I've built crystal lattice filters (and others) in the past. I also have
books on designing them,
Basically you are all correcr !
You CAN pull them with capacitors.
It WILL change the input and output impedances, more if you pull them more.
Crystal Frequency is the design parameter you use to change the filter's
operation.
So, your milage may vary, but you can pull them with capacitors, and you
might be sorry you did, depending on how far off the impedance matches end
up being. Its better to change the crystals.
73 de Gary, AA2IZ
----- Original Message -----
From: <d.e.warnick@comcast.net>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Saturday, October 10, 2009 8:32 AM
Subject: Re: [TenTec] CW filter for Omni-VI+.... What model? 250Hz,9MHz...
LOW TONE CW!??
>
>
> Just knowing the value of the capacitors is not enough. That would assume
> that identical crystals were used in both the high-tone and the low-tone
> filters. We cannot make that assumption. Therefore, the individual
> comparing the filters would have to measure the crystals used for center
> frequency at least. At that point, if the crystals were identical, we
> could continue the process of measuring the capacitors necessary to pull
> their center frequency. As Jerry's e-mail stated, that is not the purpose
> of the capacitors in this filter design.
>
>
>
> Dave
>
> WA3MKB
>
>
> ----- Original Message -----
> From: "Mike Hyder --N4NT--" <Mike_N4NT@embarqmail.com>
> To: geraldj@weather.net, "Discussion of Ten-Tec Equipment"
> <tentec@contesting.com>
> Sent: Saturday, October 10, 2009 7:32:20 AM GMT -05:00 US/Canada Eastern
> Subject: Re: [TenTec] CW filter for Omni-VI+.... What model? 250Hz,
> 9MHz... LOW TONE CW!??
>
> I divided 250 Hz by 9 million Hz and got 0.00002778. I suppose I must then
> move the decimal two places to the right, or 0.002778% to get the
> percentage
> of change needed to move a 9 MHz crystal 250 Hz. I don't vouch for the
> math
> and can't get my head around numbers like that but am sure you can. In my
> opinion, Ten-Tec would have to pay dearly for such accuracy in the
> crystals
> and might well move their frequency a quarter kHz with capacitors.
>
> My statement was based upon something I heard from Ten-Tec or general lore
> and it was based on reality rather than theory. As I dimly recall, I was
> discussing the filters and wishing I had the 500 Hz centered one. The
> other
> fellow said that the only difference between them was the capacitors. I
> should have asked further and taken notes, huh? Perhaps we can get a
> willing soul to examine his 500 Hz centered filter and report the
> capacitor
> values so we can compare them to a 750 Hz centered filter. Perhaps we can
> get a kind soul at Ten-Tec to give us the specifications for the filters.
> Either way might help us better understand what Ten-Tec did and why they
> did
> it.
>
> Mike
>
> ----- Original Message -----
> From: "Dr. Gerald N. Johnson" <geraldj@weather.net>
> To: <tentec@contesting.com>
> Sent: Friday, October 09, 2009 03:29 PM
> Subject: Re: [TenTec] CW filter for Omni-VI+.... What model? 250Hz,9MHz...
> LOW TONE CW!??
>
>
>> On Fri, 2009-10-09 at 15:14 -0400, Mike Hyder --N4NT-- wrote:
>>> Somewhere along the line I got the notion that the difference between
>>> the
>>> filters centered at 750 Hz and centered at 500 Hz was only the
>>> capacitors
>>> on
>>> the filter board.
>>>
>>> Did I make this up or can anyone shed light on it?
>>>
>>> Mike N4NT
>>>
>> More properly its the frequency of the crystals that set the center
>> frequency, the capacitors set the coupling between crystals, and the
>> bandpass. Dragging the crystals with different capacitors might work but
>> would change the filter terminal impedance that would probably cause
>> increased insertion loss. 250 Hz shift on a 9 MHz filter wouldn't
>> require a change in capacitors at all, just the crystals.
>>
>> 73, Jerry, K0CQ
>
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