[time-nuts] Triangle Waves

[Bob Camp]

Hi

My *guess* would be that magnetic pickup (power lines etc) in the L of the
L/R (or LC or even RLC) network would be a difficult problem to address. Big
toroidal coils might be one solution to that. 

Cost wise the DAC is looking better and better....

Bob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Magnus Danielson
Sent: Wednesday, February 03, 2010 4:02 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Triangle Waves

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

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