[Elecraft] NorCal40 -> K1 rx xtal filter evolution

[Wayne Burdick]

Hi Mike,

I can probably scrounge up the original Lm and other specs for you.

Varactor capacitance vs. voltage for the parts I used in the design is readily available on the web. Just search for the part datasheet.

Varactor tuning of the filter is great for varying the bandwidth, but it is a compromise in terms of loss and ripple.

73,
Wayne
N6KR


> On Apr 17, 2020, at 12:55 PM, Mike Markowski <mike.ab3ap at gmail.com> wrote:
> 
> For fun, I thought I'd make an archeological dig and take a look at how things changed from NorCal 40a to K1 since they're easy to study.  Then I realized part of it is not so easy for me.  The 40a uses a Cohn filter and I thought I'd run through analysis (not simulation) of it.  Again, just for the exercise of it while cooped up during these viral stay-at-home days.
> 
> However, the data sheets I downloaded for the HC-49 crystal don't seem to have the motional capacitance and inductance.  So, two questions arise: (1) I thought these were provided by crystal makers, the experts. Are they expected to be measured or is there some simplification in network analysis that doesn't require them?, and (2) are there Cohn filter tables like there are for Butterworth and Chebyshev?  I can't find anything on analysis after a fair bit of googling.
> 
> Moving on to the K1, receive filter bandwidth is controlled by changing capacitance via varactor diode voltage.  But that comes from the DAC and requires probing to determine voltage that in turn sets capacitance for the filter.  I can't find much on varactor controlled Cohn filters.  So (3), is there a place to read up more on it?
> 
> Tnx es 73!
> Mike ab3ap, suffering cabin fever
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