[time-nuts] FW: [Fwd: [Fwd: HP10811 problem - wrong frequency]]

[BNeubig at t-online.de]

   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: philipp80 at gmx.de <philipp80 at gmx.de>
   Antwort an: Discussion of precise time and frequency measurement
   <time-nuts at febo.com>
   An: time-nuts at 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
   philipp80 at gmx.de
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