Unit 2. Communications Principles Flashcards
- Amplitude modulation
1. Name the part of the AM signal that does not fluctuate in amplitude by the modulating signal.
The carrier.
- What frequencies does the amplitude modulated waveform contain?
Unmodulated carrier frequency, USB, and LSB.
- What part of the modulated carrier wave contains the information carrying component?
The sidebands.
- In AM, what is the relation between the bandwidth required to transmit the signal and the frequency of the modulating signal?
The bandwidth required is two times the modulating signal frequency.
- Define “percent of modulation.”
It refers to the amount of effect or change that the intelligence has on the carrier.
- Find the % mod if a 4 Vpk-pk RF signal is modulated by a 2.5 Vpk-pk audio signal.
The percent of modulation is 62.5 percent—found by dividing 2.5 by 4 and multiplying the result by 100.
- If the signal in figure 2–3, C, has an Emax of 50 mVrms and an Emin of 5 mVrms, what is its %mod?
81.8 percent.
- What are two results of over modulation?
Severe distortion and increases bandwidth of an AM signal.
- Frequency modulation
1. How does the rate of deviation relate to the frequency of the modulating signal?
They are directly proportional.
- What determines the amount of deviation of a FM carrier?
The amplitude of the modulating signal.
- To prevent interferences with other FM stations, who establishes limits on the maximum amount of deviation in FM?
The FCC establishes the maximum amount of deviation.
- What is a significant sideband?
A sideband that contains at least 1 percent of the total transmitted power.
- How do you determine the modulation index in FM?
By dividing the amount of frequency deviation by the frequency of the modulating signal.
- Where do FM sidebands get their power?
From the unmodulated carrier.
- What is the relationship between modulation index and sideband power?
A higher modulation index means more power in the sidebands. It is even possible to have all the power in the sidebands and none in the carrier. At this point, any further increase in modulation would start taking power from the sidebands and placing it back in the carrier resulting in a redistribution of power.
- Phase modulation
1. In PM, what effect does the change in carrier frequency have on modulated waveshape?
None. The frequency change in PM is incidental.
- Describe the effect the positive and negative alterations of a modulating signal have on the phase of the carrier in PM.
During the positive alternation of the modulating signal, the phase of the carrier lags behind the unmodulated carrier. During the negative alternation it leads the unmodulated carrier.
- When is the carrier at its rest frequency in PM?
During the constant amplitude part of the modulating frequency.
- What part of the modulating signal controls the amount of phase shift in PM?
Amplitude.
- What part of the modulating signal controls the rate of phase shift?
Frequency.
2–2. Digital Transmission Techniques
- Pulse code modulation
- Name the four steps of PCM.
(1) Band limiting.
(2) Sampling.
(3) Quantizing.
(4) Encoding.
- State the main purpose of the band-limiting filter.
It ensures the input to the sampler never exceeds a maximum frequency.
- Define sampling.
Converting a continuous time signal into a discrete time signal.
- What part of the pulse train is varied using PAM, PWM, and PPM?
PAM = amplitude, PWM = width or duration, and PPM = position.
- Asynchronous and synchronous transmission
1. With ATM, what is the length of each transmission unit?
One character in length.
- What part of an ATM transmission tells the receiving device that a character is coming and that the character has been sent?
The start and stop bits.
- How does the receiving device determine whether it has received a correct character?
By summing the 1 bits. If the character arrives with an even number of 1 bits, the device assumes that it has received a correct character.
- How do networks reduce the overhead cost of data transmission?
By sending messages using synchronous transmissions.
- How does synchronous transmission differ from asynchronous transmission?
Synchronous transmission blocks many characters together for transmission.
- Error detection and correction
1. What does VRC check for on each incoming character?
Odd or even parity.
- In LRC, what transmitted character does the receiver use to determine if a transmission was error- free?
BCC.
- When using the checksum method of error detection, what binary number is used to divide the sum of all the characters in order to derive the checksum?
255.
- How does the CRC method of error detection determine the dividend when computing the BCC?
CRC treats the binary ones and zeros in the frame address, control, and information fields as one long binary number.
- How effective is CRC at detecting errors in most applications?
99 percent in most applications.
- How does ARQ work?
The receiver automatically sends a retransmitted request to the sender if it finds an error in a received frame.
- Using forward error control, at which end of the transmission link are errors corrected?
Receiving end.
- Using forward error control, what is the transmitter’s function in error correction?
To transmit multiple copies of the same message to the distant end.
- Using forward error control, what is the receiver’s function in error correction?
To compare all copies of transmitted message, then reconstruct the message using the good portions of the message copies received.
