What equipment do I need to test an eye diagram for USB?

Question

I'd like to test USB full speed with a goal of testing high speed (480 Mbit/s), I have a tek scope that does 300 MHz (which I could upgrade to 500 MHz) and I'm looking at a 500 MHz differential probe. As I understand it I also need a breakout board (which I'm not quite sure is the best thing to get), but I'm looking at this board from tek and one listed here.

What are the minimum requirements for a test like this?

Is this equipment list sufficient to preform an eye diagram test for full speed USB?

Is this equipment list sufficient to preform an eye diagram test for full speed USB if I have a 500 MHz scope?

Answer 1

What are the minimum requirements for a test like this?

Minimum requirements for test equipment to use for USB 2.0 signal quality evaluation is listed at USB.org in the following place. There are links that describe electrical test procedures and tool requirement for Rohde & Schwarz, Tektronix, Agilent, LeCroy, and Yokogawa oscilloscopes.

Typically the eye evaluation software tools are offered on scopes with no less than 2GHz bandwidth. For Tektronix, the eligible scope series are TDS7254/B, TDS7704/B, CSA7404/B, TDS6604/B, TDS6804/B, TDS6404, DPO7254, DPO7354, and DPO/DSA70000. The smallest eligible oscilloscope for USB 2.0 testing is MSO/DPO5204.

For FS evaluation you don't need differential probes, the scope does it mathematically using single-ended probes.

However, the software package can't be installed on smaller bandwidth scopes, so, even if 500 MHz bandwidth is OK for FS eyes, it is unlikely that you can use this scope.

Answer 2

If your signal has a fundamental at 500MHz, and you're trying to measure its characteristics, then you're not going to do well with a 500MHz scope because the scope won't capture any of the harmonics. You will need a higher bandwidth scope and probe if you want to make an accurate measurement of what's happening.

Answer 3

To test a USB high speed transmitter, you have to test the TP3 mask:

The horizontal position of point 3 is at 37.5% UI, and point 6 is at 62.5% UI, so you are trying to measure a rise-time of about 75% of 2.08 ns, or about 1.5 ns.

A 500 MHz scope will measure a minimum risetime of about \$0.75/500\ {\rm MHz}\$, or 1.5 ns.

You can estimate the risetime you measure will be about

$$\tau_{meas}\approx\sqrt{\tau_{scope}^2+\tau_{sig}^2},$$

so a 500 MHz scope is not going to cut it. I'd look for at least 1 GHz, and 2 GHz will help if your product doesn't have much margin.

Answer 4

I ended up using a single ended measurement and using matlab to poll the textronix scope and using the instrument control toolbox:

myScope = oscilloscope
drivers(myScope)
availableResources = getResources(myScope)
availableResources{5}
myScope.Resource = 'TCPIP::xxx.xxx.xxx.xxx::INSTR'
connect(myScope)
get(myScope)
waveformArray = getWaveform(myScope, 'acquisition', true);
figure;plot(waveformArray)
enableChannel(myScope,'CH3');
enableChannel(myScope,'CH4');
Time = linspace(0,myScope.AcquisitionTime,myScope.WaveformLength);
aqs = 1000;
for i = 1:aqs
[w3, w4] = readWaveform(myScope, 'acquisition', true);
pause(0.05);
w3mat{i} = w3;
w4mat{i} = w4;
end
figure;subplot(2,1,1),hold on,legend('D+','D-'),subplot(2,1,2),hold on
for i = 1:aqs
w3 = w3mat{i};
w4 = w4mat{i};
subplot(2,1,1)
plot(Time,w3,'b'),plot(Time,w4,'r')
subplot(2,1,2)
plot(Time,w3-w4,'r')
end
subplot(2,1,1),legend('D+','D-'),ylabel('Volts');
subplot(2,1,2),legend('D+ subtracted from D-'),xlabel('Time'),ylabel('Volts');
subplot(2,1,1)
x = [8.2  21.15 69.93 75.13 69.93 21.15 8.2 ]*1e-9/2;%These come from the USB IF website specs
y = [1.65 2.5   2.5   1.65  0.8   0.8   1.65];
fill(x+1e-7,y,'r')
title('USB Full speed EYE test')
line([1e-7 1.4e-7  ],[-0.9  -0.9])
line([1e-7 1.4e-7  ],[4.4  4.4])