VXOs, Litz wire and pipe dreams

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From: Robert J. Gobrick (rgobrick@nfld.com)
Date: Tue Mar 12 1996 - 19:45:39 EST


QRP-L Gang,

Joe N2CX sets the example of what a modest QRPer is all about. What Joe
failed to tell you all is that Joe is the World Champion crystal bender as
proven by his spectacular win at last year's New England QRP Club Crystal
Bending Shootout at the ARRL Headquarters - why it's reported that he moved
that colour burst frequency on 80 meters all the way up to the Digital TV
frequencies.

So let's see if someone can actually refute what Joe has to say - I COULD
NEVER bend a crystal that much and I used to do tricks like die and tool
tapping a threaded screw into the side of a FT-243 crystal holder to have a
"real" variable VXO circuit..

Cheers 73/72 Bob VO1DRB/WA6ERB
 

>Folks,
>
>There has been some discussion lately about a VXO article appearing in the
>latest 73 magazine. Unfortunately I've deleted the messages , but I want o
>comment on what I remember of them.
>
>I've done some work with VXOs as you may remember from last year, when I told
>of a VXO contest at the NEQRP meeting. Using some ideas from Ha-Jo Brandt,
>DJ1??, it was easy to obtain a tuning range of about 15 kHz with a 3.579 MHz
>color burst crystal. Wider swings were possible, but the circuit was no
>longer under control of the crystal. Simply put, it was a VFO.
>
>Now the 73 article extolls the virtues of honeycomb coils and Litz wire to
>extend the tuning range. I agree with the honeycomb coils, since they give
>inductors with low distributed capacitance which is what Ha-Jo found he
>needed. But the use of Litz wire, I suspect has no meaning.
>
>I don't mean to be too critical of the magazine article's author, but I
>suspect that his results were the result of what I'll politely call "empirical
>design" (That means you just fiddle around with something until you get it to
>work). as opposed to a thourough desing effort. I agree with Kent Torrell
>when he says that you really need to mesure the crystal parameters and model
>the circuit to say with certainty what's happening. Don't get me wrong, we
>all eventually get to the empirical optimization stage, but usually with a
>sound starting position.
>
>On the other hand, I could be wrong. If someone can show me a reproducible
>vxo with the kind of tuning range mentioned that retains crystal stability,
>I'll be a beleiver. However I suspect that the article is another typical of
>73 these days that are more the result of strainght empiricism and hope, than
>careful experimentation and measurement.
>
>Maybe I'll try conversing with the author, and see what the's all about. No
>flames intended, I'm just trying to put on my engineer's hat (No, not the
>railroad kind!) and give a rational perspective.
>
>Comments?
>
>72/73,
>
>Joe E., N2CX
 ____________________________________________________________________
| Bob Gobrick - VO1DRB/WA6ERB/VE2DRB - Newfoundland, Canada |
| QRPer Galore - ARCI, GQRP, NORCAL, NEQRP, COQRP, MIQRP, NWQRP |
| Internet: rgobrick@nfld.com |
| bgobrick@nlnet.nf.ca |
| Compuserve: 70466.1405@compuserve.com |
|____________________________________________________________________|


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