Upcoming SlideShare
×

Thanks for flagging this SlideShare!

Oops! An error has occurred.

×
Saving this for later? Get the SlideShare app to save on your phone or tablet. Read anywhere, anytime – even offline.
Standard text messaging rates apply

# Frequency modulation

5,737

Published on

1 Comment
6 Likes
Statistics
Notes
• Full Name
Comment goes here.

Are you sure you want to Yes No
Views
Total Views
5,737
On Slideshare
0
From Embeds
0
Number of Embeds
0
Actions
Shares
0
356
1
Likes
6
Embeds 0
No embeds

No notes for slide

### Transcript

• 1. NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite ASSIGNMENT # 3 “FREQUENCY MODULATION”Cauan, Sarah Krystelle P. July 11, 2011Communications 1 / BSECE 41A1 Score: Eng‟r. Grace Ramones Instructor
• 2. FREQUENCY MODULATIONFrequency Modulation Principles While changing the amplitude of a radio signal is the most obvious method to modulateit, it is by no means the only way. It is also possible to change the frequency of a signal to givefrequency modulation or FM. Frequency modulation is widely used on frequencies above 30MHz, and it is particularly well known for its use for VHF FM broadcasting. In FM, the carrier amplitude remains constant, while the carrier frequency is changed bythe modulating signal. As the amplitude of the information signal varies, the carrier frequencywill shift in proportion. As the modulating signal amplitude increases, the carrier frequencyincreases. If the amplitude of the modulating signal, decreases the carrier frequency decreases.The reverse relationship can also be implemented. A decreasing modulating signal will increasethe carrier frequency above its center value, whereas an increasing modulating signal amplitudevaries, the carrier frequency varies above and below its normal center frequency with nomodulation. The amount of change in carrier frequency produced by the modulating signal isknown as the frequency deviation. Maximum frequency deviation occurs at the maximumamplitude of the modulating signal. The frequency of the modulating signal determines how many times per second thecarrier frequency deviates above and below its nominal center frequency. If the modulatingsignal is 100-Hz sine wave, then the carrier frequency will shift above and below the centerfrequency 100 times per second. This is called the frequency deviation rate. An FM signal is illustrated in Figure 1. With no modulating signal applied, the carrierfrequency is a constant-amplitude sine wave at its normal constant center frequency. The modulating information signal is a low-frequency sine wave. As the sine wave goespositive, the frequency of the carrier increases proportionately. The highest frequency occurs atthe peak amplitude of the modulating signal. As the modulating signal amplitude decreases, thecarrier frequency decreases. When the modulating signal is zero amplitude, the carrier will be atits center frequency point.
• 3. Now when the modulating signal goes negative, the carrier frequency will decrease. Thecarrier frequency will continue to decrease until the peak of the negative half cycle of themodulating sine wave is reached. Then, as the modulating signal increases toward zero, thefrequency will again increase. Note in Figure 1 how the carrier sine waves seem to be first“compressed” and then “stretched” by the modulating signal. a) b) c) Figure 1 The principle of frequency modulation: (a) carrier signal, (b) modulating signal (c) Modulated signal (Frequency Modulation)
• 4. Phase Modulation Another way to produce angle modulation is to vary the amount of phase shift of aconstant frequency carrier in accordance with a modulating signal. The resulting output is a PMsignal. Imagine a modulator circuit whose basic function is to produce a phase shift. Rememberthat a phase shift refers to a time separation between two sine waves of the same frequency.Assume that we can build a phase shifter that causes the amount of phase shift to vary with theamplitude of the modulating signal. The greater the amplitude of the modulating signals, thegreater the phase shifts. Assume further that positive alternations of the modulating signalproduce a lagging phase shift and negative signals produce a leading phase shift. If a constant-amplitude frequency carrier sine wave is applied to the phase shifter, theoutput of the phase shifter will be a PM wave. As the modulating signal goes positive, theamount of phase lag increase with the amplitude modulating signal. This means that the carrieroutput is delayed. That delay increases with the amplitude of the modulating signal. The result atthe output is as if the constant-frequency carrier signal had been stretched out or its frequencylowered. When the modulating signal goes negative, the phase shift becomes leading. This causesthe carrier sine wave to be effectively speeded up or compressed. The result is as if the carrierfrequency had been increased. Phase modulation produces frequency modulation. Since the amount of phase shift isvarying, the effect is as is the carrier frequency is changed. Since FM is produced by PM, thelater is often referred to as indirect FM. It is important to point out that it is the dynamic nature of the modulating that causes thefrequency variation at the output of the phase shifter. In other words, FM is only reduced as longas the phase shift is being varied. In FM, maximum deviation occurs at the peak positive and negative amplitudes of themodulating signal. In PM, the maximum amount of leading ang lagging shift occurs at the peakamplitude of the modulating signal. The faster the modulating signal voltage varies the greaterthe frequency deviation produced. Because of this, the frequency deviation produced in PMincreases with the frequency of the modulating signal. The higher the modulating signalfrequency, naturally the shorter its period and the faster the voltage changes. Higher modulatingvoltages produce greater frequency deviation. However, higher modulating frequencies producea faster rate of change of modulating voltage and, therefore, also produce greater frequencydeviation.
• 5. Deviation When the audio signal is modulated onto the radio frequency carrier, the new radiofrequency signal moves up and down in frequency. The amount by which the signal moves upand down is important. It is known as the deviation and is normally quoted as the number ofkilohertz deviation. As an example the signal may have a deviation of ±3 kHz. In this case thecarrier is made to move up and down by 3 kHz. Assume a carrier frequency of 50 MHz .if the peak amplitude of the modulating signalcauses a maximum frequency shift of 200 kHz, the carrier frequency will deviate up to 50.2 MHzand down to 59.8 MHz. The total frequency deviation is 50.2 – 49.8 = 0.4 MHz = 400 kHz. Inpractice, however, the frequency deviation is expressed as the amount of frequency shift of thecarrier above or below the center frequency. Therefore, the frequency deviation in the exampleabove is said to be 200 kHz. This means that the modulating signal varies the carrier above andbelow its center frequency to 200 kHz. The frequency of the modulating signal determines therate of frequency deviation but has no effect on the amount of deviation which is strictly afunction of the amplitude of the modulating signal.