[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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