Answer :

Principle:

A wave analyzer is an instrument designed to measure relative amplitudes of single frequency components in a complex waveform. Basically, the instrument acts as a frequency selective voltmeter which is tuned to the frequency of one signal while rejecting all other signal components. The desired frequency is selected by a frequency calibrated dial to the point of maximum amplitude. The amplitude is indicated either by a suitable voltmeter or a CRO.

Basic Wave Analyzer:

A basic wave analyzer is shown in Figure 1. It consists of a primary detector, which is a simple LC circuit. This LC circuit is adjusted for resonance at the frequency of the particular harmonic component to be measured.

Figure1: Basic wave Analyzer

The intermediate stage is a full wave rectifier, to obtain the average value of the input signal. The indicating device is a simple DC voltmeter that is calibrated to read the peak value of the sinusoidal input voltage.

Since the LC circuit is tuned to a single frequency, it passes only the frequency to which it is tuned and rejects all other frequencies. A number of tuned filters are connected to the indicating device through a selector switch.

Heterodyne Wave Analyzer:

• Wave analyzers are useful for measurement in the audio frequency range only. For measurements in the RF range and above (MHz range), an ordinary wave analyzer cannot be used.
• Hence, special types of wave analyzers working on the principle of heterodyning (mixing) are used. These wave analyzers are known as heterodyne wave analyzers.

Operation:

• In this wave analyzer, the input signal to be analyzed is heterodyned with the signal from the internal tuneable local oscillator in the mixer stage to produce a higher intermediate frequency.
• By tuning the local oscillator frequency, various signal frequency components can be shifted within the pass-band of the IF amplifier.

• The output of the IF amplifier is rectified and applied to the meter circuit.

   

Figure 2: Heterodyne Wave Analyzer

• The amplitude of the unknown component is indicated by the output meter. The output meter is calibrated by means of signals of known amplitude.
• The frequency of the component is identified by the local oscillator frequency, i.e. the local oscillator frequency is varied so that all the components can be identified. The local oscillator can also be calibrated using input signals of known frequency.
• The fixed frequency amplifier is a multistage amplifier which can be designed conveniently because of its frequency characteristics.
• This Analyzer has good frequency resolution and can measure the entire AF frequency range.
• With the use of a suitable attenuator, a wide range of voltage amplitudes can be covered. Their disadvantage is the occurrence of spurious cross-modulation products, setting a lower limit to the amplitude that can be measured.
• Two types of selective amplifiers find use in heterodyne wave analyzers. The first type employs a crystal filter, typically having a center frequency of 50 kHz. By employing two crystals in a band-pass arrangement it is possible to obtain a relatively flat pass-band over a 4 cycle range. Another type uses a resonant circuit in which the effective Q has been made high and is controlled by negative feedback. The resultant signal is passed through a highly selective 3-section quartz crystal filter and its amplitude measured on a Q-meter.

When knowledge of the individual amplitudes of the component frequency is desired, a Heterodyne Wave Analyzer is used.