[Milsurplus] [Glowbugs] Quick n dirty crystal selectivity - it no workee

[Ian Wilson]

Hue,

The ESR of older crystals can be enormous by modern standards. This doesn't
matter
much in 'normal' applications, since the Q can still be 100k or so - but to
use the crystal
as a bandpass element in the cathode circuit, it will need to have lower
ESR at the point
of resonance than the cathode resistor. You may be able to make this work
by adding an
RFC in series with the cathode resistor, and putting the crystal from the
cathode to
ground. This may also produce a nifty oscillator :)

73, ian K3IMW


On Mon, Feb 15, 2016 at 5:35 PM, Hubert Miller <kargo_cult at msn.com> wrote:

> Many years back, i had an idea for improving the selectivity of the 6 - 9
> MHz 'Command Sets' receiver.
> The IF is 2830, IIRC.  I took a crystal from an old 2-MHz boat radio, 2837
> kHz, and put it across the
> first IF cathode resistor, with the bypass cap formerly there,
> disconnected. It didn't do a darn thing,
> no change in selectivity or sensitivity at all. The idea was to be
> something like the low-Z 'transfilters'
> used in some transistor circuits. I thought later that maybe the cathode
> resistor needed an RFC in
> series, otherwise the low resistance would shunt the crystal. But i never
> did that experiment, would
> have taken more rewiring for my limited time. But - is this a workable
> idea, or is there a 'gotcha' -
> something i'm not accounting for?
>
> I did try the grid-to-plate gimmick capacitor to make the stage
> regenerative, but i didn't like that at
> all. Too much like a poor implementation of  regenerative receiver. You
> don't want the selectivity
> stage gain-controlled separately. In my experience a q-multiplier is much
> better. I did see some
> 'conversion' article probably in CQ Magazine that did build a Q-multiplier
> for the 2830 kHz IF.
> I am curious how that really worked out. f/Q is the 3db selectivity and
> you can see 2830/Q is going
> to be a LOT more than 455/Q. The crystal does have a very high Q and i
> thought in this low impedance
> point, the cathode circuit, the parallel capacitance of the crystal holder
> would be insignificant.
> Obviously i missed something. I am almost up for a rematch with this
> challenge.
>
> While i'm at it: another question to the wisdom of the members. The USAAF
> radio receiver BC-348,
> the last IF is overcoupled. I see all the other IF windings are 1.5 ohms
> DC with 250 pF resonating.
> The secondary of the last IF can is 5 ohms DC resonated with 125 pF. My
> thinking was to simply
> unwind turns from this winding and add capacitance to resonate. The
> winding is 2-pi and i still
> don't know if i'd peel off the near winding or the more distant winding -
> to lower the coupling. BUT -
> i had another idea: what if i just get another first or second IF can from
> a junker, and use that to
> replace this entire original overcoupled IF can? I know in selecting IF
> transformers, there always
> was one specified for 'diode load', but i have no idea what kind of
> impedance such a transformer
> has.
> Ideas - advice?
> tnx
> -Hue Miller
>
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