SPECTRUM ANALYSIS

The heterodyning process can be analyzed by using SPECTRUM ANALYSIS (the display of electromagnetic energy arranged according to wavelength or frequency). As shown in figure 1-18, spectrum analysis is an effective way of viewing the energy in electronic circuits. It clearly shows the relationships between the two fundamental frequencies (10 and 11 hertz) and their sum (21 hertz) and difference (1 hertz) frequencies. It also allows you to view the BANDWIDTH (the amount of the frequency spectrum that signals occupy) of the signal you are studying.

Figure 1-18. - Spectrum analysis of heterodyned signal.

TYPICAL HETERODYNING CIRCUIT

Two conditions must be met in a circuit for heterodyning to occur. First, at least two different frequencies must be applied to the circuit. Second, these signals must be applied to a nonlinear impedance. These two conditions will result in new frequencies (sum and difference) being produced. Any one of the frequencies can be selected by placing a frequency-selective device (such as a tuned tank circuit) in series with the nonlinear impedance in the circuit.

Figure 1-19 illustrates a basic heterodyning circuit. The diode D1 serves as the nonlinear impedance in the circuit. Generators G1 and G2 are signal sources of different frequencies. The primary of T1, with its associated capacitance, serves as the frequency-selective device.

Figure 1-19. - Typical heterodyning circuit.

The principles of this circuit are similar to those of the block diagram circuit of figure 1-16. Notice in figure 1-19 that the two generators are connected in series. Therefore, the resultant waveform of their combined frequencies will determine when the cathode of D1 will be negative with respect to the anode, thereby controlling the conduction of the diode. The new frequencies that are generated by applying these signals to nonlinear impedance D1 are the sum and difference of the two original frequencies. The frequency-selective device T1 may be tuned to whichever frequency is desired for use in later circuit stages. Heterodyning action takes place, intentionally or not, whenever these conditions exist. Heterodyning (MIXING) circuits are found in most electronic transmitters and receivers. These transmitter and receiver circuits will be explained in detail later in this module.

Q.14 Define the heterodyne principle.
Q.15 What is a nonlinear impedance?
Q.16 What is spectrum analysis?
Q.17 What two conditions are necessary for heterodyning to take place?