10133	2014-07-11 15:39:30	k7tfc	Series capacitance G3UUR Test Oscillator
Message	Date	From	Subject
Folks, I've just come into about 100lbs of FT-243 xtals, mostly military surplus, and many of which date to 1943 and 1944. I want to test these as I sort through them for open or shorted condition, and also their frequencies +/- 1KHz. After more than seventy years, the xtals are not likely to be at their marked frequencies. So, I'm building the G3UUR test oscillator as shown in Figs. 3.35 and 7.69. The design calls out a cap in series with the XUT (xtal-under-test) to ground. The figure of 33pF is given, plus the capacitance of a switch that bypasses the cap to ground. The difference in measured frequency between switch-open and switch-closed is used in calculating the motional parameters of the XUT. For the present, I'm not interested in the motional parameters per se, but rather just the xtal's resonant frequency. Some of these xtals are marked with expected capacitance load--20 or 30pF seems typical--but as designed I'll be measuring them with ~ 40pF in the G3UUR oscillator. Should I add a switch to select the marked values (and perhaps others) or am I just gilding the lily? As described in the book, it's the difference between the cap in-or-out frequencies that is significant, so the real resonance point may not matter. For my immediate purpose, it does matter. What do you think? Thanks and 73, Todd K7TFC
10134	2014-07-11 16:12:27	billw77aaz	Re: Series capacitance G3UUR Test Oscillator
The marked frequency of military xtals is for the sets they were made for. Some had series resonant oscillators, most parallel resonant. Series-marked xtals in G3URR with 32 pF will produce a KHz or two frequency shift from the marked frequency. Some rigs had frequency multiplication in them and the marked frequency was the output frequency, not the frequency of the crystal (FT241 for example). What matters is the frequency they operate at in "your" circuit. Unless VHF overtone or a VXO, probably a 20-32 pF load. The G3UUR load is OK, in my opinion, for sorting through a huge batch fear dead/alive, or even matching into sets of similar/same frequency. W7AAZ > Folks, > > > I've just come into about 100lbs of FT-243 xtals, mostly military > surplus, and many of which date to 1943 and 1944. I want to test these as > I sort through them for open or shorted condition, and also their > frequencies +/- 1KHz. After more than seventy years, the xtals are not > likely to be at their marked frequencies. > > > So, I'm building the G3UUR test oscillator as shown in Figs. 3.35 and > 7.69. The design calls out a cap in series with the XUT (xtal-under-test) > to ground. The figure of 33pF is given, plus the capacitance of a switch > that bypasses the cap to ground. The difference in measured frequency > between switch-open and switch-closed is used in calculating the motional > parameters of the XUT. > > > For the present, I'm not interested in the motional parameters per se, > but rather just the xtal's resonant frequency. Some of these xtals are > marked with expected capacitance load--20 or 30pF seems typical--but as > designed I'll be measuring them with ~ 40pF in the G3UUR oscillator. > Should I add a switch to select the marked values (and perhaps others) or > am I just gilding the lily? > > > As described in the book, it's the difference between the cap in-or-out > frequencies that is significant, so the real resonance point may not > matter. For my immediate purpose, it does matter. What do you think? > > > Thanks and 73, > > > Todd K7TFC > >
10135	2014-07-12 00:02:24	Todd F. Carney / ...	Re: Series capacitance G3UUR Test Oscillator

10136	2014-07-12 06:04:07	billw77aaz	Re: Series capacitance G3UUR Test Oscillator
By making the series capacitance variable and calibrated would make it more useful if you need to know the absolute accuracy of a crystals calibration. Being a small value with strays the calibration would be tricky. I once spent a Christmas vacation, stuck at Grandma's house, measuring a box of about 1000 4 MHz HC6 crystals I'd bought at a swapmeet. W7AAZ
10137	2014-07-12 06:29:08	Jim Strohm	Re: Series capacitance G3UUR Test Oscillator
Instead of switched capacitors, put in a variable cap up to maybe 50 or 60 pF, and loosely calibrate the dial scale. A clever builder would add a shorting bar so that there's only on control. ;) You probably want to use a cap with ball bearings, if available. 73 Jim N6OTQ
10139	2014-07-12 11:44:42	dave081360	Re: Series capacitance G3UUR Test Oscillator
Hi Todd, Have you checked out what has been going on with the PHSNA Yahoo group? There has been a lot of discussion about testing crystals with the PHSNA. In fact, they appear to be looking for lots of crystals for testing. If you are interested, I could send you some pictures of plots made by a NAT (Network Analyzer Terminal) interfaced to a PHSNA (Poor Ham's Scaler Network Analyzer) and some sample data recorded. All are in a single Excel spreadsheet but I don't see a way to attache it to my response in this group. If you are interested, send me an email direct and I'll send it to you. My email address is: dave_at_dnbrealty.com (disguised in case Yahoo doesn't want email addresses) 73, Dave Collins - AD7JT
10140	2014-07-12 12:00:14	Todd F. Carney / ...	Re: Series capacitance G3UUR Test Oscillator

10141	2014-07-12 12:21:35	Todd F. Carney / ...	Re: Series capacitance G3UUR Test Oscillator

10143	2014-07-13 09:32:38	billw77aaz	Re: Series capacitance G3UUR Test Oscillator
I think we've all been had by the "I can't test this but it sure looks nice" eBay con artists. Bill > â >