Generation & Detection of FM Application of FM Angle Modulation Part 2 Generation & Detection of FM Application of FM

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

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.

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

PLL Demodulator The phase detector produces an average output voltage that is linear function of the phase difference between the two input signals. This low frequency component is selected by LPF. After amplification, part of the signal is fed back through VCO where it results in frequency modulation of the VCO frequency. When the loop is in lock, the VCO frequency follows or tracks the incoming frequency.

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

Comparison AM and FM 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.

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)

Summary (cont’d)- Comparison NBFM&WBFM

Part 3 Advantages Disadvantages ANGLE MODULATION Part 3 Advantages Disadvantages

Advantages Wideband FM gives significant improvement in the SNR at the output of the RX which proportional to the square of modulation index. Angle modulation is resistant to propagation-induced selective fading since amplitude variations are unimportant and are removed at the receiver using a limiting circuit. Angle modulation is very effective in rejecting interference. (minimizes the effect of noise). Angle modulation allows the use of more efficient transmitter power in information. Angle modulation is capable of handing a greater dynamic range of modulating signal without distortion than AM.

Disadvantages Angle modulation requires a transmission bandwidth much larger than the message signal bandwidth. The capture effect where the wanted signal may be captured by an unwanted signal or noise voltage. Angle modulation requires more complex and inexpensive circuits than AM.

END OF FREQUENCY MODULATION