[R-390] Filter foolishness

[Tisha Hayes]

Quote;
Bob Wrote:
"The loaded Q of the driver coils is indeed in the vicinity of 33 (input
resistance 100K ohms, resonating cap 120 pf, 120 pf = 2916 j ohms at 455
KHz). Since the 3 db bandwidth is Fc / Q you get 455 / 33 = 13.8 KHz.
That's just for the driver coil on one end. The Q of the coil it's self,
unloaded by the filter is significantly higher.

In order to be "much wider" than 16 KHz, you might want 160 KHz on both
ends. 455/160 gives you a Q of 2.8. For an input resistance of 100K ohms,
that would be a resonating capacitance of ~ 10 pf. In order to hit
something like that you would need to tune the input to each filter. That's
not something the Collins guys wanted to do. Eight more trimmers cost
money, and take up space.

Bottom line - yes the coils do get into the bandwidth of the IF filters to
a greater or lesser degree. The response is a composite of the driver coils
and the disks. You don't want to do anything dramatic to those coils. You
will certainly mess up the passband of the wider filters if you do. The
whole filter was calculated / designed with the impact of the driver coil
Q's taken into account."
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Yes, that is pretty close to what I was seeing as well, it was a fun
exercise to work through. If you played around with significant coil
variations for the different filters you are also messing with the
insertion loss and might be blasting the headphones off of your head
whenever you turned the bandwidth switch. That would be if you were looking
at a different set of specifications for each individual filter but would
make no sense for a family of filters that should be more or less matched
to each other.

I did not believe the Q values at first and had not considered that when
first looking at things. The software tool I was using gave me the value
but I decided to not emphasize that in what I wrote. Also my values for "k"
on calculating mutually opposed impedance of the coils was extrapolated
(wing and a prayer) and I was unsure how to consider the magneto-mechanical
impact upon the magic wire for the center cheekpiece spacing and coil
length (the coils could be compressed into a smaller dimension for a
greater flux density).

I was intrigued by their choice of wire gauge, turns ratio and coil
spacing. With modern computing methods we could run through in an afternoon
what would of taken a room full of people several weeks to calculate by
hand. It just increased my admiration of what they did on what is such a
small device inside of the receiver. I had to go crack open a few books on
acoustics and speaker design to get a better understanding of the magic
wire and the dancing disks.

Tisha