rejection of unwanted stations, then the only route is to introduce multiple poles to create the new bandwidth. This can be done with crystals at 455kHz, but it will take a lattice structure to do it, and all the crystals will need tight control of their motional parameters and case capacitance.

So if you take a single crystal filter and widen it to the point where it is similar in bandwidth to an AM-appropriate IFT strip, the skirts of the crystal filter will be much wider than the IFTs. I'm afraid it's just one of those things built into the maths behind filters.

Other 455kHz filter technologies like Collins' mechanical filters and Murata etc's 'ceramic' filters are multipole assemblies and can give far better wanted signal/unwanted signal performance than any single crystal filter can possibly give for AM and SSB bandwidths. The single crystal filter is really only suited to CW reception, and it works well enough to be useful at that.

I think you won't make a significant improvement to your HRO by modifying the single crystal half-lattice. I believe others have grafted in Murata/Toko ceramic filters to get better SSB reception.

These filters may in themselves have impedances best suited to transistor circuits, but impedances can be transformed. A ceramic filter could be embedded between a pair of lowish-Q tapped IF transformers to make it compatible with existing valve circuitry. Transforming Z down, and then back up again.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline