[time-nuts] Broken Ovenaire OSC 85-50
J. L. Trantham
jltran at worldnet.att.net
Sat Jul 4 02:07:48 UTC 2009
I did not realize that this post would generate this much discussion.
While I have a BEE and MSEE, I am a practicing Cardiac Electrophysiologist
and it has been some time since I thought about such issues. I have learned
a lot though from the discussions and as medicine as we know it winds down,
interests in EE will grow.
I agree with Warren. It should be fixed before modified. However, I could
not find anything that was 'broken' with the possible exception of the XTAL.
As I tried various things, I did notice from time to time some reluctance to
oscillate suggesting that the oscillator stage was an issue. With no
obvious failure and no hope of finding a replacement XTAL, I had to choose
The real problem is size. There is no room for added components, at least
on the Oscillator Board which is inside a sealed oven. There is only room
for changing components. I would have liked to try adding a resistor and
capacitor to the emitter circuit of the amplifier transistor on the
Oscillator Board but there is no room. I have pictures if anyone is
interested. The Oscillator Board is inside a metal can about 0.5x1.5x2.0 cm
that has a heater coil wrapped around the outside and an Oven Controller
Board attached to the outside. This package is surrounded by foam with the
Output Board on one face of the foam, mounted to one wall of the outside
metal package. The bottom of the metal package has 2 feed thru's, plus a
ground connection. One is 5 VDC and the other is the 10MHz out.
While with better equipment one might be able to find a problem, it was
certainly not obvious to me.
Do XTAL's fail often? If so, by what mechanism? Slowly fade away? Sudden
death? I am not experienced enough to know this answer but in all my years
with various HAM gear, I do not recall the failure of a single XTAL.
However, none of these were in an oven.
Thanks again for all the useful information.
None the less, with a simple change of resistor, the gain in the amplifier
stage increased and, at least so far, problem solved.
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of WarrenS
Sent: Friday, July 03, 2009 7:49 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50
Here is my two cents worth
20 mv output, sure sounds like something is broken.
It should be fixed before it is modified.
The 2.49 volts on the Red input voltage seem LOW, More Vcc maybe. The "Grn"
labeled wire, FreqCtrl input should be about 1/2 VCC for testing. If you
do 'need' to modify the gain,
It would seem better to bypass the 470 ohm resistor with a cap in series
with the 47 ohms.
----- Original Message -----
From: "Mike Monett" <xde-l2g3 at myamail.com>
To: <time-nuts at febo.com>
Sent: Friday, July 03, 2009 11:50 AM
Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50
> > 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.
> 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.
> 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.
> 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"
> 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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