Experiment 6 Oscilloscope Laboratory Practical ET 304B

Objective: Learn the basic theory of operation of a typical oscilloscope. Also learn the correct procedures for using the instrument to make measurements in the laboratory.

Discussion

The oscilloscope is a valuable tool for both design and troubleshooting. Knowledge of the instrument's theory of operation aids in the correct application of the instrument in the laboratory. This knowledge will lead to correct measurements and the safe use of the instrument.

The oscilloscope is a device for displaying repetitive events in terms of a voltage on the vertical axis and time on the horizontal axis. Any response that can be converted to a time varying electric signal can be displayed on the oscilloscope. The most common use of the instrument in electrical/electronic design is to display time varying waves.

Figure 1 shows a block diagram of a typical oscilloscope's internal circuits.

Channel 1

Channel 1 Delay Vertical Preamp. Line Vertical Output Switching Channel 2 Amp. Circuits Channel 2 Preamp.

Trigger C.R.T. Circuits

External Trigger

High Sweep Generator Voltage and Power Low Voltage Horizontal Amp. Supply Power Supply

Figure 1. Block Diagram of Typical Oscilloscope. Signals enter the oscilloscope through the channel 1 and 2 inputs. The vertical preamplifiers are accurate, wide-band amplifiers connected to a voltage divider attenuator network. The incoming signals are routed through the input coupling switch to the

Spring 2002 scope04b.doc preamplifier/attenuator. At this point, a portion of the signal is diverted to the Sweep Generator by the trigger circuits. The vertical signal is then sent on the delay line. The delay line provides a short time delay so the leading edge of the desired signal can be viewed. From the delay circuit, the vertical signal is routed to the vertical amplifier. The vertical amplifier provides a voltage to the plates inside the C.R.T. that direct the electron beam vertically onto the face of the scope.

The heart of the horizontal deflection circuit is the sweep generator. The sweep generator produces a ramp wave that is synchronized to the input channels by the trigger circuit. The time base control determines the time required for the ramp wave to reach its maximum value, which controls the time it takes for the C.R.T. trace to traverse the scope face. The sweep generator drives the horizontal deflection amplifiers. These amplifiers connect to plates inside the C.R.T. that direct the electron beam horizontally onto the face of the scope.

The high voltage power supply supplies the potential that produces the electron beam in the C.R.T. The low voltage power supply powers all other sub-circuits in the instrument.

Procedure

Refer to the oscilloscope operating instructions provided in the lab and answer the following questions.

1.) What is the maximum vertical resolution of the scope's vertical input in terms of voltage? 2.) What is the maximum horizontal resolution of the scope's time base (seconds)? 3.) When set to the maximum horizontal resolution, how long does it take for the electron beam to travel across the 10 divisions of the C.R.T. screen? 4.) What is the vertical frequency response of the oscilloscope in the direct coupled mode? 5.) When the Leader LBO-5825 scope is used to measure a 10 volt peak-to-peak signal at 35 MHz it measures 3.5 divisions at 2 V/div. What, if anything, is wrong with the instrument? 6.) A square wave is being viewed using a scope that has a attenuator probe (10x). The trace at low frequencies is shown in Figure 2. What may be wrong with the test setup?

time

Spring 2002 2 scope04b.doc Figure 2. Question 6 Waveform. 7.) The LBO-5825 scope is being used by two people at the same time. Each person is using one of the vertical inputs and the oscilloscope is set for dual trace operation. Unfortunately, only one channel will stay synchronized. What is the problem and how can the triggering be changed to eliminate it. 8.) What would be a good use of the LINE position of the TRIGGER SOURCE switch? 9.) It is determined that the focus and beam rotation controls need adjusting on the scope. a.) Where are these controls located? b.) Can they be adjusted without removing the cabinet from the scope? c.) What is the function of each control? 10.) Build the circuit below and measure the phase angle between the source voltage and the resistor voltage. Use a 10 k potentiometer if the correct resistor is not available. C1

Output

10Vp-p V1 .01uF R1 f=2kHz

8.2k

11.) Describe the procedure used to measure the phase difference between the two ac signals in 10.) 12.) Describe what must be done to the scope to measure the voltage across the capacitor in the circuit above.

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10.) Phase angle = ______degrees

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