Re: [time-nuts] Triangle Waves

Tue, 02 Feb 2010 18:57:41 -0800 

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
Figure 8 (attached) from Collin's paper indicates that the jitter of a 100Hz wien bridge oscillator is of the order of a few hundred nanosec or so. 
This was taken using a 3 stage limiter and a 1 sec counter gate time.
It may be feasible to do better.
It would also appear to be feasible to produce a 10Hz sinewave with ns jitter by low pass filtering a square wave using a combination of active and passive RC filtering. 

Bruce

<<attachment: Wien_Osc_Stability.png>>

_______________________________________________
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.

Reply via email to