On Feb 2, 2010, at 9:29 PM, Joseph M Gwinn wrote: > [email protected] wrote on 02/02/2010 09:13:26 PM: > >> From: >> >> Bruce Griffiths <[email protected]> >> >> To: >> >> Discussion of precise time and frequency measurement > <[email protected]> >> >> Date: >> >> 02/02/2010 09:16 PM >> >> Subject: >> >> Re: [time-nuts] Triangle Waves >> >> Sent by: >> >> [email protected] >> >> Joseph M Gwinn wrote: >>> [email protected] wrote on 02/02/2010 08:19:26 PM: >>> >>> >>>> From: >>>> >>>> Bruce Griffiths<[email protected]> >>>> >>>> To: >>>> >>>> Discussion of precise time and frequency measurement >>>> >>> <[email protected]> >>> >>>> Date: >>>> >>>> 02/02/2010 08:20 PM >>>> >>>> Subject: >>>> >>>> Re: [time-nuts] Triangle Waves >>>> >>>> Sent by: >>>> >>>> [email protected] >>>> >>>> Joseph M Gwinn wrote: >>>> >>>>> [email protected] wrote on 02/02/2010 07:20:24 PM: >>>>> >>>>> >>>>> >>>>>> From: >>>>>> >>>>>> Bruce Griffiths<[email protected]> >>>>>> >>>>>> To: >>>>>> >>>>>> Discussion of precise time and frequency measurement >>>>>> >>>>>> >>>>> <[email protected]> >>>>> >>>>> >>>>>> Date: >>>>>> >>>>>> 02/02/2010 07:27 PM >>>>>> >>>>>> Subject: >>>>>> >>>>>> Re: [time-nuts] Triangle Waves >>>>>> >>>>>> Sent by: >>>>>> >>>>>> [email protected] >>>>>> >>>>>> 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 capacitorto 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 >>> >> Yes one shouldn't lose sight of the goal which isnt a perfect triangular > >> wave, but merely a low jitter one. >> 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. > > I've lost track of our jitter objective, and why we need to achieve it. > > Also, if the intent is to measure the inherent jitter of a ZCD circuit, we > may be better off using a really clean sinewave, as it will be easier to > generate a clean enough sinewave than trianglewave. > > The fact that we will use a triangle or trapezoid in practice will change > the numbers somewhat, but the ranking of proposed circuits by their > sinewave jitter should carry over correctly, so long as the same > fundamental frequency is used. > > Joe Gwinn >
The objective is to check out a limiter that will have performance at better than 10 ns level when driven with a beat note in the 5 to 10 Hz range. A signal 5 to 10X better than the limiter target performance would be adequate. That puts the desired signal in the vicinity of 1 ppb avar at 1 second. Ideally the signal source would also provide a stable square wave "trigger" to drive the start channel of a counter or the sweep of a scope. I 'm sure some level of troubleshooting on the limiter will be needed eventually. Generating a good trigger off of a slow waveform isn't all that easy. The trigger also may be helpful in verifying the performance of the signal source it's self. Bob > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
