[time-nuts] Triangle Waves

[Magnus Danielson]

Bruce Griffiths wrote:
> Joseph M Gwinn wrote:
>>> 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.
>>>      
>> Yes, there would need to be some kind of drift compensation (I favor a
>> opamp servoloop), but given that we are trying to measure ZCD jitter
>> (versus long-term wander), isn't this good enough?  The distortion 
>> will be
>> small and stable, and so will not cause jitter.
>>
>> Joe Gwinn
>>    
> Yes one shouldn't lose sight of the goal which isnt a perfect triangular 
> wave, but merely a low jitter one.

Indeed.

> The major problem is the Johnson noise of the resistors used in the 
> integrator.
> 
> If for example one uses a simple RC filter using 25k plus 10uF and 
> drives it with a 10Hz square wave the output noise at dc is about 
> 20nv/rtHz.
> The output slew rate with say a 5V amplitude square wave is about 1V pp 
> and the zero crossing jitter due to Johnson noise is on the order of 3ns.

Hmm... what about an LR replacement of the RC chain over the op-amp?

Just a thought. It is a bit uncommon, but is essentially the same in 
function. Could prove interesting since the LR balance could possibly 
allow for lower noise selections.

Cheers,
Magnus