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NorCal40 -> K1 rx xtal filter evolution

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Mike Markowski-2

NorCal40 -> K1 rx xtal filter evolution

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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wayne burdick

Re: NorCal40 -> K1 rx xtal filter evolution

Administrator

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 <[hidden email]> 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
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
> Message delivered to [hidden email]

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Mike Markowski-2

Re: NorCal40 -> K1 rx xtal filter evolution

Thanks, Wayne. If not too much trouble that would be great. Or even a
run down of how you got the values. Maybe I'm just looking in the wrong
place. And I guess the ripple doesn't matter too much for a cw-only qrp
rig?

It would be very interesting to read a technical "why we changed this"
from rig to rig over time. But I guess that leads to problems with
tipping off the competition and being a bit of a time sink, too!

73,
Mike ab3ap

On 4/17/20 4:15 PM, Wayne Burdick wrote:

> 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 <[hidden email]> 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
>> ______________________________________________________________
>> Elecraft mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/elecraft
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:[hidden email]
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>> Message delivered to [hidden email]
>