[time-nuts] Broken Ovenaire OSC 85-50

[Bruce Griffiths]

Warren

I agree, it's certainly possible that the external circuitry should have
a higher operating voltage than 5V.

A crystal current of 50uA to 1mA or so might perhaps be expected for a
typical 10MHz overtone crystal.
Unless some non standard crystal cut with exceptionally high ESR is used.

The oscillator transistor dc emitter current appears consistent with a
crystal current of around 1mA or so.
Can use Spice to check but there is no need for elaborate starting
techniques when simulating this circuit as it has plenty of excess gain
if the simulated Q isn't too large (50,000 or so is easy) (only ESR has
much effect on the crystal current).
Elaborate simulations looking for ppm changes when the active device
models aren't that good can be counterproductive.
Doing this with any confidence requires enormous trust in the
convergence of the numerical algorithms employed.
All flavours of Spice do have problems with accuracy and convergence
when the condition number of the solution matrix is large enough.
Often this is because the simulated circuit is unstable so an educated
guess as to what to change to preserve stability/convergence is required.
Occasionally Spice can even predict noise that is much larger than that
actually measured (it can have trouble when one has both positive and
negative feedback loops).

Bruce

WarrenS wrote:
>> Thus one cannot arbitrarily increase Vcc without considering the consequences ...
>>     
>
> True, but with the AC voltages as low as they are, I do think it is safe to think that the crystal current should be higher.
> I was not suggesting to arbitrarily rise the Vcc, I was asking any chance it is at the wrong voltage???
> Even though 2.49 volts is plenty of Vcc to design an osc circuit to run on, It seems like a low, funny voltage to design for, on  what I assume is an old product.
>
> 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 8:36 PM
> Subject: Re: [time-nuts] Broken Ovenaire OSC 85-50
>
>
>   
>> WB6BNQ wrote:
>>     
>>> I agree with Warren.  The VCC seems way too low.  Interestingly, noone else seems to think so,
>>> or did not notice it.
>>>
>>> Bill....WB6BNQ
>>>
>>>
>>>   
>>>       
>> It was noticed, but it is consistent with the resistor values used in
>> the circuit.
>> The crystal current is determined by the dc emitter current of the
>> oscillator transistor.
>> Thus one cannot arbitrarily increase Vcc without considering the
>> consequences for the crystal current etc.
>>
>> Bruce
>>     
>>> 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.
>>>> 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
>>>>     
>>>>         
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>>>       
>>
>>
>>
>>     
>
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