Oops I think its about 300ppm there... D.
-----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of David C. Partridge Sent: 07 September 2008 14:56 To: 'Discussion of precise time and frequency measurement' Subject: [time-nuts] Ancient OCXO in scope calibrator. I'm having some problems with an ancient Bulova OCXO in a Tektronix 184 'scope calibrator. This is an crystal mounted on an octal valve base with a heater winding wrapped round it, and a common or garden variety thermostat which is supposed to switch at 75 celsius. I've not measured the switching temperature, as I don't have the means but the outer case does get fairly warm (40 plus degrees?). The oven is turning on and off. It is used in a circuit with a 7587 Nuvistor tetrode (yes it's a valve/tube circuit). The crystal is connected cathode to G1 with a trimmer capacitor of 3-12pF. The signal at the cathode is supposed to be about 70V p-p. I'm measuring the frequency of the signal after the transformer stage which couples it to the first stage of a countdown board. For the first minute or so after turn on from cold, it sits below 10MHz and is fairly stable and climbing as the oven warms up, and I can adjust the frequency up towards 10MHz with the capacitor (but not all the way), then suddenly, at the stage where it is starting to look as if it will soon stabilise at about the right frequency, it jumps to way over 10MHz and the lowest I can get it down to with the capacitor is about 10.0003xxx MHz where xxx is not very stable at all - in fact it can vary up to to 10.0005xxx and down to 10.0002xxx. I've tried freeze spray on most of the components round there to no effect. If I try to probe the signal at the cathode of the nuvistor even with a high impedance active probe with a P6201 with a 100x attenuator (about 1pF loading IIRC) the oscillation just drops dead. Now for calibrating 'scopes, it doesn't need to be any more accurate than it is (30ppm) - but ... Do any of the collected mavens have an explanation for the behaviour, and recommendations for fixing things? Cheers Dave _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
