Figure 2-9.Basic heterodyne meter (block diagram).
Q-7. In a heterodyne-type frequency meter, what is the difference between the oscillator frequency and
the unknown frequency?
For all practical purposes, the point of zero beat can be assumed when the clicks are heard at
infrequent intervals. Figure 2-10 illustrates the zero-beat concept. Maintaining a condition of absolute
silence in the earphones is extremely difficult when you are making this measurement. When the
incoming signal is strong, the clicks are sharp and distinct. When the signal is weak, the zero-beat
condition is evidenced by a slowly changing "swishing" or "rushing" sound in the headset. After the zero
beat is obtained, the dial reading corresponds to the frequency measured.
Figure 2-10.Graph of sound heard in earphone when zero beating.