[sdiy] Oscillator jitter measurements

[Czech Martin]

I assume there are two design problems in this circuit.

1.) the designer of the 311 took great care to seperate the output
open collector current from the differential input stage.
Otherwise pin 4 could
be connected to pin 1 internally. He did not do this, and you should
not attempt to do it, either.
The 311 app notes will tell you
about that. After all, a comparator is a very high gain circuit
without any feedback for gain reduction, thus very susceptible
to oscillations that would never be seen with a normal op amp.
At the same time it produces fast transients, especially with
the output stage, an "interesting" mixture.
A good start would be a good groundplane board.
Or copper foil. You need this also for the reference point
of the input stage supply decoupling capacitor.

Althoug it sounds simple, it is not so simple. Perhaps some
experiments have to be made in order to set up things properly.
This is because of the sometimes unexpected parasitics
of your components and the board. Even the way the PSU and
cables are made could have an influence, so I'm sorry that I can not
give you a 100% solution.

The 20MHz bandwidth limit of the scope can be an obstacle
when observing glitches, and those can turn your measurement
into rubbish. I think that during such a measurement a 50 ohm
scope output for monitoring and adjustment is a must.

There is nothing worse then a computer aided measurment which
runs merily with bogus signals, producing a lot of bogus
data (which is perhaps not so obvious in the first days of
thinking, wondering and analysis).

2.) What is the 1nF between + and - doing?
You have added a 2M2 for hysteresis, but you slow it down
with this large capacitive load. Delete this cap.
Better give the 2M2 a few pF in parallel in order to 
overcome the + input parasitics. The faster the input, the less
time in linear mode where all this nasty oscillating things can happen.
If you want to reduce the bandwidth of the input (why?)
you should do it at the input.
And watch out for the maximum differential mode input specs.

The basic problem is that you and me are used to the slow, closed loop
behaviour of a 741, 071 or similar. I.e. bandwidth about 3MHz or lower. 
But comparators are different. They need to be very fast
and have in some cases 10x bandwidth or more. If we use our slow linear
op amp habits with comparators chips, we are in trouble.

m.c.

-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Fredrik Carlqvist
Sent: Freitag, 18. Juni 2004 13:26
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] Oscillator jitter measurements


Hi all!


I have constructed a jitter measurement device (called the jitterbug, actually ;-) with a resolution down to 100ns. I have done some initial measurements on the synths I have at home. No conclusive results yet. But the jitterbug works very well. My plan is to measure all different VCO topologies I can get my hands on and present the results on a web page with statistical analysis, distribution diagrams and everything. 
 
However, I need your help with the trigger circuit. It uses a LM311. There is an image of the current design here:
 
http://www.minod.com/jitter/schemo.gif
 
The problem is that it has to be trimmed in with an oscilloscope before each measurement as to remove oscillation and false triggers. My scope goes up to 20MHz and has no memory, so it is very difficult to see when the trig pulse is ok. It is not impossible though. 
 
I want the trigger circuit to be useable without using a scope and without a very pure input waveform if possible. Right now I have to hack into the VCO core itself, since the slopes at the audio output of the synth often is not very sharp. Most synths cannot open the filter enough it seems. 
 
So, for the benefit of science and our future VCO constructions please help me with the trigger circuit!
 
 
Fredrik C