[time-nuts] Broken Ovenaire OSC 85-50

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jul 3 23:25:02 UTC 2009


Mike Monett wrote:
>   (This is  a repost to see if I can figure out why  the  original was
>   scrubbed. Sorry for the duplicate.)  
>
>   > An update on the Broken Ovenaire OSC 85-50.
>
>   > I prepared  a 'schematic' of the Output Board  and  the Oscillator
>   > Board (attached) and I have lots of pictures of the  external unit
>   > and the insides if anyone is interested.
>
>   > I resoldered  all connections and replaced all transistors  on the
>   > Output Board  and  the  Oscillator  Board  all  to  no  benefit. I
>   > measured all  the components with an LCR meter and found  the 0.01
>   > uF bypass  on the 330 ohm resistor in the emitter  circuit  of the
>   > output transistor  of the Output Board to be low and  with  a high
>   > ESR. I replaced this with about a 20% increase in output amplitude
>   > but still  inadequate. I replaced the rest of the 0.01 uF  caps on
>   > the output  board  with   no   additional  benefit.  I transiently
>   > disconnected the  Red  wires from the  Oven  Controller  board and
>   > there was no increase in output or significant increase in voltage
>   > to the Oscillator Board.
>
>   > Therefore, it  appeared  that a 'low output  crystal'  (if  such a
>   > thing exists)  was the only logical explanation that I  could come
>   > up with. That seeming to be the case, there appeared to be  only 4
>   > options. 1. Toss the OCXO (sorry, too much effort so far). 2.
>
>   > Build an  external   amplifier   (seemingly   too  much additional
>   > effort). 3. Try to adjust on the bias of the oscillator transistor
>   > to achieve  a higher output (seemed too 'iffy'). Or  4.  Lower the
>   > value of  the  resistor in the emitter circuit  of  the Oscillator
>   > Board to  get  more gain out of the last stage  in  the Oscillator
>   > Board.
>
>   > I replaced  the  470 ohm resistor with a 47 ohm  resistor  and the
>   > amplitude increased to about 0.4 V P-P into a 50 ohm load  and was
>   > sufficient to make it a usable OCXO again.
>
>   > I reassembled, resealed with Epoxy and all seems well so far.
>
>   > If anyone wants pictures or other info, please let me know.
>
>   > Thanks for all the suggestions and help.
>
>   > Joe
>
>   Joe,
>
>   Congratulations on getting your system to work!
>
>   A couple  of  things. First, trying to measure the  currents  in the
>   circuit with a ferrite toroid won't do you much good. You don't know
>   what the  currents  should  be,  and  the  secondary  of  the toroid
>   transformer requires a termination resistor. The value  changes with
>   the turns ratio.
>
>   Just from looking at the circuit, the RF currents will  be extremely
>   low. This  requires a large number of turns on the  secondary, which
>   will probably resonate at or below the 10MHz operating frequency due
>   to stray  capacitance  from the connection to the  scope.  So  it is
>   unlikely you will get any useful progress in this direction.
>
>   However, from the values on your schematic, the output  tank circuit
>   resonates at  9.602MHz with a Q of 9.6. So the tank is  already well
>   below resonance, which attenuates the output voltage.
>
>   Any stray capacitance you add to the circuit will bring the resonant
>   frequency lower, further aggravating the loss in signal.
>
>   The output  tank  is tapped with the 75pF and 91pF  in  series. This
>   further attenuates the signal.
>
>   I'd change the circuit to a single capacitor across the tank  with a
>   small trim capacitor to tune it to resonance.
>
>   To get  the signal into 50 ohms for distribution, I'd add  a limiter
>   if you can tolerate a square wave output, or a good emitter follower
>   if you  need a sine wave. Take the output from the collector  of the
>   2N2369 to get the maximum signal amplitude.
>   

Blindly adding a wide bandwidth limiter will degrade the phase noise if
the input signal slew rate is too low.
In such cases ts better to use a cascade of limiters each with an output
filter and a well defined gain at the zero crossing.
The output filter and the gain (at zero crossing) of each stage is
selected to minimse the jitter at the output.


Bruce





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