Construction and Working of Radio Frequency Signal Generator

Construction and Working of Radio Frequency Signal Generator Points : construction and working of radio frequency signal generator The block diagram of a signal generator is shown in fig. An RF oscillator is employed for generating a carrier wave form whose frequency can be adjusted typically from about 100KHz to 30 MHz. Carrier wave frequency can be varied and indicated with the help of 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.
1) Frequency of output voltage changes with the change in supply voltage so regulated power supply is used.
2) 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.
3) Temperature also causes change in oscillator frequency, so temperature-compensating devices are used.
4) Q — factor of LC circuit should be very high, say about 20.000. This can be achieved by employing quartz crystal oscillator in place of LC 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 the meter indicates it. I

Modulation level can be adjusted up to 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 measurement. 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 very important that the amplitude of the output signal remains constant as the RF frequency is varied.