[time-nuts] Broken Ovenaire OSC 85-50
WarrenS
warrensjmail-one at yahoo.com
Sat Jul 4 06:37:04 UTC 2009
Bruce
4ma? Interesting, Maybe IF the Osc was running, BUT
My DC calculations of the present values I can read on Schematics come to 1.3ma + 0.26 = 1.58 ma load on the 510 ohm
With it NOT osc, which is almost the present case,
the Only Dc current drawn is.
1.75 V on 20K = 0.08ma (Q1 bias)
1.04 V on 2.2K = 0.47ma (Q1 collector)
0.365V on 470 ohm. = 0.77 ma (Q2 emitter)
Osc board =1.32 ma
Buffer board
(2*4.7k) at 2.49 V = 0.26 ma
Also note that the 4.7 K ohm pot and 4.7K divider voltages show excess current draw somewhere, maybe on the Grn lead bypass cap..
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 11:08 PM
Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50
> 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
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>> _______________________________________________
>>>>>> time-nuts mailing list -- time-nuts at febo.com
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>>>>>> and follow the instructions there.
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>>
>>>> _______________________________________________
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>>>>
>>>>
>>>>
>>>
>>>
>>>
>>>
>>
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