This document discusses methods of generating frequency-modulated (FM) signals. There are two basic methods: direct FM, where the carrier frequency is directly varied by the information signal using a voltage-controlled oscillator; and indirect FM, where the information signal modulates the phase of the carrier signal. FM generation can be achieved easily using a linear voltage-to-frequency converter like the LM566C integrated circuit, which generates a square or triangular FM wave that can be filtered into a sine wave. The LM566C is capable of FM frequencies up to 1 MHz. Noise has little effect on FM signals because FM receivers use limiter circuits that clip amplitude variations while preserving the frequency modulation carrying the information.

1. FM Generation
A. D. Patel Institute Of Technology
EC :- Sem V Present By :- Pratik Makadia
(130010111017)
19 October
2015
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2. FM SIGNAL GENERATION
• They are two basic methods of generating frequency- Modulated signals
1. Direct Method
2. Indirect Method
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3. DIRECT FM
𝑓𝑖 = 𝑓𝑐 + 𝑘 𝑓 𝑚(𝑡)
• In a direct FM system the instantaneous frequency is directly varied with the
information signal. To vary the frequency of the carrier is to use an
Oscillator whose resonant frequency is determined by components that can
be varied. The oscillator frequency is thus changed by the modulating signal
amplitude.
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4. INDIRECT FM
𝒙 𝒕 = 𝑨 𝒄 𝐜𝐨𝐬(2𝝅𝒇 𝒄 𝒕 + 𝜽(𝒕))
𝜽 𝒕 = 𝟐𝝅𝒌 𝒑 𝒎(𝒕)
𝜽 𝒕 = 𝟐𝝅𝒌 𝒇
𝟎
𝒕
𝒎 𝝉 𝒅(𝝉)
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5. FM Generation
• FM generation is quite easy. All that is needed is a linear voltage-to-frequency
converter.
• If a voltage-controlled capacitor is placed in the feedback loop of an
oscillator and the modulation signal is applied across this capacitor, then the
oscillator will change in frequency and the resulting output signal is an FM
signal.
• The LM 566C to generate the FM signal.
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6. Fig:- FM generation using the LM 566C. The LM 386 drives the LED
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7. • The LM 566 is a linear voltage-to-frequency converter which can generate an
FM signal up to 1 MHz .
• The LM 566 delivers either an FM square wave or an FM triangular wave.
• The square/triangular wave can be converted into a “good” sine wave with
the use of a simple RC filter.
• The input impedance for the modulation signal is 1 MΩ and the output
impedance of the square/triangular wave is 50 Ω.
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8. NOISE IN FM SIGNAL
1. Noise is essentially amplitude variations. An FM signal, on the other hand,
has a constant carrier amplitude.
2. Because of this, FM receivers contain limiter circuits that restrict the
amplitude of the received signal.
3. Any amplitude variations occurring on the FM signal are effectively clipped
off. This does not hurt the information content of the FM signal. Because
of the clipping action of the limiter circuits, noise is almost completely
eliminated.
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