[time-nuts] Broken Ovenaire OSC 85-50
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Sat Jul 4 06:08:55 UTC 2009
Warren
My Spice model of the dc levels in the oscillator indicate a current
more like 4mA in the 510 ohm resistor.
Bruce
WarrenS wrote:
> I just saw your schematic of the rest of the buffer circuit. Sorry I missed it early.
>
> The Osc Vcc is shown driven from a 510 Ohm resistor, and with the total current thru the 510 res at under 2 ma,
> Looks like Osc Vcc should be at more like 4.0 volts instead of 2.49 volts.
> If I'm correct about that (this time) then, It has got to be one of three things.
> The 510 ohm is wrong, the 5 volts is wrong, or the load on the 510 ohm is wrong.
> It should be easy enough to measure which one it is.
> The voltage at the junction of the two 4.7K resistors are WRONG.
> Looks like it is caused by an excess current draw on the Grn lead.
> Check the direction and leakage of the voltage controlled cap diode and bypass.
> If all else is right, Look for the cause of the extra loading current on the 510 ohm,
>
> WS
>
> ****************
>
> ----- 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 9:55 PM
> Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50
>
>
>
>> 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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>>>
>>
>>
>>
>>
>
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