Hi, The observed failure is not due to a problem with the oven control. If the frequency is stable within a few ppm (after warm-up), then the crystal will not be defective too. As mentioned before, this frequency _is_ the resonance frequency of the so-called B-mode of the SC-cut crystal used:. The frequency of this mode is strongly temperature sensitive (that is why it is sometimes used intentionally for temperature sensing. To force the oscillator to operate on teh desired 3rd overtone C-Mode at 10.000 MHz, a trap consiting of C5, C6, L2 and L3 is used between base and emitter. C5 acts as a DC blocking capacitor only, so the trap is basically a series resonant circuit C6, L3 paralled by an additional coil L2. This trap has therefore a parallel resonance given by C6 and (L2+L3) and a series resonance given by L3, C6. The design is such that the parallel resonance lies between the (undesired) fundamental mode and 10.000 MHz, while the series resonance lies between 10.000 MHz and the B-Mode frequency 10.887 MHz. In this way the trap beahves like a capacitor only in the narrow band in the vicinity of 10.000 MHz, while its reactance is inductive at the fundamental mode and at athe B-Mode of the crystal. Only when it is capacititve, the colpitts oscillator shows a negative input resistance to the crystal, i.e. allows the crystal to oscillate. The position of the series resonance is rather critical, ecause the gap between C- and B-mode is rather narrow. Therefore the Q of L3 should be rather high. In your case the series resonance condition may not be kept, either because L3 or C6 may be defective or because its value may have "aged", or due to a loose solder joint. Wish you good success Bernd DK1AG -------- Original-Nachricht -------- Betreff: [time-nuts] HP10811 problem - wrong frequency Datum: Sun, 13 Jan 2008 19:37:48 +0100 Von: [EMAIL PROTECTED] <[EMAIL PROTECTED]> Antwort an: Discussion of precise time and frequency measurement <time-nuts@febo.com> An: time-nuts@febo.com Hello all together, i have a problem with my HP10811 ocxo. I connected it properly to 24V, 12V and to my counter to measure its frequency. The measured frequency is 10.887 MHz, but it should be 10MHz (also noted on the 10811). A 5MHz tcxo is measured correctly, so i think the counter is okay. Can anyone help me ? Thanks a lot... Best regards [EMAIL PROTECTED] _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.