In this article, the detailed explanation of a signal generator is given. The principles of signal modulation, the block diagram of an AM signal generator and the measures needed to achieve a stable frequency output is explained below.
Like an oscillator, a signal generator is also a source of sinusoidal signals. The main difference between a signal generator and an oscillator is that a signal generator is capable of modulating its sinusoidal output signal with other signals. When signal generators are used for producing an unmodulated sinusoidal output they are said to be producing continuous height wave [CW] 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 Generator – Applications
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.
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.
- Regulated power supply is used as a change in the frequency of output voltage brings a change in the supply voltage.
- 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.
- The L-C oscillator should be replaced by a quartz crystal oscillator so as to make the Q-factor of L-C circuit as high as 20,000.
A modulation oscillator is also used for generating an audio-frequency modulating signal. This oscillator is highly stable. Different techniques are used in the modulation oscillator in order to change the frequency and the amplitude of the signal being generated.
There are also other steps taken in this oscillator to get various types of waveforms such as the square, triangular waves or pulses. An output amplifier is used to take in the radio-frequency and the modulation-frequency signals. The output amplifier is basically a wide-band amplifier. A meter is used to adjust and indicate the percentage of modulation.
A control device is used to adjust the modulation level up to 95%. The signal then reaches the output of signal generator after it is fed to an attenuator. Output meter is provided to read the final output signal.
Another important factor that determines the signal generator performance is the accuracy to which the frequency of the RF oscillator is known. 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. If more accuracy is 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 important specification of a signal generator is its amplitude stability. As the radio frequency is always varied, it is very important that the amplitude of the output signal remains constant.