[time-nuts] Broken Ovenaire OSC 85-50
bruce.griffiths at xtra.co.nz
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
> 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
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.
> 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.
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