The signal generator, like an oscillator, is a source of sinusoidal signals but the signal generator is also capable of modulating its sinusoidal output signal with other signals. This is the main difference between the two instruments (signal generator and oscillator). When the signal generators are employed for producing an unmodulated sinusoidal output they are said to be producing CW (continuous height wave) signal. When the produced output signal is modulated, the modulating waveforms may be either externally applied sine-waves, square waves, triangular waves, pulses or more complex signals, as well as internally generated sine-waves. Amplitude modulation (AM) or frequency modulation (FM) may be used. Nor­mally amplitude (AM) modulation is employed. Principles of amplitude modulation (AM) and frequency modulation (FM) are illustrated in the figure shown below.

Signal Modulaton

Signal generators are primarily employed for providing appropriate signals for calibra­tion, testing and troubleshooting of the amplifier circuits used in communication, electron­ics such as radio and television amplifiers. They are also employed for measurement of char­acteristics of antennas and transmission lines.

Block diagram of a signal generator is shown in the figure below.

AM sigal generator -Block Diagram

An RF oscillator is employed for generating a carrier waveform whose frequency can be adjusted typically from about 100 kHz to 30 MHz. Carrier wave frequency can be varied and indicated with the help of a range selector switch and a vernier dial setting. Range is selected by employing frequency dividers. Frequency stability of oscillator is kept very high at all frequency ranges.

Following measures are taken in order to achieve stable frequency output.

• Frequency of output voltage changes with the change in supply voltage so regulated power supply is used.
• Buffer amplifiers are used to isolate the oscillator circuit from output circuit so that any change in the circuit connected to the output does not affect the frequency and amplitude of the oscillator output.
• Temperature also causes change in oscillator frequency, so temperature compen­sating devices are used.
• Q-factor of L-C circuit should be very high, say above 20,000. This can be achieved by employing quartz crystal oscillator in place of L-C oscillator.

An audio-frequency modulating signal is generated in another very stable oscillator, called the modulation oscillator. Provision is made in the modulation oscillator for changing the frequency and the amplitude of the signal being generated.

In this oscillator provision is also made to get various types of waveforms such as the square, triangular waves or pulses. The radio-frequency and the modulation-frequency sig­nals are fed to a wide-band amplifier, called the output amplifier. Percentage of modulation ₍ can also be adjusted and it is indicated by the meter.

Modulation level can be adjusted upto 95% by a control device. The output of the am­plifier is then fed to an attenuator and finally the signal goes to output of signal generator. Output meter is provided to read the final output signal.

The accuracy to which the frequency of the RF oscillator is known is an important specification of the signal generator performance. Most laboratory type models are usually calibrated to be within 0.5 – 1.0% of the dial setting. This accuracy is usually sufficient for most measurements. For greater accuracy, if needed, a crystal oscillator, whose frequency is known to be within 0.01% or better, may be used as an internal RF calibration source.

Another key specification of signal generators is their amplitude stability. It is very important that the amplitude of the output signal remains constant as the RF frequency is varied.