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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