Bruce Griffiths wrote:
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 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
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.
Bruce
Make that 25K plus 2uF producing ~2V pp quasi triangular wave with a
slew rate of around 40V/s and a noise bandwidth of about 5Hz producing a
zero crossing jitter of around 1.1ns due to Johnson noise from the 25K
resistor.
To ensure this isnt degraded by the logic supply noise an extremely low
noise logic supply (at least for the output stage) is necessary.
Bruce
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