[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
More information about the Time-nuts_lists.febo.com
mailing list