Notes on jitter and timing measurements with new tek scope
Given a week or so to play with a rental TDS6154C 15GHz oscilloscope, a few
SMA cables & connectors were ordered and then I did a couple measurements to see how
well it works.
Timing sensitivity
The TDS6154C has a “fast rise pulse” output. Two short (6-inch) SMA-to-BNC
adapter cables were used plus a BNC “tee” in the middle to connect the pulse output to
channel 1 as shown in Figure 1.
SMA-to-BNC
SMA-to-BNC
BNC "tee"
Figure 1 - using the Tek scope's own fast output
The “tee” was used to provide a connection point for an impedance discontinuity later.
Figure 2 shows the signal as measured, plus the FFT of the waveform as calculated by the
scope. The “fast rise pulse” output has a fundamental frequency of ~1.5kHz and so the
fundamental peak is buried at the left edge of the FFT, out of view. Unfortunately I had
the cursors in the wrong place to make this obvious in the photo, but the 10% - 90% rise
time is 0.5 divisions, or 250ps. The specs on the output pulse are for a 200ps rise time, so
we can estimate the bandwidth of the cabling system & scope as being 0.35/50E-12, or
about 7GHz. The cables are 12GHz parts, but we have the BNC bits, so that’s
reasonable.
A second measurement was made by inserting a BNC “barrel” into the “tee”
center connection. The open barrel creates an in-phase reflection. Assuming typical RG58 propagation velocity (0.66c, or 7.8E12 in/sec) and a barrel conductor length of 1.5in,
the expected round-trip delay is about 385ps. Figure 3 shows a round-trip delay of
365ps, so that says the scope is working well for direct timing resolution.
Figure 2 - signal rise time and spectrum for setup of Figure 1
Figure 3 - Reflection caused by addition of barrel to tee
Jitter Measurement
The TDS6154C software package contains demonstration versions of the jitter
analysis software that can be run a small number of times before they are disabled. The
software should be good for one or two more invocations before it is exhausted. Once
invoked it may be left running for days without counting as another license checkout.
To measure the jitter I re-used the fast pulse output but ran the signal through a
relatively long piece of RG-58 cable. This was mostly for the convenience of being able
to connect the same signal to both this scope and another for comparison, but the cable
also adds more capacitance, smoothing the rise and making measurements slightly more
difficult. The setup is shown in Figure 4. The SMA-to-BNC cables and the BNC “tee”
are the same as used previously. The jitter package generates the usual statistics,
measuring cycle-to-cycle time, duty cycle, etc., and providing min/max/average
calculations. A histogramming feature is provided which allows easy comparison to
other scopes.
SMA-to-BNC
SMA-to-BNC
BNC "barrel"
BNC "tee"
16ns long RG-58
Figure 4 - Setup for jitter measurement
Two problems were immediately noted. The first is typical. After hooking up the
signal and manually adjusting the channel gain and horizontal sweep rate to reasonable
settings, I then hit the “autoset” button in the jitter package to let the scope find its own
gain & sweep speed. Multiple attempts from various starting positions all failed
miserably, generating error messages saying that the scope was unable to find a
gain/sweep setting that it could use to measure statistics. While disappointing, this was
not a surprise; few scope auto-set features ever seem to work right. After resetting the
scope manually for gain, threshold point and sweep rate, the statistics were easily
captured.
The second functional problem is one of software. The jitter controls suggest that
one should take data and then ask for a plot, but this doesn’t work. Doing so will only
histogram the last few data points. If you want to see the histogram of all data, the
plotting function must be invoked first, then you have to clear the accumulated data, and
then start taking new data. The histogram builds as more samples are collected. The help
files are, as you might expect, mute on the subject.
Figure 5 shows the histogram of jitter as collected over about 400 samples. The
scope’s own test pulse has a trimodal distribution, indicative of a normal period plus a
sideband caused by some other modulation. The error is about ±3ns out of 1ms. The
histogram has controls to zoom in and out, and one place cursors on it, although I was
unable to manually set the horizontal scale of the histogram to integer numbers of ns per
division. Perhaps with more time this could be figured out, but after a short operational
period it looks like the cursors are the preferred method of measuring where the peaks
are.
Figure 5 - Histogram to cycle-to-cycle width as measured by Tektronix scope
To compare the results with that of another scope, I used the same piece of RG-58
to connect the TDS6154C’s test output to a LeCroy Wavepro 960 we have at ANL. This
setup is shown in Figure 6. Following the same sequence, the auto-set function of this
device failed just as miserably as the more expensive scope’s did. Once more manually
setting everything up, a similar histogram was developed. The LeCroy scope’s
histogramming function is slightly more user-friendly than that of the Tek device,
however it still fails to display the histogram with any arbitrary horizontal units.
However, a larger failing is that the LeCroy scope can’t use the cursors on the calculated
waveform.
Display issues aside, however, it can be seen in Figure 7 that the histogram of the
Wavepro 960 is, qualitatively, not significantly different than that of the TDS6154C. The
peaks are in the same places and the outliers are in the same relative positions. However,
the peaks are broader, indicating that the sampling clock of the Wavepro960 isn’t quite as
clean as that of the TDS6154C; the expectation is that the width of the peaks is a vector
sum of the jitter of the input signal and the jitter of the sampling clock of the scope.
TEK
SMA-to-BNC
SMA-to-BNC
BNC "barrel"
BNC "tee"
16ns long RG-58
LeCroy
Figure 6 - Setup for comparison jitter measurement using LeCroy Wavepro 960
Figure 7 - Jitter histogram from Wavepro 960
Vertical accuracy and range
A couple quick tests with open and 50-ohm terminated inputs shows that the
TDS6154C has quite a bit of internal noise that does not scale with input range. The
noise appears to be artifacts from the input filtering algorithm that’s used along with the
ADC. The manual indicates there is an internal DSP that can be turned on and off to
reduce this to some degree, but it’s bad enough that I cannot recommend this scope for
accurate low-level voltage measurements.
In similar vein the maximum input range for the channel inputs is 1V p-p, and
warnings about damaging the inputs are plentiful. While the scope certainly has
bandwidth and timing, it is by no means a general-purpose instrument good for
everything.
Additional comments
It would have been interesting to see what the jitter measurement software did
with a very clean clock input with a faster edge transition, but I didn’t have one close at
hand to try. Obviously the slower edge intentionally created by the use of the RG-58
limited the jitter resolution (peak width in histograms) obtained by either scope. For
entertainment I tried to obtain jitter measurements of the TDS6154C’s test output on a
TDS3054B digital scope but couldn’t resolve the jitter to better than 10s of nanoseconds.
Conclusions
The TDS6154C is a nice scope for timing measurements, but it really doesn’t
outclass other instruments unless there is a need for jitter measurements at scales
markedly less than ±1ns, or a necessity to measure timing in the less than 500ps realm.
The real power of the device is based in its very long record length. It’s worth a month’s
rent for those occasional times that a clock signal must be very carefully characterized
but the cost/benefit analysis does not favor outright purchase.