[time-nuts] Broken Ovenaire OSC 85-50
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sat Jul 4 04:55:31 UTC 2009
WarrenS wrote:
> A couple more thoughts;
>
> Is the VCC (Red wire) current draw correct?.
> Should be 1.3 ma (per the schematic values) with the 470 Ohm and
> more like 7 plus ma with the 47 ohm.
>
> AND I don't remember seeing, IS the Freq real close AND can it be tuned by the cap and Green wire voltage?
> If both answers are YES, at least most things are working correctly.
>
> As Bruce said:
>
>> "if the resistor values are correct this is the voltage(s) that one should get"
>>
> With the resistors values shown, I agree that all the other voltages are correct given the 2.49 volts,
> BUT the 2.49 volt is NOT controlled by any of the values on that schematic, it's just an input.
> AND the output amplitude is not yet being controlled by current cut off.
>
> Something that needs to be asked is are all these the correct values.
> It sounds like you got them by measuring and not from a "should be" schematic.
>
> One last, way out thought.
> It was my belief that very good osc have some sort of AGC to control the amplitude,
> ANY chance that the VCC is used as the input to control the amplitude of the Osc output?
> If so, could it be that the external AGC that is what broken and supplying too low a VCC voltage to the circuit?
>
> ws
>
>
The Colpitts crystal oscillator limits the crystal current by being
periodically cutoff for part of the cycle.
There is an optimum duty cycle (usually something like 25%) that
corresponds to minimum phase noise.
The oscillator dc collector current determines the crystal current and
hence the output amplitude.
It is possible to perform AGC by varying the oscillator Vcc but the
output amplifier schematic appears to have no circuitry for this.
Some circuits actually do this, but the oscillator transistor is still
cutoff for part of the oscillator cycle.
The oscillator VCC is fed from a tap in a resistive divider network
(located on the external (to the oven) buffer board) connected between
the 5V supply and ground.
Usually one just varies the emitter (or collector) dc current by
selecting a resistor value.
Bruce
> *****************
> ----- Original Message -----
> From: "Bruce Griffiths" <bruce.griffiths at xtra.co.nz>
> To: "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
> Sent: Friday, July 03, 2009 8:21 PM
> Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50
>
>
>
>> WarrenS wrote:
>>
>>> 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.
>>>
>>>
>> Not so, if the resistor values are correct this is the voltage that one
>> should get.
>> The question that remains: Is the voltage what the designer intended?
>> With the Colpitts oscillator used limiting in the oscillator transistor
>> (usually by current cutoff) is used to limit the amplitude.
>> Just increasing the voltage without checking the resultant crystal
>> current will be within permissable limits can be counter productive.
>>
>>
>>> 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.
>>>
>>> ws
>>>
>>>
>> Bruce
>>
>>> ********************
>>> ----- 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
>>>>
>>>> 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"
>>>>
>>>> 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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>>
>>
>>
>>
>
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