In a previous post, we described A Robust Method for Measuring Clock Jitter with Oscilloscopes 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 post and the next, we’ll show how to make use of the clock period track function to match jitter variations to possible sources of jitter, in particular to voltage variations on the clock power rail. The offset voltage of a function generator powers a clock signal source. By creating a known variation in the function generator output, we can match that to the resulting clock jitter to calculate the clock jitter sensitivity to rail voltage changes. A known clock jitter sensitivity value can help you predict how a design will respond to rail voltage changes.
As in our previous post, the clock is a 5-stage ring oscillator based on the the 74AC14 hex inverter, powered by a 5 V rail. The test instrument is a WavePro HD 12-bit, 4-Ch, 8 GHz, 20 GS/s, 5 Gpts oscilloscope with 50 ps time resolution and 60 fs intrinsic sample clock jitter, which is used to measure a square wave clock signal between 10 and 66 MHz. For this experiment, we also use a 5 V DC clean power source to test what our clock jitter is with an “ideal” rail, and a function generator to generate a perturbing signal that will put noise on our 5 V power rail so we can test how the clock jitter responds to it.
1. Measure Clock Jitter Using a Clean Power Source
Figure 2. Measuring clock period with clock powered by a clean source. |
2. Create a Power Source with a Known Voltage Perturbation
Next, we connect the output of the function generator to another oscilloscope input channel. In our examples, C2 (pink) is the clock signal and C3 (blue) is the function generator output.
Figure 3. Function generator output square wave. |
The function generator is set to output a 200 mV peak-to-peak, 10 kHz square wave with a 5 V offset. We connect the function generator’s sync output to the oscilloscope’s Ext. input so that we can set a 50% Edge trigger on the function generator output. Figure 3 shows the function generator output simulating a 5 V power rail with a small square wave perturbation.
3. Connect the Perturbed Power Source to the Clock
Figure 4. The function generator output connected to the clock. Note the drop in voltage. |
Our power trace is now off the screen, so we readjust the V/div and Offset to get it back on the screen as shown in Figure 4. The Mean measurement parameter (P4 on C3) reads the mean amplitude of the power source.
You may wonder why the mean output level has dropped from 5 V to 3.3 V. When the power source is connected to the clock, the voltage decreases significantly, due to the loading of the power source by the ring oscillator.
Figure 5. A 400 mVpp oscillation related to the clock appears on the power trace. |
4. Investigate the Relationship Between the Clock and Power Source
Since this oscillation happened after the power source was connected to the clock, and not before, it is obviously somehow related to the clock. As it happens, the ring oscillator load changes with every output state change, causing the oscillation.
Figure 6. The noise on the power rail and the clock output are synchronous. The 48 MHz frequency noise is from the clock. |
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