AM/FM Signal Generator Block Diagram and Working

AM/FM Signal Generator

The A.M./F.M. signal generator serves the purpose of generating the amplitude modulated signals as well as the frequency modulated signals. It is a two in one generator. The A.M. section will be useful in testing A.M. receivers. The F.M. section will be useful in testing the F.M. receiver circuits. A block diagram of such an A.M. / F.M. signal generator is shown in Figure.

Block Diagram of an A.M./F.M. Signal Generator  

The block diagram of an A.M./F.M. signal generator is show in Figure below.

AM/FM Signal Generator Block Diagram

Working of A.M./F.M. Signal Generator

The R.F. signal is generated from 100 kHz to around 200 MHz. This is done by an R.F. oscillator. This oscillator will be a well-designed oscillator. The frequency of this oscillator is tunable in different bands of frequency using a range selector switch. Provision will be made to effect frequency modulation of audio signal. This is done using a varactor diode. The varactor diode will be across the tuned circuit of the R.F. oscillator. Variation of the bias on the varactor diode causes frequency modulation. This variation will be produced by the application of the modulating signal to the varactor diode circuit, when the F.M. mode of modulation is selected through the modulation mode switch.

The modulator stage modulates the audio signal in the amplitude modulation mode. In frequency modulation mode its modulating input will be disabled. So in the F.M. mode it only acts as an R.F. amplifier amplifying the frequency modulated signal. However in the amplitude modulation mode, its input is the modulating signal supplied through the modulation mode selector switch.

Hence it acts as a modulator circuit. The amplitude modulated signal, or the amplified frequency modulated signal (depending on the setting of the modulation mode switch) will be applied to the R.F. amplifier. This enhances the magnitude of the signal. 'Through an attenuator (coarse and fine) the output reaches the output socket.

The audio frequency signal for modulation is generated from a stable audio frequency oscillator. The waveform of this signal is made as pure as possible. The controls for the audio signal amplitude, amplitude of R.F. signal are provided. The provision to control the modulation index is effected by providing a monitor for audio signal level, as well as R.F. signal level. A single indicating instrument can be used with a switch to monitor both the levels.

In the amplitude modulation mode, the audio signal is directly connected to the modulator from the audio frequency oscillator. Amplitude modulation takes place. The amplitude modulated signal will as usual be present at the output terminals.

If the modulation mode switch is put on to F.M. side the audio signal will reach the varactor diode circuits. Hence the (carrier) R.F. signal will be frequency modulated. This reaches the modulator. The modulators modulating input is disconnected in this mode.

Hence it acts as only an R.F. amplifier and gives in its output an amplified radio frequency signal. The signal is further amplified and given to the output terminals through an attenuator.