EE 420L - Lab 2 - Operation of a Compensated Scope Probe

Jonathan K DeBoy
deboyj@unlv.nevada.edu
6 Feburary 2015

Pre-lab work

 

Introduction

We learned about how probes and cables used for measurements of the circuits we create can affect them. The lab is designed to show us how to properly adjust compensated oscilloscope probes.

Experiment 1 - Compensations Levels of a Probe

Spice Simulations

Figure 1.2

We measured the capacitance of our probe using a multimeter capable of sub-nanoFarad accuracy. The points of measurement were from the tip of the probe to ground (Ctip in series with Ccable) and from the connector middle conductor to ground (Ccable alone).
 
 

Experimental Data

Figure 1.4   Figure 1.3   Figure 1.4
                          Figure 1.1 - Undercompensated                            Figure 1.2 - Overcompensated                  Figure 1.3 - Appropriatly Compensated

Above are the measured wave forms of the same circuit with the same probe, just at different levels of compensation. The first figure is under-compensation where we can see that the Ctip is too small and we have a slower curve on the changes from high and low. The second figure is the same probe, but over compensated, meaning that Ctip is too small and holds an excess charge on build ups like switching from low to high, giving a ringing effect. The last one has been compensated appropriately so we see a nice, crisp waveform.

 

Experiment 2 - Empirical Cable Capacitance Measurement

Experimental Data

Figure 2.4

From our measurement in the lab, we found the point in time at which the voltage across the capacitor was at 64% of the input voltage and assumed that that was after one time constant (explained more below in hand calculations). We measured 0.3387V at Vo from a 0.98968V at Vin at a time of 128.3 ns. With this information, we can calculate the capacitance of the cable.

Hand Calculations

Figure 1.2

Armed with simple RC circuit knowledge, we know that it takes exactly one time constant for an RC circuit to discharge its voltage by 64%. So by devising an experiment with a pulse generator and a cable (not probe), we can use the inherent capacitance of the coaxial cable for our C and select a resistor at our choosing (1k).

 

Experiment 3 - Cable vs Probe

 

Experimental Data

Figure 3.4     Figure 3.5

Figure 3.1 - Cable (1x) Measurement                                             Figure 3.2 - Probe (10x) Measurement

Using a cable, no compensation, means that we are placing a 120pF capacitor across the measuring location to ground. Since the impedance of a capacitor decreasing with higher frequencies (100MEG Hz in this case), this capacitor will act like a short to ground, bypassing the resistor in the Oscilloscope required for measurements, indicating an amplitude of about 0 on screen. The compensated probe has a capacitance in series with this Ccable, significantly reducing, however not eliminating, this effect.

   

Conclusion & Discussion

This was a great experiment for us to get introduced into the magic of probe compensation, and I feel that this should have been introduce a little earlier than a 400 level class but hey, I'm not running the department. As for designing a test point on a printed circuit board (PCB) to be used with a measuring cable of known length (known capacitance), we can place a capacitor in series from the point of desired measurement to reduce the overall capacitance of the cable (compensation circuitry at the point). To diminish frequency dependencies, we can place a slight inductance in series as well. In fact, I have a suspicion that oscilloscopes use a high pass filter and an inductor to better correct for high frequency measurements.

 
 
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