Hi Mike,
I have noticed that most, if not all, of the crystals in Ten-Tec filters
have colored markings on them that appear to have been made with a felt
tipped pen. My theory is that someone at the facility where the filters
are assembled (maybe not at Ten-Tec) was tasked with testing each
crystal, probably in a special test fixture oscillator to determine
their exact resonant frequency. The crystals were then marked with
colors which represented the last few significant figures indicated on
the frequency counter. Then the right crystals could be picked for
building each filter. By buying crystals in big batches, and then
sorting them in to exact frequencies, they could get accuracy in
hundreds of Hertz without paying the crystal manufacturer for that kind
of accuracy. Instead you pay labor for the guy who sits at the bench
with the pile of crystals, test oscillator and frequency counter,
testing crystal after crystal.
This is all supposition on my part. I wonder if there is someone in the
crystal filter business who can say whether my guess is reasonable.
Ken N6KB
Mike Hyder --N4NT-- wrote:
> 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
>>
>
> _______________________________________________
> TenTec mailing list
> TenTec@contesting.com
> http://lists.contesting.com/mailman/listinfo/tentec
>
>
_______________________________________________
TenTec mailing list
TenTec@contesting.com
http://lists.contesting.com/mailman/listinfo/tentec
|