[time-nuts] Triangle Waves

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Feb 3 01:19:26 UTC 2010


Joseph M Gwinn wrote:
> time-nuts-bounces at febo.com wrote on 02/02/2010 07:20:24 PM:
>
>    
>> From:
>>
>> Bruce Griffiths<bruce.griffiths at xtra.co.nz>
>>
>> To:
>>
>> Discussion of precise time and frequency measurement
>>      
> <time-nuts at febo.com>
>    
>> Date:
>>
>> 02/02/2010 07:27 PM
>>
>> Subject:
>>
>> Re: [time-nuts] Triangle Waves
>>
>> Sent by:
>>
>> time-nuts-bounces at febo.com
>>
>> Magnus Danielson wrote:
>>      
> [snip]
>    
>>> Just a reality check question here... a simple triangle oscillator is
>>> very easily created by two op-amps, one for an integrator and one for
>>> Schmitt trigger operation. If you want better long-term stability open
>>>        
>    
>>> the loop and insert a 10 Hz from your favourite divider chain of a
>>> trusted 10 MHz or so. Would such a design be limiting your measurement
>>>        
>    
>>> goals considerable, and would any flaws be reasonably to overcome by
>>> better design?
>>>
>>> Cheers,
>>> Magnus
>>>
>>>        
>> For beat frequencies in the 1-100Hz range one only need verify the ZCD
>> jitter and delay variations etc., to within a few nanosec.
>> In the short term such jitter tantalisingly close to what a well
>> designed audio oscillator is capable of.
>> Unfortunately the trigger jitter in most counters is very large for
>> frequencies in this range so verifying the low jitter of an audio
>> oscillator requires using a ZCD or equivalent.
>>      
> Would integration of a 50% duty cycle square wave generate an adequate
> triangle wave?  Modern opamps make pretty good low-noise integrators,
> although one would need to use a good integration capacitor to ensure
> linear ramps.
>
> The square wave would come from a simple binary divider chain, which will
> clean many things up and ensure a stable duty cycle, whatever the nature
> of the original signal source.
>
> Joe Gwinn
>
>    
The integration function requires a low frequency cutoff (either a 
servoloop or a resistor shunting the integration capacitor) to avoid 
integrator saturation.
This inevitably distorts the triangle wave, however it should be 
possible to reduce the triangular wave distortion by predistorting the 
integrator input current.

Bruce






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