[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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