Lab 2 – EE 420L
Authored by: Daniel Senda
Email: sendad1@unlv.nevada.edu
Spring 2019
Due: 02-06-2019

 

1) Introduction

This lab is intended to help students learn how to properly test circuitry using a correctly compensated oscilloscope probe and the importance on setting equipment up properly.

2) Pre-Lab Description

The pre-lab required the student to complete the following before proceeding with lab:

-       Watch scope probe video and read review.

-       Change parameters of scope probe simulation to get a good understanding of a 10:1 probe.

-       Understand operation and analysis of simple RC circuits.

-       Understand and know bode plots.

-       Read the entire lab write-up before going to class.

3) Description of Lab Procedures

In order to get accurate readings from an oscilloscope, the user is required to use of the appropriate probes that are compensated correctly. If a probe is not compensated correctly, it will give the user distorted results. The first part of the lab procedures had the student undercompensate, overcompensate, and correctly compensate a 10:1 probe.

Undercompensated probe:

Overcompensated probe:

Correctly compensated probe:

The oscilloscope was used was set to read a 10:1 probe. If the oscilloscope were to be on the 1:1 probe setting, it would still read the signal but the scale of the reading would be off by a factor of 10. Having the correct setting is important to get correct readings. (Picture of probe):

The following schematic is of a 10:1 oscilloscope probe. It shows the 9MΩ resistor and 10-20pF capacitor (which can typically be adjusted to properly compensate the probe) that are found in the tip of the probe. The cable of the probe also has a capacitance that is about 90pF per meter. The other end of the probe connects to the oscilloscope, which usually has a 1MEG resistance and a 15pF capacitance.
LTspice schematic of compensated probe circuit:

 

The following calculations show how the Vscope voltage is ten times smaller than Vtip. Also, note that the probe tip capacitance should be adjusted to 45pF (in this case) to get a precise 10:1 probe reading.

To show that the voltage at Vscope is 1/10th of the voltage at Vtip, the following assumptions were made:

Variable

Vtip

Ctip

Rtip

Ccable

Cscope

Rscope

Vscope

Value

1V

45pF

9MΩ

90pF

15pF

1MΩ

Will solve








 

The next lab procedure was to create an experiment circuit to determine the capacitance of the a cable. A simple RC circuit was made, with a resistor value of 100kΩ. The piece of cable was put into the circuit as a capacitor. The next step was to sweep the frequency of the input source until the student was able to see the charge and discharge time of the capacitor formed by the cable. The RC time constant was measured, and then the capacitance was found through hand calculations. The student verified the calculated capacitance with a capacitance meter.

LTspice schematic of circuit:

Time constant (τ = 8.4µs) measured through the oscilloscope:

Note, the frequency that allowed to capture charging and discharging of the capacitor was 10kHz.

From the τ measurement, the following calculations were made to solve for capacitance:


Capacitance of cable measured through LCR meter:

As can be seen, the calculated capacitance was really close the measured capacitance value of the cable.

 

Following the procedures, the student’s next task was to build a voltage divider using two 100k resistors. The input of the divider was required to be a 1MHz pulse going from 0 to 1V. The student had to measure the output two ways, using a regular cable and using a correctly compensated probe.

LTspice circuit of voltage divider:

Measurements from the oscilloscope of Vout (output) of the voltage divider are shown below:
Uncompensated cable:                                                    Compensated probe:
           
Analyzing the pictures, the uncompensated cable has a distorted signal reading while the compensated probe has a clean signal reading. In conclusion, anyone who uses an oscilloscope should use a compensated probe when measuring to get more precise signal readings (especially at higher frequencies).

 

Lastly, the student was required to discuss the proper implementation of 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 on the board.
- In order to test points on a board with an uncompensated cable without loading the circuitry, the student must take design precautions into consideration. First, the student should implement a resistive and capacitive load (like is seen at the tip of a compensated probe) on the circuit board itself. The student can also incorporate a variable capacitor instead of a fixed capacitor so the capacitance can be adjusted for different lengths of cable. This test-point design would no longer require a compensated probe because the compensation would be already taken care of on the circuit board.

This concludes lab 2. (Lab was backed-up on an external drive)

 

Additional Links

→ Return to listing of lab reports
→ Daniel’s CMOS homepage
→ Dr. Baker’s CMOS homepage