3-1
CHAPTER 3
QUANTITATIVE MEASUREMENTS
LEARNING OBJECTIVES
Upon completion of this chapter, you will be able to do the following:
1. Explain the purposes and benefits of performing quantitative measurements.
2. Identify the various methods of performing impedance measurements.
3. Identify the various methods of performing power measurements.
4. Identify the various methods of performing frequency measurements.
INTRODUCTION TO QUANTITATIVE MEASUREMENTS
You have already studied the basics of performing electronics measurements and how to determine if
a component is or is not functioning properly. This chapter will cover techniques used in measurements of
specific impedance, frequency, and power. These measurements are extremely important to you in
evaluating the performance of a piece of electronic equipment.
IMPEDANCE MEASUREMENTS
Impedance measurements are often used during routine test procedures. Impedance-measuring
equipment, such as impedance bridges, are mainly used in determining the capacitance and inductance of
component parts. However, the values of combined circuit constants also may be obtained and used in
direct calculations of impedance. An impedance measurement effectively totals the inductive and
capacitive reactance together with the resistance in a circuit. In addition, impedance measurements are
useful in testing and analyzing antenna and transmission line performance and for determining the figure
of merit (Q) of electrical parts and resonant circuits.
Q meters are impedance-measuring instruments that determine the ratio of reactance to resistance of
capacitors or inductors and resistors. Details of Q meters and impedance bridges as well as a number of
other methods of measuring circuit impedance are described in the following paragraphs. Also discussed
are methods of measuring the impedance of antennas and transmission lines.
BRIDGE METHODS
Bridges are among the most accurate types of measuring devices used in the measurement of
impedance. In addition, bridges are also used to measure dc resistance, capacitance, and inductance.
Certain types of bridges are more suitable for measuring a specific characteristic, such as capacitance or
inductance. Basic schematics for the various bridge circuits are shown in figure 3-1. The bridge circuits
shown are similar in that they usually contain two branches in the measuring circuit, two branches in the
comparing circuit, a detector circuit, and a power circuit, as shown in figure 3-2. The bridge shown in
figure 3-2 is actually the dc Wheatstone bridge; however, the general principles of circuit operation for ac
remain the same.
