[time-nuts] Broken Ovenaire OSC 85-50
WarrenS
warrensjmail-one at yahoo.com
Sat Jul 4 07:24:08 UTC 2009
I get a 'if working correct' Vcc of about 3.5V
If the Vcc is 2.49 volts, I calculate a 3ma unaccounted for current draw from something.
So I have same conclusion, one or more of the following is wrong.
510 Ohm too high, 5 volts too low, un-shown current draw thru Leakey caps or extra circuits or bad data.
(plus the pot is (and should be) more likely a 10 K 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 11:58 PM
Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50
> Warren
>
> The Vcc dc current doesn't change much if the circuit is oscillating (at
> least it shouldn't if the circuit operating correctly - the active
> devices aren't supposed to saturate as thats adds too much phase noise).
>
> My simulations were over the 50-100C rang although temperature doesn't
> affect the currents and voltages too much.
> They also don't include loading of the frequency control pot wiper.
>
> There is no major inconsistency other than the effect of the
> unanticipated loading of the EFC pot wiper.
> The simulated oscillator Vcc is actually about 3V when everything is
> working properly.
> NB its important to use models that accurately reflect the correct
> transistor junction area just blindly tossing in 2N3904's increases the
> current somewhat as the Vbe is lower for a given current than the actual
> transistors. The simulated oscillator Vcc is then closer to 2.5V.
>
> Earlier circuit schematics didn't include as many actual measured dc
> voltages.
>
> Bruce
>
> WarrenS wrote:
>> 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.
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>> _______________________________________________
>>>>>> 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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