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 observed in earphone when zero beating.

The manufacturer's calibration book is a very important part of the frequency meter package; in fact, the book is so important that it bears the same serial number as the heterodyne-type frequency meter itself. Contained in this book is a list of the dial settings and the corresponding frequencies produced by that meter at those dial settings. Operating instructions for the meter are also included.

Absorption Wavemeter

WAVEMETERS are calibrated resonant circuits used to measure frequency. The accuracy of wavemeters is not as high as that of heterodyne-type frequency meters; however, they have the advantage of being comparatively simple and can be easily carried.

Q.8 What equipment uses a calibrated resonant circuit to measure frequency?

Any type of resonant circuit can be used in wavemeter applications. The exact kind of circuit used depends on the frequency range for which the meter is intended. Resonant circuits consisting of coils and capacitors are used for VLF through VHF wavemeters.

The simplified illustration of an absorption wavemeter, shown in figure 2-11, consists of a pickup coil, a fixed capacitor, a lamp, a variable capacitor, and a calibrated dial. When the wavemeter's components are at resonance, maximum current flows in the loop, illuminating the lamp to maximum brilliance. The calibrated dial setting is converted to a frequency by means of a chart, or graph, in the instruction manual. If the lamp glows very brightly, the wavemeter should be coupled more loosely to the circuit. For greatest accuracy, the wavemeter should be coupled so that its indicator lamp provides only a faint glow when tuned to the resonant frequency.

Figure 2-11. - Absorption wavemeter circuit.

FREQUENCIES ABOVE THE AUDIO RANGE

The signal frequencies of radio and radar equipments that operate in the UHF and SHF ranges can be measured by resonant, cavity-type wavemeters or resonant, coaxial-line-type wavemeters. When properly calibrated, resonant-cavity and resonant-coaxial line wavemeters are more accurate and have better stability than wavemeters used for measurements in the LF to VHF ranges. These frequency-measuring instruments are often furnished as part of the equipment. They are also available as general-purpose test sets.

Although many wavemeters are used in performing various functions, the cavity-type wavemeter is the type most commonly used. Only this type is discussed in some detail.

Cavity Wavemeter

Figure 2-12 shows a typical CAVITY WAVEMETER.

The wavemeter is of the type commonly used for the measurement of microwave frequencies. The device uses a resonant cavity. The resonant frequency of the cavity is varied by means of a plunger, which is mechanically connected to a micrometer mechanism. Movement of the plunger into the cavity reduces the cavity size and increases the resonant frequency. Conversely, an increase in the size of the cavity (made by withdrawing the plunger) lowers the resonant frequency. The microwave energy from the equipment being tested is fed into the wavemeter through one of two inputs, A or B. The crystal rectifier then detects (rectifies) the signal. The rectified current is indicated on current meter M.

Figure 2-12. - Typical cavity wavemeter.

Electronic Frequency Counters

Another device used to measure frequencies above the audio range is the ELECTRONIC FREQUENCY COUNTER. Since this instrument will be covered in detail in a later chapter, only a brief description is provided at this time.

The electronic frequency counter is a high-speed electronic counter with an accurate, crystal-controlled time base. This combination provides a frequency counter that automatically counts and displays the number of events occurring in a precise time interval. The frequency counter itself does not generate any signal; it merely counts the recurring pulses fed to it.

WAVEFORM ANALYSIS

WAVEFORM ANALYSIS can be made by observing displays of voltage and current variations with respect to time or by harmonic analysis of complex signals. Waveform displays are particularly valuable for adjusting and testing pulse-generating, pulse-forming, and pulse-amplifying circuits. The waveform visual display is also useful for determining signal distortion, phase shift, modulation factor, frequency, and peak-to-peak voltage.

Waveform analysis is used in various electrical and electronic equipment troubleshooting. This section will briefly discuss the oscilloscope and spectrum analyzer to provide you with basic knowledge of this test equipment.

Q.9 Name two instruments used to analyze waveforms.