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Correct answer is (a) True

Easy explanation: Time-division duplexing (TDD) means that signals are transmitted simultaneously on a single CHANNEL by interleaving them in different time slots. Each time slot may contain one data word, such as 1 byte from an A/D CONVERTER or a D/A converter. As long as the serial data RATE is high ENOUGH, a user will never know the difference. The primary benefit of TDD is that only one channel is needed. It saves spectrum space and cost.

Correct choice is (d) 336kHz

The explanation is: Serial DATA rate = SAMPLING rate x no. bits/frame = 3.5 KHZ x 96 = 336 kHz.

The correct ANSWER is (a) True

To explain: All modern telephone systems use digital transmission via PCM and TDM. The only place where analog signals are still used is in the local loop—the connection between a telephone company’s central office (CO) and the subscriber’s telephone, KNOWN as the customer PREMISES equipment (CPE). All local and long-distance CONNECTIONS are digital.

Right ANSWER is (C) 96

Easy EXPLANATION: Here, Data channels = 16 and WORD length is 6 bits.

Bits per frame = 6 x 16 = 96.

Right option is (a) 15

To elaborate: 16 (total no. of channels) – 1 (channel USED for ID) = 15 (for data).

The CORRECT option is (a) The PARALLEL binary data is converted into serial before transmission

The EXPLANATION: The analog signal is converted into a DIGITAL signal. Since the converted digital signal is parallel bits of data, it has to be converted to serial before it can be MULTIPLEXED and transmitted.

Correct answer is (d) Not EASILY recoverable

To ELABORATE: When signals have been degraded because of noise, attenuation, or distortion, all the receiver has to do is to determine whether a PULSE was transmitted. Amplitude, width, frequency, phase shape, and so on do not affect reception. THUS PCM signals are easily RECOVERED and rejuvenated, no matter what the circumstances.

The correct ANSWER is (b) Time division multiplexing

Easy explanation: The most popular form of TDM uses pulse-code modulation (PCM), in which multiple channels of digital data are transmitted in serial form. Each channel is assigned a time SLOT in which to transmit one binary word of data. The data STREAMS from the VARIOUS channels are interleaved and transmitted sequentially.

Right ANSWER is (d) Fast data transfer

The explanation is: Since the time AVAILABLE for transmission is shared by all the signals that are modulated, Time DIVISION multiplexing is not fast when compared with other multiplexing techniques. However, for applications such as TELEMETRY, with a high sampling rate, the speed of time division multiplexing is sufficient to meet the REQUIREMENTS.

The correct option is (b) False

To explain I would say: After clock pulses of the proper frequency have been obtained, it is necessary to synchronize the multiplexed channels. This is usually done with a special synchronizing (sync) pulse applied to one of the input channels at the transmitter. In the four-channel system discussed previously, only three ACTUAL signals are transmitted. The fourth channel is used to transmit a special pulse WHOSE CHARACTERISTICS are unique in some way so that it can be EASILY RECOGNIZED.

Right OPTION is (a) Clock recovery circuits

The BEST explanation: Clock recovery circuits are USED to REMEDY the synchronization problem encountered in demultiplexing. The clock pulse is derived from the transmitted signal so that synchronization errors are REDUCED.

Correct option is (d) Clock recovery circuits

Easiest explanation: INSTEAD of using a free-running clock oscillator set to the identical FREQUENCY of the TRANSMITTER system clock, the clock for the demultiplexer is derived from the received PAM signal itself. A circuit called the clock recovery circuits are TYPICAL of those used to generate the demultiplexer clock pulses.

The correct choice is (a) Trigger all AND GATES at clock frequency

To elaborate: The one-shot multivibrator is USED to trigger all the decoder AND gates at the clock frequency. It produces an output PULSE whose DURATION has been set to the DESIRED sampling interval.

Right OPTION is (c) 2.5ms

Easiest explanation: Time PERIOD for one frame = 1/100 = 0.01s = 10ms. During that 10-ms frame period, each of the four CHANNELS is SAMPLED once. Each channel is thus ALLOTTED 10/4 = 2.5 ms.

The correct choice is (a) Commuter

Easy explanation: Multiplexers in EARLY TDM/PAM telemetry systems used a form of rotary switch known as a commutator. Multiple switch SEGMENTS were attached to the various incoming signals while a high-speed brush rotated by a DC motor rapidly SAMPLED the signals as it passed over the contacts.

Correct answer is (b) Low pass filter

Explanation: the analog signal is converted to a series of constant-width pulses WHOSE amplitude follows the shape of the analog signal. The original analog signal is recovered by PASSING it through a low-pass filter. In TDM using PAM, a circuit called a multiplexer (MUX or MPX) samples multiple analog signal sources; the RESULTING pulses are interleaved and then transmitted over a single channel.

Correct answer is (a) True

The best EXPLANATION: In serial TRANSMISSION, the data is sent via a SINGLE cable. When a clock pulse is applied to the shift register it TRANSMITS the information bit by bit in allocated time slots.

The correct option is (a) Time slots

Explanation: In FDM, MULTIPLE signals are transmitted over a single channel, each signal being allocated a PORTION of the SPECTRUM within that bandwidth. In time-division multiplexing (TDM), each signal occupies the entire bandwidth of the channel. However, each signal is transmitted for only a brief time. In other words, multiple signals take turns transmitting over the single channel.

The correct choice is (a) Linear MIXER

Best explanation: The modulator outputs CONTAINING the SIDEBAND information are added algebraically in a linear mixer; no modulation or generation of sidebands takes place. The resulting output signal is a composite of all the modulated subcarriers. This signal can be used to MODULATE a RADIO transmitter or can itself be transmitted over the single communication channel. Alternatively, the composite signal can become one input to another multiplexed system.

The correct answer is (b) Bandpass filters

Explanation: A RECEIVER PICKS up the SIGNAL and demodulates it, recovering the composite signal. This is sent to a group of bandpass filters, each centered on one of the carrier FREQUENCIES. Each filter passes only its CHANNEL and rejects all others. A channel demodulator then recovers each original input signal.

The CORRECT choice is (a) Subcarrier

Best explanation: Each signal to be transmitted feeds a modulator circuit. The CARRIER for each modulator (fc) is on a different frequency. The carrier frequencies are usually EQUALLY spaced from one ANOTHER over a specific frequency range. These carriers are referred to as subcarriers.

Correct ANSWER is (a) FCC

Best explanation: The BANDWIDTHS of radio channels vary, and are usually determined by FCC REGULATIONS and the type of radio service involved. Regardless of the type of channel, a wide BANDWIDTH can be shared for the purpose of TRANSMITTING many signals at the same time.

Right answer is (c) Code division MULTIPLEXING

The explanation is: Another form of MULTIPLE accesses is known as code-division multiple access (CDMA). It is widely used in cell phone SYSTEMS to allow many cell phone subscribers to use a common bandwidth at the same time. This SYSTEM uses special codes assigned to each user that can be identified. CDMA uses a technique called spread spectrum to make this type of multiplexing POSSIBLE.

The correct choice is (c) Analog signal

To explain I would say: The two most COMMON types of MULTIPLEXING are frequency-division multiplexing (FDM) and time-division multiplexing (TDM). Two variations of these basic METHODS are frequency-division multiple ACCESS (FDMA) and time-division multiple access (TDMA). In general, FDM systems are used for analog information and TDM systems are used for digital information.

Correct option is (c) Variable FREQUENCY oscillators

Easy explanation: The local OSCILLATOR is usually a variable frequency oscillator or a frequency SYNTHESIZER so that its frequency can be adjusted over a relatively wide RANGE. As the local-oscillator frequency is changed, the mixer translates a wide range of input frequencies to the FIXED IF.

Right choice is (a) True

Easy explanation: Multiplexing is the process of simultaneously transmitting TWO or more INDIVIDUAL signals over a single COMMUNICATION channel, cable or wireless. In effect, it increases the number of communication channels so that more information can be TRANSMITTED.

Correct answer is (d) Less power consumption

The best explanation: The USE of RF amplifiers in the initial stage of signal selection REDUCES the effect of noise.RF amplifiers improve sensitivity, because of the extra gain; improve selectivity, because of the ADDED tuned circuits; and improve the S/N RATIO. Further, spurious signals are more effectively rejected, minimizing unwanted signal generation in the mixer.

The correct choice is (a) LNA

Easiest explanation: LNA or low noise amplifier is placed in between the ANTENNA and the mixer. The oscillator SIGNAL is relatively strong and some of it can leak through and appear at the INPUT of the mixer. The RF amplifier between the mixer and the antenna isolates the TWO, SIGNIFICANTLY reducing any local oscillator radiation.

The CORRECT answer is (a) True

The explanation: A sensitive and selective receiver can be made using only AMPLIFIERS, selective filters and a demodulator. This is called a tuned radio frequency or TRF receiver. EARLY radios used this design. However, such a receiver does not USUALLY deliver the kind of performance expected in modern COMMUNICATIONS applications.

Right option is (b) Superheterodyne receivers

The explanation is: Superheterodyne receivers convert all INCOMING signals to a LOWER frequency, known as the intermediate frequency (IF), at which a single set of amplifiers and filters is used to PROVIDE a FIXED level of sensitivity and SELECTIVITY. Most of the gain and selectivity in a superheterodyne receiver are obtained in the IF amplifiers.

The CORRECT option is (b) BIT error rate

The explanation is: For analog signals, the signal-to-noise ratio is the main CONSIDERATION in analog signals. For digital signal transmission, the bit error rate (BER) is the main consideration. BER is the number of ERRORS made in the transmission of many SERIAL data bits.

Correct option is (a) MDS

Best explanation: The MDS or Minimum Discernible SIGNAL is the input signal level that is approximately equal to the average internally generated noise value. This noise value is called the noise floor of the RECEIVER. MDS is the amount of signal that would produce the same AUDIO POWER output as the noise floor signal. The MDS is usually expressed in dBm.

Correct CHOICE is (a) SELECTIVITY and sensitivity

For explanation: A communication RECEIVER must be able to identify and SELECT the desired signal from thousands of others present in the FREQUENCY spectrum (selectivity) and to provide sufficient amplification to recover the modulating signal (sensitivity). A receiver with good selectivity will isolate the desired signal in the RF spectrum and eliminate or at least greatly attenuate all other signals.

The correct answer is (c) 1,59,96,800Hz

Explanation: The frequency can vary as MUCH as 200 Hz for every 1 MHz of frequency or 200×16=3200Hz.

The MAXIMUM frequency is 16,000,000 – 3200 = 1,59,96,800 Hz.

Correct choice is (c) 269.65MHz

The best explanation: VARIATION = 300ppm = 0.03%

This variation is multiplied by the FREQUENCY multiplier CHAIN, yielding

0.03 PERCENT x 18 =60.54 percent. Now, 268.2 MHz x 0.0054 = 1.45 MHz.

Thus, the frequency of the transmitter output is 268.2 + 1.45 MHz. The maximum frequency is

268.2 + 1.45 = 269.65 MHz.

Correct answer is (a) 1,60,03,200Hz

The BEST explanation: The frequency can vary as much as 200 Hz for EVERY 1 MHz of frequency or 200×16=3200Hz.

The maximum frequency is 1,60,00,000 + 3200 = 1,60,03,200 Hz.

Correct choice is (b) 268.2MHz

For explanation: Total frequency multiplication factor 2 x 3 x 3 = 18

Transmitter OUTPUT frequency = 14.9 MHZ x 18 = 268.2 MHz.

The correct option is (b) Crystal OVENS

The best I can explain: greater STABILITY of crystal frequency can be achieved by MOUNTING the crystal in sealed, temperature-controlled chambers known as crystal ovens. These devices maintain an absolute CONSTANT temperature, ENSURING a stable output frequency.

Right option is (b) Crystal OSCILLATOR

To elaborate: The transmitter MUST remain on the assigned frequency. It must not drift off or wander from its assigned VALUE despite the many operating CONDITIONS, such as wide temperature variations and CHANGES in power supply voltage, that affect frequency. The only oscillator capable of meeting the precision and stability demanded by the FCC is a crystal oscillator.

The correct CHOICE is (a) VARIABLE FREQUENCY oscillator

Best explanation: A variable frequency oscillator (VFO) is used to provide CONTINUOUS tuning over a desired range. Using VFO different carrier frequencies can be generated.