PART 3:GENERATION AND DETECTION OF ANGLE MODULATION GENERATION OF DIRECT/INDIRECT FM PLL

Generation of FM Two major FM generation: Direct method: straight forward, requires a VCO whose oscillation frequency has linear dependence on applied voltage. Advantage: large frequency deviation Disadvantage: the carrier frequency tends to drift and must be stabilized. example circuit: Reactance modulator Varactor diode

VOLTAGE VARIABLE CAPACITOR (VVC) A VVC or varactor is a silicon diode that when reversed biased acts as a variable capacitor. Varying the reverse bias voltage changes the capacitance. Increasing the voltage decreases the capacitance and visa versa. The useable capacitance range is from about 5 pF to 100 pF.

FM WITH A VVC +V RFC FM output Crystal Modulating signal in VVC

FM WITH A VVC 1. This circuit is a crystal-controlled oscillator. 2. The divider provides reverse bias for the VVC. 3. When the modulating signal goes positive, the bias increases and the capacitance drops. 5. When the modulating signal goes negative, the frequency decreases. 4. The crystal and oscillator are pulled higher in frequency.

Generation of FM (cont’d) ii) Indirect method: Frequency-up conversion. Two ways: Heterodyne method Multiplication method One most popular indirect method is the Armstrong modulator

Armstrong modulator Integrator Balanced modulator Down converter Frequency multiplier (x n) Crystal oscillator Phase shifter Vc(t) fc Vm(t) fm

Armstrong modulator For example: Let fm =15Hz and fc= 200kHz At frequency deviation= 75kHz,it need a frequency multiplication by a factor, n, n=75000/15=5000; So it need a chain of four triplers (34) and six doublers (26), ie:n= (34) x (26)=5184, But, n x fc=5000 x 200kHz=1000MHz So, down converter with oscillating frequency=900MHz is needed to put fc in the FM band of 88MHz-108MHz

FM Detection/Demodulation FM demodulation is a process of getting back or regenerate the original modulating signal from the modulated FM signal. It can be achieved by converting the frequency deviation of FM signal to the variation of equivalent voltage. The demodulator will produce an output where its instantaneous amplitude is proportional to the instantaneous frequency of the input FM signal.

FM detection (cont’d) To detect an FM signal, it is necessary to have a circuit whose output voltage varies linearly with the frequency of the input signal. The most commonly used demodulator is the PLL demodulator. Can be use to detect either NBFM or WBFM.

TYPES OF FREQUENCY DEMODULATORS A demodulator converts the carrier frequency variations into the original modulating signal. To detect an FM signal, it is necessary to have a circuit whose output voltage varies linearly with the frequency of the input signal. There are six common FM demodulators: Foster Seeley Discriminator Ratio Detector Pulse Averaging Discriminator Quadrature Detector Differential Peak Detector Phase-Locked Loop (PLL) The quadrature detector and PLL are the most common.

PLL Demodulator fVc0 V0(t) VCO FM input Phase detector Low pass filter Amplifier fVc0 VCO Vc(t)

PHASE-LOCKED LOOP In In VCO Error Amplifier Phase detector LPF VCO The VCO locks onto the input signal. In VCO

PLL OPERATION A PLL is a feedback control circuit that compares an input signal to the output of a voltage controlled oscillator (VCO) and adjusts the VCO frequency if it differs from the input signal. If the VCO and input frequencies are different, the phase detector produces an error signal that is filtered into a DC voltage that controls the VCO frequency.

PLL DEMODULATOR If the input to a PLL is an FM signal, the VCO will track or follow the input. The low pass filter output will be the original modulating signal. The PLL demodulator is the best FM detector as it serves as a filter for good selectivity and noise reduction.

PLL PROVIDES FM DETECTION Error Amplifier Out In Phase detector LPF VCO

PLL Demodulator Let instantaneous freq of FM Input, fi(t)=fc +k1vm(t), and the VCO output frequency, f VCO(t)=f0 + k2Vc(t); f0 is the free running frequency. For the VCO frequency to track the instantaneous incoming frequency, fvco = fi; or

PLL Demodulator f0 + k2Vc(t)= fc +k1vm(t), so, If VCO can be tuned so that fc=f0, then Where Vc(t) is also taken as the output voltage, which therefore is the demodulated output

FREQUENCY SHIFT KEYING(FSK) FSK is a variation of FM used to transmit binary data. In FSK, a binary 0 is transmitted as a lower carrier frequency while a binary 1 is transmitted as a higher carrier frequency. FSK is used in radio modems.

FREQUENCY SHIFT KEYING(FSK)

PART 4: ADVANTAGES/DISADVANTAGES OF ANGLE MODULATION APPLICATIONS OF ANGLE MODULATION SUMMARIES

ADVANTAGES OF FM OVER AM Less sensitivity to noise. Noise is amplitude variations that are eliminated in the receiver. Capture effect. Only the strongest signal on the receive frequency gets through. Transmission efficiency. More efficient transmitter amplifiers can be used with FM.

Advantages of FM over AM(cont’d) Its the SNR can be increased without increasing transmitted power about 25dB higher than in AM Certain forms of interference at the receiver are more easily to suppressed, as FM receiver has a limiter which eliminates the amplitude variations and fluctuations. The modulation process can take place at a low level power stage in the transmitter, thus a low modulating power is needed. Power content is constant and fixed, and there is no waste of power transmitted There are guard bands in FM systems allocated by the standardization body, which can reduce interference between the adjacent channels.

DISADVANTAGES OF FM Uses excessive spectrum space. As a result, FM can only be used at the high radio frequencies (VHF and above).

Application of FM used by most of the field VHF portable, mobile and base radios in exploration use today. It is preferred because of its immunity to noise or interference and at the frequencies used the antennas are of a reasonable size.

Summary of angle modulation (what you need to be familiar with)

Summary (cont’d)

Summary (cont’d) Bandwidth: Actual minimum bandwidth from Bessel table: b) Approximate minimum bandwidth using Carson’s rule:

Summary (cont’d) Multitone modulation (equation in general):

Summary (cont’d)

END OF ANGLE MODULATION END OF PART 3& 4. END OF ANGLE MODULATION