Correct Answer : Continuous wave transmission
Explanation : The oscillator generates the carrier and can be switched off and on by a telegraph key to produce the dots and dashes of the International Morse code. Information transmitted in this way is referred to as continuous-wave (CW) transmission. Such a transmitter is rarely used today.
Correct Answer : Oscillator
Explanation : An oscillator, in most applications a crystal oscillator, generates the final carrier frequency. A modulator simply adds the information signal and an antenna transmits the signal.
Correct Answer : Duplexer
Explanation : A duplexer is a device that is placed just before the antenna in a transducer system. It is basically a switch which allows both transmission and reception but only one at a time.
Correct Answer : 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 Answer : 268.2MHz
Explaination : Total frequency multiplication factor 2 x 3 x 3 = 18Transmitter output frequency = 14.9 MHz x 18 = 268.2 MHz.
Correct Answer : Variable frequency oscillator
Explanation : A variable frequency oscillator (VFO) is used to provide continuous tuning over a desired range. Using VFO different carrier frequencies can be generated.
Correct Answer : Crystal Oscillator
Explanation : 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.
Correct Answer : 1,60,03,200Hz
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 Answer : Crystal ovens
Explanation : 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.
Correct Answer : Selectivity and sensitivity
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.
Correct Answer : LC tuned circuits
Explanation : Initial selectivity in a receiver is normally obtained by using LC tuned circuits. By carefully controlling the Q of the resonant circuit, we can set the desired selectivity.
Correct Answer : Sensitivity
Explanation : A communication receiver’s sensitivity, or ability to pick up weak signals, is mainly a function of overall gain, the factor by which an input signal is multiplied to produce the output signal. In general, the higher the gain of a receiver, the better its sensitivity.
Correct Answer : Variable frequency oscillators
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.
Explanation : A duplexer is a device which acts as a switch and stops the high power transmission signals from entering into the receiver, which is tuned to handle very low power for better sensitivity. When the duplexer fails, the high power signals enter the receiver where the Low noise amplifier is damaged.
Correct Answer : Less power consumption
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.
Correct Answer : LNA
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.
Correct Answer : MDS
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 Answer : Bit error rate
Explanation : 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 Answer : Superheterodyne receivers
Explanation : 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.
Correct Answer : Sum and difference of frequencies
Explanation : The key circuit is the mixer, which acts as a simple amplitude modulator to produce sum and difference frequencies. The incoming signal is mixed with a local oscillator signal to produce this conversion.
Correct Answer : 143
Explaination : Total channels = 860/6 = 143.33 or 143.
Correct Answer : FM/FM system
Explanation : Generally the individual signals which require multiplexing are frequency modulated. These signals are then added up by the mixer and the resulting output signal is again frequency modulated before transmission.
Correct Answer : 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.
Explanation : Continuous wave transmission such as morse code, multiplexing is not necessary since only two voltage levels are present and each bit is sent one by one. Also, only one information signal is transmitted whereas in telemetry, TV and satellite communications numerous information is transmitted hence multiplexing is required.
Correct Answer : Analog signal
Explanation : 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 Answer : Code division multiplexing
Explanation : 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.
Correct Answer : Spatial multiplexing
Explanation : Spatial multiplexing is the term used to describe the transmission of multiple wireless signals on a common frequency in such a way that they do not interfere with one another. One way of doing this is to use low-power transmissions so that the signals do not interfere with one another. When very low power is used, the signals do not travel very far. The transmission distance is a function of the power level, frequency, and antenna height.
Correct Answer : Linear mixer
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.
Correct Answer : FCC
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.
Correct Answer : Subcarrier
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 : 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.
Correct Answer : 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 : Commuter
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 : Trigger all AND gates at clock frequency
Explanation : 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.
Correct Answer : Clock recovery circuits
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.
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 Answer : Time division multiplexing
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.
Correct Answer : The parallel binary data is converted into serial before transmission
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 : Not easily recoverable
Explanation : 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.
Correct Answer : 336kHz
Explaination : Serial data rate = sampling rate x no. bits/frame = 3.5 kHz x 96 = 336 kHz.
