How FM Modulator Works? Frequency of resonance depends on the value of inductance and capacitance Additional capacitance will increase the capacitance, thus reduce the resonance frequency For FM, we want the frequency increase/decrease wrt to amplitude of modulating signal. How??
6 How Do These Modulators Works? The tuned circuit is part of the oscillator used to generate the carrier freq so, if the capacitance changes, then so will the carrier freq. This is demonstrated in Figure below. To produce a freq modulated carrier, it is needed to find a way of making the info signal increase and decrease the size of the capacitance and hence control the carrier freq. How to achieve? – using a device called Varactor Diode and then by using a transistor. Figure: Frequency Modulated Carrier
7 Varactor Diode Varactor diode is a semiconductor diode that is designed to behave as a voltage controlled capacitor. When a semiconductor diode is reverse biased, no current flows and it consists of two conducting region separated by a non- conducting region. This is very similar to the construction of the capacitor. Recall, the reverse biased diode has a capacitance of
8 Varactor Diode By increasing the reverse biased voltage, the width of the insulating region can be increased and hence the capacitance value decreased. Thus, if the info signal is applied to the varactor diode, the capacitance will therefore be increased and decreased in sympathy with the incoming signal. -more capacitance. -less capacitance.
Varactor Modulator Circuit Tuned circuit sets the operating frequency of the oscillator C1 is a DC blocking capacitor to provide DC isolation between the oscillator and the collector of the transistor. L1 is an RF choke which allows the info signal through to the varactor but blocks the RF signals.
The info signal is applied to the base of the input transistor and appears amplified and inverted at the collector. This low freq signal passes through the RF choke (L1) and is applied across the varactor diode. The varactor diode changes its capacitance in sympathy with the info signal and therefore changes the total value of the capacitance in the tuned circuit. The changing value of capacitance causes the oscillator freq to increase and decrease under the control of the information signal. The output is therefore an FM signal. The Operation of the Varactor Modulator
The value of capacitance of varactor at the centre of its linear range is 40-pF. This varactor will be in parallel with a fixed 20-pF capacitor. What value of inductance should be used to resonate this combination to 5.5MHz in an oscillator? Example 1
The incoming signal is passed through a phase- shifting circuit. The degree of phase shift that occurs is determined by the exact freq of the signal at any particular instant. The rules are: If the carrier is unmodulated, the phase shift is 90°. If the freq carrier increase, the phase shift is GREATER than 90°. If the freq carrier decreases, the phase shift is LESS than 90°. Phase comparator circuit is use to detect the changes in the phase of the signal by comparing the phase of the original input signal with the output of the phase shifting circuit.
It then produces a DC voltage level which depends on the result of the comparison according to the following rules: Phase shift = 90°, no change in DC voltage level. Phase shift > 90°, result in increased DC voltage level. Phase shift < 90°, result in decreased DC voltage level. As the phase change, the DC voltage level moves up and down and re-creates the audio signal. A low pass filter is included to reduce the amplitude of any high-freq ripple and also blocks the DC offset. Consequently the signal at the output closely resembles the original input signal.
16 The Phase-Locked Loop (PLL) Detector (Reference) Error Voltage Tuned Voltage used to control the VCO Figure: Block Diagram of PLL Detector PLL is a closed loop feedback control system in which either the frequency or the phase of the feedback signal is the parameter of interest.
17 The Phase-Locked Loop (PLL) Detector When there is no external input signal (FM signal, f i ), the VCO operates at the preset frequency (natural/free-running frequency, f n ) The VCO’s natural freq is determined by external component. It is normally set (locked) to IF center freq. When FM signal applied to the PLL, the phase comparator compares the f i with the VCO output freq. Phase comparator produced error voltage that is proportional to the freq difference (f d = f 0 -f i ) After several cycles around the loop, the VCO’s freq will be equal to FM signal freq. And the loop is said to have acquired freq locked. Once the loop is freq locked, the phase difference between the external input and the VCO’s output is converted to a dc bias voltage. The error voltage is filtered, amplified and applied back to the input of the VCO. Therefore, the error voltage is also proportional to the freq deviation demodulated info signal
18 The Phase-Locked Loop (PLL) Detector A PLL operate in three different modes: Free running Capture Tracking In the free running mode, the input frequency is not close enough to the VCO frequency and the PLL runs at the free running frequency determined by the tuning circuits of the VCO. The error voltage is outside the range of the VCO. As the input frequency gets closer to the VCO frequency, the error voltage reaches a value at which it can begin to change the VCO frequency. This is the capture mode. The error voltage will continue to decrease as the VCO frequency gets closer to the input frequency. Finally, when the VCO is operating at the same frequency as the input, the PLL is in the tracking mode. The VCO will track changes in the input frequency as long as the input frequency remains in a range of frequencies known as the hold-in range.
Noise performance and SNR improvement In FM/PM, limiters reduce noise thus improve SNR ratio during demodulation. In AM, once signal is contaminated with noise, it cannot be remove. Capture Effect FM/PM receiver can differentiate between 2 signals received with the same frequency. The receiver will capture (locked on) the stronger signal and eliminates the weaker signal. In AM, if signals are received at the same freq, all of them will be demodulated and heard. Power Utilization and efficiency In FM/PM, total power remains constant regardless if modulation is present (power is taken from carrier and redistributed to SBs). In AM, total power is the constant carrier power plus SBs powers.
Disadvantages of Angle Modulation Wide bandwidth of the transmission High quality angle modulation produces many side freq, thus it require wider BW than AM. Eg. Commercial AM radio band => 10kHz of BW Commercial FM radio band => 200kHz of BW Circuit complexity and cost Modulation and demodulation cct required for FM/PM are complex than those for AM i.e expensive. But now, it is almost comparable due to advent of IC technology.