EMRFD Message Archive 15371
Message Date From Subject 15371 2019-08-14 11:54:04 Leinweber, Glen crystal oscillator tempco Pulled out the standard circuit G3UUR crystal-characterizing oscillator tomeasure some old 10MHz HC-49 pulled from very old hard drive interfaces.
I had built G3UUR on a small PCB, 2N3904's. Used two 470pf polystyrenecapacitors between base-emitter, and emitter-gnd.Stuck in my crystal, and left it on overnight. Seemed that frequency wasremarkably stable while outside temps dropped from 29C to 19C.
Curious, seemed far more stable than a cheap canned Xtal osc.
Those polystyrenes supposedly have about -150ppm/C tempco. Couldthis G3UUR be fortuitously temperature-compensated or nearly so?
So made a crude oven (far more crude than one built by Wes in the fogof times past). A tin can wrapped in foam packing material except forits bottom end, in a pot of water hot-to-cold. An old mercury thermometerto measure temperature.Starting with hot water, took it down from 45C to 10C, probably too fast.Through that range, frequency varied 10 Hertz (nominal 9997228).One turnover temp was found at 35C. Another was approx. 13C.That's one stable oscillator: could use more negative tempco.
Checked a large parts supplier. No polystyrenes. Any caps with negativetempco? Only 50kV and $$$. All others marked "obsolete", "no stock". Sigh.Harbour any you can find.
15372 2019-08-14 13:42:25 Jim Kortge Re: crystal oscillator tempco
Glenn and others,
15376 2019-08-24 08:41:02 w0gk Re: crystal oscillator tempco Re: NTC caps, I've bought polystyrene caps from West Florida Components.Digikey has some NTC ceramics in useful values. Use their component search feature to find them. You may need to figure out the EIA letter code equivalents for N150, N220, etc.Also, Dan's used to have some NTC ceramic trimmers. These are useful for dialing in temperature compensation. Don't know if they are still available. As I recall, they weren't advertised as NTC, but were marked "N300" and seem to behave as NTC.The problem I've run into with polystyrene caps is that the available values are often too big to be useful for temperature compensation. Generally, values less than than 100pF are what I've needed.You mentioned Wes, W7ZOI. His article in QST a number of years ago is a great resource for tweaking oscillator temperature compensation in an organized, orderly manner. From your mention, I assume you've seen that article, if not it's worth tracking down.Randy,W0GK