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

[Hubert Miller]

Thanks for the data point, Ian; this is something i need to keep in mind. I suddenly remembered an incident which i have wondered about for a long time,

which you may have helped explain. 
Many years ago, i was repairing – upgrading a BC-312 receiver – mostly improving the tube bias voltages on the RF and IF.

I decided i’d try to get a little more crystal filter selectivity. The filter load is just a resistor. I lowered the resistor by about half – but noticed NO selectivity

change whatsoever. I wondered why. But if the ESR of this 465 kHz crystal was rather high, then changing the load would have minimal effect. At least that’s

the best explanation i have seen in lo, these many years.

-Hue 

 

 

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 <mailto: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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