[time-nuts] Triangle Waves

[Joseph M Gwinn]

time-nuts-bounces at febo.com wrote on 02/02/2010 08:19:26 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 08:20 PM
> 
> Subject:
> 
> Re: [time-nuts] Triangle Waves
> 
> Sent by:
> 
> time-nuts-bounces at febo.com
> 
> 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, whateverthe 
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.

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