Hi Henry et al,
Further to the discussion about Collins mechanical filters, below is a short
piece I wrote for my local club mag a couple of years ago that explains
roughly how they work. I can't remember the sources for the info (and I'm
not and engineer/expert/deity) so corrections are welcome.
-------------
Collins Mechanical Filters
In the '50s and '60s, the Collins Radio Company, now part of the giant
Rockwell Corporation, made the finest amateur radio equipment available, and
their legendary "S-line" range of separates was the ultimate choice for the
wealthy amateur. Collins pulled out of the amateur market in the early '80s,
but the sharp-eyed among you may have noticed recently a small "Collins
Inside" logo appearing in some adverts from the likes of Yaesu and AOR. This
indicates the inclusion of one or more Collins Mechanical Filters. But what
is a mechanical filter?
The Collins Mechanical Filter (CMF) is a mechanically resonant device which
receives electrical energy, converts it into mechanical vibration, filters
out unwanted frequencies, then converts the mechanical vibration back into
electrical energy at the output. If you opened a CMF (not a good idea given
their cost and intricacy) you would find three basic elements: transducers
which convert electrical oscillations or vice versa, mechanically resonant
discs each around the size of a new 5p piece, and disc coupling rods or
wires running alongside the stack of discs. These discs, with Qs of around
10,000, endow the CMF with its exceptional steep-skirted selectivity.
Mechanical filters are electrically and mechanically stable and they resist
ageing, breakdown and drift even with extreme temperature changes or long,
continuous service. For example, frequency shift of a typical CMF holds
between 1.5 and 2 parts per million/degree C over a -25degC to +85degC
range. The CMF also achieves a flat-topped frequency response
characteristic, being built with a 60 to 6dB shape factor as low as 1.2:1.
In addition to electrical and mechanical conversion, the transducer also
provides proper termination for the mechanical network. The discs determine
the centre frequency of the CMF. Since each disc represents a parallel
resonant circuit, increasing the number of discs increases skirt selectivity
of the filter. Skirt selectivity is specified as shape factor, which is the
ratio (bandwidth 60dB below peak)/(bandwidth 6dB below peak). By varying the
mechanical coupling between the discs, i.e. making the coupling rods larger
or smaller, the bandwidth of the filter is varied. Because the bandwidth
varies approximately as the total area of the coupling wires, the bandwidth
can be increased by either using larger or more coupling rods.
"BreakTHROUGH" - Aberdeen Amateur Radio Society, August 1996
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73
Al, GM4BAP
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