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Jitter measurement

B

budgie

Jan 1, 1970
0
Well, it's probably jitter and "wobble" as well ....

I have a PLL with a 10MHz xtal oscillator, comparison frequency 10kHz, reference
source being a GPS receiver which outputs 10k. This all works very well, but I
was hoping to somehow quantify the residual phase variation, and without
requiring particularly elaborate gear.

I've done visual comparisons on the CRO with the OCXO timebase oscillator in my
frequency counter, which showed no discernible wobble in the phase relationship.

I've also fed the buffered 10MHz into the counter which resolves to 1Hz (1 sec
gate) and the variation in this "sampling" is the 1Hz you'd expect in a sampled
stream (10,000,000 or 10,000,001).

I've also listened to the 10MHz on a narrowband FM receiver 9comms test set) and
can't discern any change in the residual instrument noise when the oscillator is
on or off, but that only indicates that the comms test set has a noise floor.

I can work out the freq/voltage slope at the varicap and watch the control
voltage on a CRO, but that is going to incorporate additional effects into the
calculation.

Any suggestions welcome.
 
J

John_H

Jan 1, 1970
0
budgie said:
Well, it's probably jitter and "wobble" as well ....

I have a PLL with a 10MHz xtal oscillator, comparison frequency 10kHz, reference
source being a GPS receiver which outputs 10k. This all works very well, but I
was hoping to somehow quantify the residual phase variation, and without
requiring particularly elaborate gear.

I've done visual comparisons on the CRO with the OCXO timebase oscillator in my
frequency counter, which showed no discernible wobble in the phase relationship.

I've also fed the buffered 10MHz into the counter which resolves to 1Hz (1 sec
gate) and the variation in this "sampling" is the 1Hz you'd expect in a sampled
stream (10,000,000 or 10,000,001).

I've also listened to the 10MHz on a narrowband FM receiver 9comms test set) and
can't discern any change in the residual instrument noise when the oscillator is
on or off, but that only indicates that the comms test set has a noise floor.

I can work out the freq/voltage slope at the varicap and watch the control
voltage on a CRO, but that is going to incorporate additional effects into the
calculation.

Any suggestions welcome.

My favorite technique is to use a digital oscilloscope in X-Y mode. A
stable reference that's locked to the signal to be measured gets split
and filtered into a sine and cosine with reasonably inexpensive RF
components such as those from minicircuits.com (or passives for the
lower frequencies). The reference sine and cosine feed 2 channels of
the scope in XY mode resulting in a circle. The signal with the jitter
goes to the trigger. With one point per trigger, the scope shows a
spot, a smear, or a "smile" showing the amount of jitter within one
reference cycle (degrees within the 360 degree circle). The scope I
used a dozen years ago had a timebase jitter of about 75 ps, limiting
the measurement.

The requirement to lock on to the incoming signal means a little extra
work. A VCXO with a low frequency cutoff tracks the incoming signal
below the cutoff but only delivers the VCXO noise above the cutoff.
With this reference, you can watch the jitter.

Or use a SERDES and an FPGA to take the incoming signal and resolve it
down to less than 1ns of time interval resolution per clock.
 
R

Rene Tschaggelar

Jan 1, 1970
0
budgie said:
Well, it's probably jitter and "wobble" as well ....

I have a PLL with a 10MHz xtal oscillator, comparison frequency 10kHz, reference
source being a GPS receiver which outputs 10k. This all works very well, but I
was hoping to somehow quantify the residual phase variation, and without
requiring particularly elaborate gear.

I've done visual comparisons on the CRO with the OCXO timebase oscillator in my
frequency counter, which showed no discernible wobble in the phase relationship.

I've also fed the buffered 10MHz into the counter which resolves to 1Hz (1 sec
gate) and the variation in this "sampling" is the 1Hz you'd expect in a sampled
stream (10,000,000 or 10,000,001).

I've also listened to the 10MHz on a narrowband FM receiver 9comms test set) and
can't discern any change in the residual instrument noise when the oscillator is
on or off, but that only indicates that the comms test set has a noise floor.

I can work out the freq/voltage slope at the varicap and watch the control
voltage on a CRO, but that is going to incorporate additional effects into the
calculation.

Any suggestions welcome.

Having a look at the noise of the VCO control voltage
may be one way, but if not done carefully may introduce
additional jitter. The usual method is to measure the
slopes of the carrier. EG have it running on 10MHz and
measure from 10Hz to 1MHz beside the carrier. Then you
can integrate the slope to get ps_rms-jitter.

Rene
 
K

Ken Smith

Jan 1, 1970
0
Well, it's probably jitter and "wobble" as well ....

I have a PLL with a 10MHz xtal oscillator, comparison frequency 10kHz, reference
source being a GPS receiver which outputs 10k. This all works very well, but I
was hoping to somehow quantify the residual phase variation, and without
requiring particularly elaborate gear.

Can you make a second system?

In theory, the 10k from two different GPSes should be the same frequency.
The scope display of both outputs should have no slip action so you can
blow up the timescale to see the jitter.
 
M

Mike

Jan 1, 1970
0
budgie said:
Well, it's probably jitter and "wobble" as well ....

I have a PLL with a 10MHz xtal oscillator, comparison frequency 10kHz,
reference
source being a GPS receiver which outputs 10k. This all works very well,
but I
was hoping to somehow quantify the residual phase variation, and without
requiring particularly elaborate gear.

Residual phase variation relative to what? In other words, what is your
reference point for the measurement?

If your reference is the 10kHz signal from the GPS receiver (your PLL
input), then use the 10kHz signal as the scope trigger, and display the
10MHz output. If there's no residual phase variation, you'll see a clear
edge of the 10MHz output every time the scope triggers.

-- Mike --
 
B

budgie

Jan 1, 1970
0
Can you make a second system?

In theory, the 10k from two different GPSes should be the same frequency.
The scope display of both outputs should have no slip action so you can
blow up the timescale to see the jitter.

What is unknown is how the 10kHz signal is derived. It is possible that the
pulse count is 10,000 per sec but the duty cycle varies, or a number of other
possibilities.. The data sheet isn't revealing, and the manufacturer doesn't
want to know legacy receivers, so there is no detail on the derivation. (Having
seen how the 50/60Hz was derived from colour-burst crystals in the old Fairchild
5369 devices, I am leery of making assumptions In that family, M cycles were
counted in the high state, and N in the low state, with M<>N. IIRC they also
varied M across one output cycle to achieve "proper" division - the 5369EYRN
produced 50Hz from a 3.579545 crystal by this form of "fudged" non-integer
division.)

For this reason, I see it as conceivable that two devices may show jitter but
still deliver zero slip.

That is why I used the OCXO from the frequency counter timebase in a CRO
comparison - the jitter or variation in the counter timebase would certainly be
independent of the GPS-derived oscillator's variation. That disclosed no
discernible perturbation to a slow but steady slip, indicating that neither was
particularly bad.

What I am trying to achieve is some improvements in the area of the loop filter
and the oscillator itself. With a methodology for quantifying the variation,
this becomes a more scientific process.
 
B

budgie

Jan 1, 1970
0
Residual phase variation relative to what? In other words, what is your
reference point for the measurement?

Please see my reply to Ken Smith's post. I'm not after a relative measure, I'm
after an absolute - i.e. compared to a pure jitter-free ideal source.
If your reference is the 10kHz signal from the GPS receiver (your PLL
input), then use the 10kHz signal as the scope trigger, and display the
10MHz output. If there's no residual phase variation, you'll see a clear
edge of the 10MHz output every time the scope triggers.

Certainly the case. And that would disclose any irregularities in the 10kHz
stream also

But as I mentioned in the other reply, I am after a method to quantify the
variation.
 
K

Ken Smith

Jan 1, 1970
0
budgie said:
5369 devices, I am leery of making assumptions In that family, M cycles were
counted in the high state, and N in the low state, with M<>N. IIRC they also
varied M across one output cycle to achieve "proper" division - the 5369EYRN
produced 50Hz from a 3.579545 crystal by this form of "fudged" non-integer
division.)

Yes, I've done stuff like this to get non-integer frequency ratios. If
you have reason to believe that this is the sort of thing they have done,
it may be worthwhile to try to figure out what they have really done.

Does the 10KHz hold steady if you trigger from the 1PPS pulse? If so, you
will know that the pattern repeats from one second to the next. This will
help to assure you that there isn't going to be much below 1HZ.

You can buy a really good OCXO for a few hundreds of US$.

or:

Assuming you can make three of your PLL circuits, you could use them to
help look at the noise in the 10KHz etc. (You can use two but it takes
more work and you have to assume some stuff)

If you lock multiple identical PLL circuits onto the 10KHz, you can see
the noise they introduce by comparing their outputs. Exactly how to
compare them, I haven't thought through yet. (See below)

If you set the loop filter in the PLLs to be very slow, you will know that
they can't track high frequency noise on their inputs. If you have more
than one, you can see how much high frequency noise they make. By using
all combinations of the three, you can solve for the noise for each one.

If you compare two with two different bandwidths, you may be able to see
the noise in the input. (If the PLLs are too noisy you won't be able to
see it) If you know the noise performance of your PLLs you can solve for
the noise in the input signal that comes through in the band the two PLLs
don't share.

Thinking about comparing:

We need a phase detector that has a low noise and a high gain and that
doesn't have its over-range point near the point where the two edges
line up.

Assuming we have VCOs running at a multiple, we can make a phase shifted
signal for use in an XOR type phase detector. I see a problem with this
because the noise from the power supplies appears on the output of the XOR
and this could seriously limit the noise floor.

Perhaps a system with some sort of clean tri-stating would work. You only
enable the output during a short period near the edge.
 
K

Ken Smith

Jan 1, 1970
0
Residual phase variation relative to what? In other words, what is your
reference point for the measurement?[/QUOTE]

Here's a silly idea. I post it only because there may be the spark of a
good idea in here.

Get about 6KM of coax. Feed the 10KHz into one end and compare the two
ends of the cable with a scope.

At 10KHz a modest fraction of what you put in will make it to the far end.
 
P

Phil Hobbs

Jan 1, 1970
0
Ken said:
Residual phase variation relative to what? In other words, what is your
reference point for the measurement?


Here's a silly idea. I post it only because there may be the spark of a
good idea in here.

Get about 6KM of coax. Feed the 10KHz into one end and compare the two
ends of the cable with a scope.

At 10KHz a modest fraction of what you put in will make it to the far end.
[/QUOTE]

This is basically a giant delay-line discriminator. I wonder if the
phase stability of a big reel of coax is better or worse than a quartz
crystal's? A quartz crystal discriminator with a Q of 10**6 would do
much the same thing in a smaller space for much less money, and would
avoid the problem of multiples of 2pi delay in a long cable. The
sensitivity can be surprisingly high even at very small deviations.

Cheers,

Phil Hobbs
 
B

budgie

Jan 1, 1970
0
Yes, I've done stuff like this to get non-integer frequency ratios. If
you have reason to believe that this is the sort of thing they have done,
it may be worthwhile to try to figure out what they have really done.


I don't have any reason to believ they have done this, but I don;'t know exactly
how they derive 10k from the GPS signal so I can't rule it out.
Does the 10KHz hold steady if you trigger from the 1PPS pulse? If so, you
will know that the pattern repeats from one second to the next. This will
help to assure you that there isn't going to be much below 1HZ.

The 1pps edge is synced to the 10kHz, but that really only requires that every
second there are 10,000 cycles on the 10k line - not that the individual cycles
are themselves uniform. Sort of like the way OUR electricity generation
frequency is +/- a certain frequency tolerance, but as midnight approacheth the
frequency is driven to ensure that at midnight the total number of cycles in the
24-hours is correct.

At one stage I wondered about timing each cycle of the 10k reference pulses
using a higher speed clock and period counting, but the gating would need to be
synced to the 10k pulse - and at the end of the day this would only remove the
uncertainty over the "regularity" of the 10k stream.

You can buy a really good OCXO for a few hundreds of US$.

I *think* the one in the frequency counter is fairly clean and
wobble/jitter-free. Certainly the side-by side (and Lissajou) comparisons on
the CRO with the PLL unit suggest neither has any issues - if either one had a
problem then it would be evident, even though it wouldn't be clear which.
or:

Assuming you can make three of your PLL circuits, you could use them to
help look at the noise in the 10KHz etc. (You can use two but it takes
more work and you have to assume some stuff)

If you lock multiple identical PLL circuits onto the 10KHz, you can see
the noise they introduce by comparing their outputs. Exactly how to
compare them, I haven't thought through yet. (See below)

If you set the loop filter in the PLLs to be very slow, you will know that
they can't track high frequency noise on their inputs. If you have more
than one, you can see how much high frequency noise they make. By using
all combinations of the three, you can solve for the noise for each one.

If you compare two with two different bandwidths, you may be able to see
the noise in the input. (If the PLLs are too noisy you won't be able to
see it) If you know the noise performance of your PLLs you can solve for
the noise in the input signal that comes through in the band the two PLLs
don't share.

Thinking about comparing:

We need a phase detector that has a low noise and a high gain and that
doesn't have its over-range point near the point where the two edges
line up.

Assuming we have VCOs running at a multiple, we can make a phase shifted
signal for use in an XOR type phase detector. I see a problem with this
because the noise from the power supplies appears on the output of the XOR
and this could seriously limit the noise floor.

Perhaps a system with some sort of clean tri-stating would work. You only
enable the output during a short period near the edge.

Comparing is something I have mused over. I can't discern any wobble or jitter
visually on the CRO comparison, but that only means that it is below the
threshold of visual discernment (sic). Similarly, I can't see any signs on the
loop filter output. Neither of these "easy" comparison approaches lets me
quantify, and hence precludes their use in optimising.

This PLL is far different from those of wide(r)-band synthesisers I am more
familar with. It doesn't have a wide acquisition requirement, so the loop
filter can be over-damped and is ultra-slow.

Being an XOR comparator (which obviously has its own contribution to jitter), at
quadrature lock its output (and the filter output to the varicap) is half-rail.
The varicap control line has a switch to select SETUP or OPERATE functions. In
setup, a half-rail divider voltage feed the VCO which is trimmed to 10MHz. This
is just to centre the PLL operation. The 10MHz VCXO has about a 80Hz range from
rail to rail (non-linear of course but we're not trying to generate linear FM/PM
signals) so that's about 16Hz/volt average slope

I don't for a moment accept the premise that what I have to date is perfect, but
maybe I'm trying to go too far with a simple setup. Maybe it IS *good enough*
as a cal source for the workshop.
 
K

Ken Smith

Jan 1, 1970
0
The 1pps edge is synced to the 10kHz, but that really only requires that every

Do you mean is synced by circuits or appears to have a stable timing
relationship. I'm going to assume the latter.
second there are 10,000 cycles on the 10k line - not that the individual cycles
are themselves uniform.

If they are using a non-integer divider or the like, this ensures that
there isn't going to be modulation below the 1Hz mark from that. You seem
to have missed this point in my posting.

[...]
I *think* the one in the frequency counter is fairly clean and
wobble/jitter-free. Certainly the side-by side (and Lissajou) comparisons on
the CRO with the PLL unit suggest neither has any issues - if either one had a
problem then it would be evident, even though it wouldn't be clear which.

No, this just means you didn't look closely enough :>. Everything has
noise. In this case it may be so low that you don't care about it but you
can be sure it is there somewhere.

[... phase compare ...]
Comparing is something I have mused over. I can't discern any wobble or jitter
visually on the CRO comparison, but that only means that it is below the
threshold of visual discernment (sic).

Most analog scopes still have a X10 horz button. Try using it. Since you
are repeating fast, the trace should still be bright. Also, it can help
to turn off the lights.

The flicker in the lights beats with the sweep of the scope and causes
some visual artifacts. These usually look like troubles that are not
there and not hide troubles.

[...]
This PLL is far different from those of wide(r)-band synthesisers I am more
familar with. It doesn't have a wide acquisition requirement, so the loop
filter can be over-damped and is ultra-slow.

Over damping is good. You want a large phase margin to avoid noise
peaking near the gain cross over.
Being an XOR comparator (which obviously has its own contribution to
jitter), at quadrature lock its output (and the filter output to the
varicap) is half-rail.

You could use a tri-stating phase detector to reduce this. Another trick
assuming that you have the divide by twos at the bottom of the counter is
to use an XOR to construct the inverse of the ideal locked signal.

---- ----
Q(n-1) ---- ---- ----

-- -- -- --
Q(n-2) -- -- -- -- --

---- ----
XOR -- ---- ----

Compare the XOR to the output of your phase detector and you'll get the
idea.

The varicap control line has a switch to select SETUP or OPERATE
functions. In setup, a half-rail divider voltage feed the VCO which is
trimmed to 10MHz. This is just to centre the PLL operation. The 10MHz
VCXO has about a 80Hz range from rail to rail (non-linear of course but
we're not trying to generate linear FM/PM signals) so that's about
16Hz/volt average slope

In fact the VCO may be fairly linear. (not good enough for audio) If the
varicap is connected to the turned circuit through a smallish capacitance
to reduce its effect, this tends to make things more linear. If instead
it is in parallel with a largish capacitance, the circuit is less linear.
 
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