[R-390] Filters

[Bob Camp]

Hi

Ok, that gets us off into another rabbit hole.

The FM filters were indeed amazing things. To me the most amazing part of it was watching the line girls (in that era, yes indeed the entire line was staffed by "female associates") align one. 16 - 24 stagger tuned poles, very simple tools. No automation, just manual tune up. They went from nothing to a perfect filter in under a minute. Truly amazing. When one mentioned aligning an R-390 to the same people, the reply was - "bring it in, we all worked on them to…" (the rest of the concerning the relative skills of a certain junior engineer compared to the line assembly team is not suitable for a family oriented list). 

Sorry for the digression …

The Q of the resonators in a big pot core filter is much lower than the Q of the resonator disks in a mechanical filter. That makes the LC filter very good for a nice broadband FM IF filter. Another minor issue is the coupling between resonators. The mechanical's have a very tough time doing the sort of linear phase that an FM IF wants. Simple answer - there's a reason you don't see mechanical filters in FM radios. 

The flip side is that the high Q / low coupling of the mechanical makes them just the right thing for more narrow filters. SSB is more tolerant of phase / delay ripple than FM. Mechanical filters are fine for an R-390A. 

A narrow mechanical will have less insertion loss than a narrow LC filter (higher Q counts). With a gain stage or two you could fix that. It would make straight drop in a bit more complex. 

Lots of rabbits to chase….

Bob

On Feb 18, 2013, at 1:09 AM, Drew P. <drewrailleur807 at yahoo.com> wrote:

> Bob Camp wrote: 
> 
> Another thing the filters share a background with are the Vibrasponder / Vibrasender PL tone reeds used in FM two way gear.
> 
> [snipped]
> 
> Another IF filter solution similar to that used in some old FM two way gear could be used - a big bunch of cascaded LC tuned circuits like the old "Permakay" filters.  These would occupy much space and hence would limit you to perhaps 2 bandwidth choices within the space inside the filter cover. Bandwidth curve shape would not be as good as the mechanical filters, but would be much better than a flat zero non-functional filter. 
> 
> I am following this thread with interest as I have a Stewart-Warner with a defunct 4 kc "mechanism a philharmonic".
> 
> Drew
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