Measure jitter in a PLL
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 4/1/2005 2:00:00 AM
| Download the complete paper in Word format: Jitter generation and measurement with off-the-shelf test equipment |
Communications systems rely on phase-locked loops (PLLs) to lock onto and extract clocks embedded in data streams. Jitter in a signal is the enemy, the characteristic that can cause a PLL to lose a lock. PLLs can compensate for some jitter, but they have limits. To find those limits and select a PLL for a communications system, you must evaluate its jitter performance.
Communication test sets can generate input signals for testing a PLL, but what do you do if you don't have one? Fortunately, you can characterize a PLL with test equipment that's probably already on your test bench—two signal generators with phase or frequency modulation and a digital oscilloscope. One signal generator creates the base clock signal, and the other generates controlled jitter through modulation. Using these tools, you can measure a PLL's jitter tolerance, jitter transfer, and jitter generation.
To perform the test, use the test setup in the figure. Increase the jitter amplitude of the test signal in the smallest amounts possible until the PLL either loses its lock or generates an alarm signal. Then, repeat the procedure with a constant jitter amplitude but with increasing jitter frequency. Compare the measured jitter tolerance values against the jitter tolerance you are trying to meet.
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Use this test setup to evaluate a PLL’s jitter performance. |
You can learn the full details of this test setup from a paper written by Slobodan Milijevic, an engineer at Zarlink Semiconductor. In his paper, Milijevic explains how to use the signal generators and scope to characterize a PLL; he opens with a jitter tutorial followed by a description of the test setup and test procedure. He also explains how to calculate the jitter you need for the tests. You can download the paper, "Jitter generation and measurement with off-the-shelf test equipment," in Word format
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