----- Original Message -----
From: "Nick Kennedy" <
nick-wa5bdu@ suddenlink. net>
To: <
qrp-l@mailman. qth.net>
Sent: Monday, December 18, 2006 8:03 AM
Subject: [QRP-L] THE ARRL KILLED MY DOG!!!
> Actually, that's not true. But I needed an attention-getting subject
> line to compete with all the other stuff going on. My real subject is:
>
> Cast Adrift: VFO stability or lack thereof
>
>
> I'm piddling with the PotLuck project to see if I can actually build
> something by late May. I was thinking of a sort of generic 20M CW
> transceiver. But I'm a little wary since with a 4.915MHz IF, the VFO
> needs to be up over 9MHz. Will I have drift problems?
>
> First, what's my criterion for acceptable drift? Well, I'm not after
> state of the art; I figure if the guy on the other end doesn't notice
> I'm drifting, then for practical purposed, I'm not drifting. So maybe
> less than 100 Hz max in a 5 minute transmission, just sort of a rough
> number.
>
> Decided to try a basic Colpitts circuit from SSDRA, but with a varicap
> added for tuning and also base biasing, which oddly, was not shown in
> the book. The device is a 2N3904 and I built it Manhattan style. Caps
> are NPO except some kind of poly-whatever for bypassing Vcc. The
> inductor in the tank is a dash-7 (white) iron powder toroid.
>
> On initial testing, it drifted like crazy. No need to measure, because
> we're talking kilocycles in a short time. Part of the issue is that the
> output comes off the emitter, which is also where the critical voltage
> divider connects. So just the cable to the counter added lots of drifty
> C. Monitoring in the receiver is the best way to view drift. I do
> realize I'll need some serious buffering in the final version. But it
> still drifts too much with no load physically connected.
>
> OK, first round of bright ideas. I'm using a double sided board. Maybe
> the capacitance to the bottom is having some effect. So I connected the
> top and bottom foils together. Also, there are lots of sneaky little
> warm and cold air currents in the house in December. So I put the thing
> in a candy tin with tight fitting lid. I wrapped the board in bubble
> wrap for insulation and brought the supply leads (regulated 8VDC) in
> through a small hole. Wow! Now the drift is much worse. Just
> approaching the can with my hand makes it go nuts. So as a slight
> revision, I solder the common lead to the can at the point of entry.
> Now the drift is merely horrible. Also disconnected the varicap. And
> the trimmer next to the inductor replaced with a fixed NPO. No help.
>
> Maybe one of the caps is suspect. So I just replace the whole lot--the
> two voltage dividers, the blocking/coupling cap from tank to base, the
> tank range-setting cap, the output cap, the blocking cap from the
> varicap--all from a different batch. Still drifts too much. Some has
> to be blamed on all the soldering going on, but even after an hour or
> two warm-up and stabilization, it's no good.
>
> Next bright idea is the Manhattan pads. I generally use them about 3/16
> in diameter or so, but at the base I used a rectangular chunk about 1/4
> by 3/8 inch due to all the connections. I had measured earlier that a
> 1/4 inch diameter pad would have C of about 1 pF. Not a lot in general
> terms, but also not very temperature stable. I had built up a
> spreadsheet that simplifies all the series and parallel C in my circuit
> to the equivalent in parallel with the inductor and finally figures the
> resonant frequency with the tank coil. So how much difference would say
> 0.1pF make at the base? Turns out it was over a kilocycle. So maybe
> all these pads, especially the big one, are causing problems. I got
> some neat little gold terminals on Teflon base from Dan's, so I busted
> off the base pad, drilled a hole and epoxied in a standoff terminal to
> replace it. Back to testing, drift may have improved, but it's still
> way too much. Maybe 500 Hz in 10 minutes or so. But I still have some
> more pads in critical spots.
>
> That's where I am now. I'm waiting for the glue to dry on a terminal
> replacing the pad at the emitter / voltage divider junction. I also got
> an electrical 4-inch handy-box to mount the thing in, for magnetic
> shielding and a bit of thermal isolation. This replaces the candy tin.
>
> Any suggestions? It's always something I've left out, although I've
> tried to tell all. Supply is 8VDC regulated. Biasing is for about 4mA.
> Biasing resistors are 11k and 15k, emitter resistor is 1000 ohms.
>
> 72--Nick, WA5BDU
>
> No dogs were harmed in the composing of this post.
>
>
>
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