On Tue, 2007-01-09 at 16:05 -0500, Jack Wigal wrote:
> I have a TT #217 filter (9MHz IF, 8 pole, 500Hz) that has a bad crystal. TT
> doesn't repair them, so I'm gonna play with it if I can find parts. Does
> anyone have a bad one they would sell cheap? I also wonder if I can convert
> it to a 219 (6 pole, 250Hz), so has anyone tried that. If any of you have a
> 219 laying around, please send me the capacitor sizes on the filter. I have
> already purchased another filter to replace the dead one, so now I'm trying
> to learn something. I'm also open to any suggestions you may have. Since
> most folks probably don't find this interesting, it may be best to email me
> directly.
>
> Thanks,
> Jack, kr8z
>
I believe the Tentec crystal circuit is one of series crystals with a
shunt "coupling" capacitor to ground between each pair of crystals. If
so you get narrower bandwidth by increasing the values on the shunt
capacitors. With identical crystals you can get a useful filter using
all the same value capacitors. But if you consult some crystal filter
design references such as the tome by Zverev, you can design with a
better time response which takes different valued capacitors. There can
also be capacitors in series with the individual crystals working to
adjust their resonant frequency. 250 or 500 Hz would probably use all
the same crystal frequency though that's more likely for 250 Hz than 500
Hz. On way of spreading the bandwidth is to use crystals of different
frequency and then thy may be spread a bit much for narrowing to 250 Hz.
To add complication, I'd expect the 500 Hz filter to use lower Q
crystals than those needed for the 250 Hz filter and that difference can
make the narrowing more difficult. And remember that changing the
bandwidth also changes the filter impedance so those matching capacitors
will need adjustment to get a predicted filter response.
Since you may be into experimental filter construction, there's no
inherent need to drop to 6 poles if 7 of the 8 crystals are good.
I noticed in my Corsair II that the IF shift was so effective at making
it a narrow receiver that the narrow IF filter wasn't all that needed
though I do have a 500 Hz filter for the second IF.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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