Key Concepts in Signal Modulation and Transmission
Modulation and Its Types
Modulation is a process where the time function that describes the information signal is multiplied by another signal, most often called a carrier.
- Angular Modulation
- Amplitude Modulation
- DSB-SC (Double Sideband Suppressed Carrier)
- AM (Amplitude Modulation)
- SSB (Single Sideband)
- VSB (Vestigial Sideband)
- Frequency Modulation
- NBFM (Narrowband Frequency Modulation)
- VBFM (Very Narrowband Frequency Modulation)
- Phase Modulation
- Narrow Band
- Broadband
Selectivity
Selectivity measures the ability of the receiver to accept a particular frequency band and reject others.
Sensitivity
Sensitivity is the minimum RF signal that can be detected at the receiver input and produce a useful modulated information signal.
Dynamic Range
Dynamic range is the difference in dB between the minimum input signal and the maximum input signal without causing distortion.
Fidelity
Fidelity is the measure of a communication system’s ability to produce at the receiver output an exact replica of the original signal without distortion.
Noise Figure
This parameter measures the performance of a communication system in the presence of noise. The overall noise figure indicates how much noise the system introduces.
Rejection Factor
The rejection factor is the relationship between the bandwidth at -60 dB and the bandwidth at -3 dB.
What is DSB-SC?
Double sideband modulation with suppressed carrier (DSB-SC) uses a technique that shifts the frequency spectrum of the signal f(t) to transmit, by multiplying f(t) by a sinusoidal signal whose frequency corresponds to the translation required.
Synchronous Detection
In synchronous detection, the receiver input signal is first multiplied with a sinusoid that is generated locally and then passes through a low-pass filter. The bandwidth is the same as the message, or something bigger. It is assumed that the local oscillator (LO) is precisely synchronized with the carrier, both in phase and frequency, hence the name synchronous detection.
Types of Transmission
- Simplex System: One transmits and the other receives information. The transmission of information is in one direction. Example: Broadcasting.
- Half-Duplex System: It transmits and receives information, but not simultaneously. Example: Walkie-talkie.
- Duplex System: The systems transmit and receive information simultaneously. Example: Telephony.
Why is AM Modulation Linear?
AM modulation is linear since it involves a multiplication process between an information signal and a carrier signal. The amplitude of the carrier is varied linearly so that it changes according to variations in the level of the modulating signal.
Single Sideband and How it is Generated
Single sideband (SSB) modulation is the suppression of the carrier and one of the sidebands, thus transmitting only one sideband containing all the information. Once the SSB signal is captured at the receiver, the carrier is reinserted to demodulate the signal and transform the information signal again.
It is generated by two methods:
- Filter Method: This consists of removing one of the sidebands of a DSB-SC signal using a suitable bandpass filter.
- Phase Shift Method: In this case, the single sideband signals are generated with an indirect method of diverting the phase spectrum.
How Can an FM Signal be Generated?
FM signals can be generated in two ways:
- Indirect FM: First, the modulating signal f(t) is integrated, and then the carrier signal is phase-modulated to produce a narrow band. Then, frequency multiplication is used to increase the modulation index. This is based on the ease of generating PM signals.
- Direct FM: In this method, the carrier frequency is varied directly according to the input modulating signal.
Fundamental Differences Between PM and FM
- In FM, information changes appear in the frequency variation Wc(t).
- In PM, the variations of the signal information are generated in the phase variation.
- Additionally, the AM modulation index is directly proportional to the amplitude of the modulating signal and independent of frequency. In FM, the modulation index is directly proportional to the modulating signal amplitude and inversely proportional to frequency.
Key Differences Between SSB and VSB
In a single sideband system, one of the bands of a DSB-SC is eliminated through a bandpass filter accordingly. In a vestigial sideband system, rather than completely eliminating one sideband (like USB), it supports a gradual cut.
Effect of Modulation Rate on Transmit Power
It has no effect because the transmission power depends on the amplitude of the carrier.
Carrier Signal Increase on FM Bandwidth
It is not affected since the bandwidth does not depend on the carrier signal; it depends on the modulation index and modulating frequency.
Differences Between Narrowband and Broadband FM
In narrowband FM, much less information is transmitted than could be transmitted in broadband FM.
A narrowband FM signal occupies the same bandwidth, Bw = 2wm, while a broadband FM signal occupies Bw = 2 * n * Wm.
Advantages of Carrier Reinsertion in AM
The advantage is the ability to use an envelope detector in the receiver to determine f(t).