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, whatever the 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.
Bruce
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