Figure 1. Clock jitter measured as a variation of clock signal absolute period. |

In this post, we’ll demonstrate a robust method for measuring clock jitter using an example from Dr. Eric Bogatin’s webinar, “The Impact of Power Rail Noise on Clock Jitter.”

The clock in our examples is a 5-stage ring oscillator which generates a square wave signal between 10 and 66 MHz. The test instrument is a WavePro HD 12-bit, 4-Ch, 8 GHz, 20 GS/s, 5 Gpts oscilloscope with 60 fs sample clock jitter.

In the process, we make a series of oscilloscope sample clock tests and timebase adjustments as consistency checks. While measuring jitter is less about absolute accuracy than about the relative precision of measuring the time interval from cycle to cycle, a fundamental part of that is ensuring the absolute accuracy of the oscilloscope’s timebase.

### 1. Test Oscilloscope Sample Clock Jitter

Figure 2. Testing oscilloscope sample clock accuracy using a known signal source. |

5 V signal and apply it to the oscilloscope on C1. We set a 50% Edge trigger and a fixed sample rate of 20 GS/s (Figure 2). After measuring the known source frequency, rise time and period using parameters, we calculate the fractional uncertainty in PPM using the formula: (Δf/f)*10^6. With a mean frequency of 30.00028 MHz, Δf =280 Hz, so the fractional uncertainty in PPM on a 30 MHz signal is 280/30*10^6 = 9.333, indicating the absolute accuracy of the oscilloscope timebase is under 10 PPM. Pretty good.

### 2. Estimate Expected Clock Period

Next, we switch the trigger to the clock source channel, and adjust the V/div and Time/div until two periods of the signal are visible on the graticule. Figure 3 shows the waveform of the ring oscillator on C2. The period is a little more than four divisions, which at 5 ns/division is about 21 ns. The rise time is a little less than two minor divisions, each of which represents 1 ns, so about 1.5 ns.

### 3. Measure Clock Period Using Parameters

Figure 3. Measuring clock signal rise time, frequency and period. |

### 4. Calculate the Standard Deviation of the Clock Period

The standard deviation (sdev) is a measure of the spread of values about the mean. By definition, in a Gaussian distribution, 68% of all measured values are within ±1 standard deviation from the mean. The period sdev value is a good figure of merit for clock jitter, which is a measure of variation from the mean.

With statistics on, the sdev of every parameter measurement is already calculated. Our clock period sdev measurement is 6.18 ps. However, since the oscilloscope’s intrinsic sample clock jitter is specified at 60 fs, and we have ascertained an accuracy of ~9 PPM, the measured period jitter at 6.18 ps is much higher than the fundamental limit of the oscilloscope. The measurement is likely “real.”

### 5. Increase the Timebase

Figure 4. Measuring clock rise time, frequency and period over a long acquisition. The parameters are measuring the full acquisition, not the zoom overlaid on it. |

### 6. Track and Histogram the Period Measurement

Figure 5 shows both the track and the histogram of our period measurement. The track function is displayed over the acquired waveform, while the histogram is plotted in a separate grid.

Figure 5. Statistical analysis of period measurement using tracks and histograms. |

### 7. Lower Sample Rate Until Measurement Degrades

As a consistency check, we investigate how the oscilloscope’s sampling rate affects the measured clock jitter. This is done for “situational awareness,” to ensure that the instrumentation is not affecting the measurements. We lower the sample rate by successive steps until the measurement visibly degrades. From 20 GS/s, we step to 10 GS/s, 5 GS/s and 2.5 GS/s with little change in the measured period sdev or the shape of the histogram. Only when we reach 1 GS/s can we see a significant change (Figure 6).

At 1 GS/s, the edges of the clock signal are not well enough defined by the number of samples to measure the period accurately. There are only about 1.3 samples on the edge (the bright dots on the zoomed clock waveform in Figure 6). As a general rule, you should use the highest possible sampling rate for jitter measurements. However, this confirms that even at a fraction of our maximum sampling rate, measurement accuracy is good, and at 20 GS/s the jitter measurement is quite trustworthy.Want to try this with your clock source? Download our step-by-step tutorial, A Robust Method for Measuring Clock Jitter.

You can also watch Dr. Eric Bogatin demonstrate in the on-demand webinar, “The Impact of Power Rail Noise on Clock Jitter.”

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