Recitation section number (or day/time):___________________________

PHYS345 Second Midterm Exam November 24, 1998

This is a closed book exam. One 3"x5" note card is permitted; this card should be turned in with your exam papers.

Programmable calculators and graphing calculators may be used during this exam.

Since this exam booklet may be separated for grading; it is important to:

Show ALL work on problem sheet and only on that sheet.

Please read questions carefully.

Credit may be lost inadvertently if solutions are not neat and orderly.

Be careful with units, signs, and significant figures.

1. (20 points)

Justify your answers in all cases!

  1. A 20 ns, 5.0 V pulse is launched into one end of 10 m coax cable having a characteristic impedance of 50 ohm. What is the amplitude of the pulse observed at the other end of the cable if it has been terminated with a 75 ohm resistor?

  2. Use Karnaugh mapping to find a suitable combination of two-input gates for this truth table.

     BA 00  01  11  10 
     00    0 1 0 0
     01    1 1 0 1
     11    1 1 0 1
     10    0 1 0 0

  3. An advocate of two-input logic gates has assembled the combination of gates shown. Construct its truth table and suggest a single three-input gate that can replace this mess.

    A B C
    0 0 0
    0 0 1
    0 1 0
    0 1 1
    1 0 0
    1 0 1
    1 1 0
    1 1 1

    Uninteresting combination of three gates

2.   (20 points)

The flashing lights seen on barricades at street work and construction sites are driven by relaxation oscillator circuits such as the one shown in the figure, with N being a neon lamp. The lamp is nonconducting until the voltage across it (and the capacitor also) rises to a value of 45 V whereupon it "ignites," dropping its resistance almost instantly to zero and discharging the capacitor. Then the lamp again becomes nonconducting and the process repeats.

If the emf of the battery is 60 V and C = 0.10 microfarad, what should R be for the lamp to flash every 1.5 sec?

Flashing neon lamp circuit

3.   (20 points)

A tachometer transducer is producing a signal of 50 mV at 20,000 rpm; the tachometer has a resistance of 10k. A signal amplifier is being used to amplify the signal for display on an inexpensive oscilloscope; the amplifier has a gain of 40dB and an input impedance of 100k. What is the voltage of the signal at the oscilloscope?

It turns out that there is a noise source of 10 mV at 60 Hz arising from pickup from a neighboring power transformer. What is the ratio of the signal voltage to the noise voltage at the oscilloscope in dB?

A technician decides to ac couple the tachometer to the amplifier with a capacitor of 0.005 microfarad to suppress the 60 Hz noise. What is the new signal-to-noise ratio?

4.   (20 points)

Using D-type flip-flops and a single two-input logic gate, design a 2-bit state counter that will execute the sequence in this state diagram:

Show the type of gate and the connections to the flip-flops needed for this counter on the schematic below:

2-bit counter

2,1,0 3-state diagram

5.   (20 points)

We have worked exclusively with D flip-flops to date. The behavior of the D FF may be summarized in its truth table: Truth table for D-type flip-flop
There is another important flip-flop, known as the JK flip-flop which may summarized by a different truth table: Truth table for JK-type flip-flop
Apply the same strategies that you have used with D FF circuits to analyze the timing diagram of this circuit: 2-bit counter using JK flip-flops
Timing diagram
Complete the state diagram to show the behavior of this 2-bit counter: 2-bit state diagram

Last updated November 24, 1998.
Copyright George Watson, Univ. of Delaware, 1998.