Lab 2 - EE 420L 

Steven Leung

2/3/15

Leungs@unlv.nevada.edu 

 

Prelab

 
  

Lab description:  

 Lab Requirements
 
             
Introduction:
 
The purpose of compentating probes is that when measuring with for example a wire (coaxial), the wire itself has a capacitiance and if one were to probe with a wire, they are adding a capacitor to the circuit. The effect of this is that it will introduce a RC time constant to the circuit if you are probing across a resistor causing some sort of delay and resulting in your circuit not changing as expected. For example if you are expectating a square wave output but just use a wire (coxial cable) to probe the output to the oscilloscope, the signal that you will get will not be a perfect square wave but one that has a significantly longer rise time. the solution to this is adding a capacitor in parallel to a resistor to "compensate" for the capacitiance in the wire. The resistor is picked depending on the input resistance of the scope and is picked to the specification that after considering a voltage divider, the output is .1 of the input (for 10x probe) or .01 of teh input (for 100x probe).
     
 
                                 
              Figure 1 (Under compensated)                                                   Figure 2 (Over copensated)                                                Figure 3 (Perfectly compensated)
 
Since the capacitor that is connected at the probe tip needs to be picked depending on the capicitance of the wire and the internal capacitiance of the scope, it is usually a variable capacitor that can be adjusted with a screwdriver. Figure 1 shows an undercompensated probe in which the signal is overshooted which means the capacitor is too large. Figure 2 shows an over compensentated probe in which the signal is undershooting and this means the capacitor is too small. Lastly, Figure 3 shows a pervectly compensated scope probe where there is no over shoot or undershoot.
 
Notice that in Figures 1-3, the probe setting on the oscilloscope is 10x becuase we are using a 10x probe.
 
 Compensated Probe Tip Circuit (10x)
 
                  
                                                 Figure 4                                                                                                                                                   Figure 5      
 
Figure 4 shows the skamatic for a compensated probe tip which uses resistors and capacitors to model the capacitiance of the wire, input capacitiance of the scope, and input resistance of the scope. The part of the circuit where C_tip is parallel with R1 is the circuit inside the tip of the probe that is used to compensate for the wire capacitiance, input capacitiance, and input resistance. Figure 5 shows the simulation results of the skamatic in figure 4. Notice that the rise time of the scope input and the probe tip are very close and that the scope input is exactly .1 of the probe tip becuase this is the design for a 10x probe.
 
Hand calculations to match simulation:
 
 
                                                                                                   Figure 6
 
Measuring the capacitiance in a cable
 
To measure the capacitiance in a cable, we made a simple RC circuit wtith a 1 MEG resistor in series with a cable (coxial) to represent the capacitor.  The input of this circuit was a 1V square wave. Using the results from the output of this circuit we can calculate the value of  capacitor in the RC circuit and therefore the capacitiance of the cable. (see figure 7 and 8) Figure 7 show the time time it takes for the voltage across the capacitor to charge to 1/2 of the pulse voltage.  The value of the capacitor is calculated to be 149 pF. When measuring the capacitiance of the wire with a multimeter, the result was 128 pF which is close enough to our calculations.
 
   
           
                                                              Figure 7                                                                                                                          Figure 8
 
 
Difference between measuring with a cable (coxial) versus a compensated probe
 
If we consider a simple 100K 100K resistor voltage divider with a 1MEG Hz square wave signal, using a cable (coxial) cable to measure the output of the voltage divider will be similar to adding a large capacitor parallel to the second resistor. What will happen at higher frequencies (the time the source is on or off is shorter) such as 1MEG Hz is that since the capacitor is so large that it will not have enough time to charge and the only thing we will see on the olliscope is a flat 0V DC line.(figure 9) The capacitor will start to charge but before it charges to a point where we can see it on the scope, the square wave is not off and it begins to discharge. On the other hand, if we were to use a compensated probe, the capacitiance that we are adding parallel to the second resistor is significantly reduced. The result of this is that with a smaller capacitiance, even at high frequencies it is able to charge and therefore we are able to see the change in voltage of the output of the scope as the square wave changes from +/- 1V.(figure 10)
 
 
                        
                                            Figure 9                                                                                                                                Figure 10
 
 
Test point on a printed circuit board
 
 In order to implement a test point on a printed circuit board so that a known length of cable  could be connected directly to the board and not load the circuitry is to add the probe tip circuit before the test point. The input resistance and capacitiance of the scope is known along with the capacitiance of the wire (since length is known) . Therefore by picking if you want a 100x attenuation or 10x attenuation (pick 9MEG or 99 MEG resistor), we can simulate the circuit and find the capacitiance value that will perfectly compensate the measurement and add just that part to the PCB. This is similar to putting the probe tip on the circuit itself (or PCB) and then when probing with a coxial cable, it is like completing an oscilloscope probe. 
 
 
Return to EE 420L Labs