[time-nuts] Broken Ovenaire OSC 85-50

Fri Jul 3 22:58:17 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.
>   
Uncalibrated speculation isnt helpful.
Estimates of the actual current would be more helpful than mere hand waving.
Tektronix current probes don't seem to suffer from such limitations.
If the current is very low then a low noise preamp is also necessary.
>   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.
>
>   
This is usually a bad idea.
Unless the circuit components have been altered, the designer intended
that the collector load be capacitive.
Using a resonant circuit tuned to resonance at the crystal frequency as
a load inevitably degrades the amplifier phase shift tempco and the
phase noise.
A detuned tank avoids the dc voltage drop and the flicker phase noise
associated with just using a collector resistor as a load.
The capacitively tapped circuit increases the current in the load.
A common base amplifier could be used with some advantage in the output
buffer but there are better circuits.
>   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.
>
>   
Emitter followers are not usually a good idea as they are somewhat
intolerant of short circuits (accidents do happen) and capacitive loading.
There are single transistor circuits with better reverse isolation than
an emitter follower.
>   Your original  post  mentions an output amplitude  of  20mV.  If the
>   normal amplitude is around 2V, this represents a loss of  40dB. This
>   is a huge loss in signal. The circuit obviously worked at  one time,
>   so there may well be some other hidden problem.
>
>   It is  possible the crystal is damaged, but this  seems  unlikely. A
>   crystal oscillator probably won't even start if the signal  level is
>   down 40dB.
>
>   You can  check  the oscillator and crystal in  SPICE.  Normally, the
>   high Q  of  the crystal will make the analysis very  slow.  It could
>   take many  hours  for   the   simulation  to  begin  oscillating and
>   stabilize at the final amplitude. The transient analysis  requires a
>   very fine  time step for accuracy, and you could run  out  of memory
>   before the simulation was complete.
>
>   
Not so (although some Spice variants may still suffer from this problem)
this may once have been true with a slow PC.
It depends on the actual oscillator circuit some circuits start faster
if one sets up a suitable initial condition such as an initial current
in the inductor in the crystal equivalent circuit but you have to get
the current right. With some oscillator circuits doing this can slow the
simulated oscillator startup.
>   I have developed a much faster way of analyzing a crystal oscillator
>   in SPICE.  Instead  of requiring tens or  hundreds  of  thousands of
>   simulated cycles,  this method gives accurate results in only  a few
>   dozen cycles.  For more information, please see  "SPICE  Analysis of
>   Crystal Oscillators"
>   
This isn't new its been around for decades.
>   http://pstca.com/spice/xtal/clapp.htm
>
>   You can  estimate the value of the crystal ESR by finding  the  Q of
>   your crystal and working backwards.
>
>   I'm attaching a gif of your schematic for reference. This is rotated
>   90 degrees and enhanced in LView Pro to improve the contrast.
>
>   Please let me know if you have any questions.
>
>   Thanks,
>
>   Mike
>
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>   
Bruce