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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
