In a related tutorial, we described A Robust Method for Measuring Clock Jitter as variation in a clock signal's period. Clock jitter is characterized by the standard deviation (sdev) of the clock period measurement. The track function of the clock period sdev shows us the variations in jitter over time, synchronous with the waveform source.
In this tutorial, we'll make use of the oscilloscope track function to match clock jitter variations to possible sources of jitter. The tutorial will investigate how variations in the voltage level of a power rail, including ripple and noise, affect the clock jitter level. A known variation in the power rail voltage is created using a function generator. The offset voltage of the function generator provides the power to the clock signal source. The square wave output of the function generator is the perturbing signal. We'll measure both the perturbing voltage and resulting clock jitter to determine the clock jitter sensitivity to rail voltage changes.
You can perform these procedures in your lab using any clock signal source that is powered by a 5 V rail.
Besides your clock signal source, you will need:
- A real-time oscilloscope capable of sampling at least 5x the signal bandwidth based on the signal rise time (bandwidth = 5*(0.35/trise) for oscilloscopes with single-pole frequency response)
We used a WavePro HD 12-bit, 4-Ch, 8 GHz, 20 GS/s, 5 Gpts oscilloscope with 60 fs sample clock jitter to measure a square wave signal between 10 and 66 MHz. We recommend at least 2-Ch, 2 GHz, 20 Gs/s with track, histogram and low-pass filter functions.
- Function generator , 50 Ω output impedance, 10 kHz square wave, 200 mV amplitude, 10 V offset range
- Clean power source , 5 V DC
- Two 50 Ω coaxial cables to input signals from clock and function generator, or equivalent single-ended probe to input your clock signal
- Coaxial tee adaptor
The clock source used in our examples is a 5-stage ring oscillator based on the 74AC14 hex inverter powered by a 5 V rail, shown embedded in a PCB in Figure 2. The logical state of the stage 5 output is inverted from the original stage 1 input, resulting in oscillation with a period equal to 10 times the propagation delay of each hex inverter stage. The specified propagation delay for a power level of 5 V is from 1.5 ns to 10 ns, so the clock period is anywhere in the range of 15 ns to 100 ns, for a frequency anywhere from 66 MHz to 10 MHz. This extreme variation is why it is so important to test clock jitter in situ for every application.
1. Recall the oscilloscope's default setup to bring it into a known state.
2. Connect your clock signal source to the clean 5 V power source and an oscilloscope input channel.
3. On the oscilloscope:
a. Set the clock channel to 50 Ω input termination, 1 V/div vertical scale.
b. Set the timebase to 5 ns/division.
c. Set a 50% Edge trigger on the clock channel.
4. Use parameters with statistics on to measure the clock signal rise time, frequency and period.
The display should be similar to Figure 2. This setup shows the period of the clock signal (P3) when measured with a stable power source. The period sdev is a figure of merit for the clock signal jitter.
5. Connect the output of the function generator to an oscilloscope input channel.
In our examples, C2 (pink) is the clock signal and C3 (blue) is the function generator output.
6. Set the oscilloscope channel that is inputting the function generator to:
a. 1 MΩ input termination
b. 200 mV/div with 5 V offset
7. Set the oscilloscope timebase to 20 µs/div, fixed sampling rate 10 GS/s or greater. We sampled at 20 GS/s.
8. Set the function generator to output a 200 mV peak-to-peak, 10 kHz square wave with a 5 V offset.
9. Connect the function generator's sync output to the oscilloscope's Ext. input.
10. Set a 50% Edge trigger on the Ext. trigger source.
The function generator display should be similar to Figure 3.
11. While maintaining the connection from the function generator to the oscilloscope, use a coaxial tee adaptor to replace the stable power input to the clock with the function generator output.
Note: From here on, we'll refer to the function generator output as the power source and its trace as the power trace. The power trace will now be off the screen.
12. Readjust the V/div and Offset to get the power trace back on the screen as shown in Figure 4.
13. Apply the (Vertical group) Mean measurement parameter with statistics on to the power trace and read the mean amplitude of the power source.