This set of Avionics Multiple Choice Questions & Answers (MCQs) focuses on “Time Division Multiplexing”.
1. Frequency division multiplexing : Frequency slots :: time division multiplexing : ?
a) Time slots
b) Coded information
c) Pulsed information
d) Band 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.
2. Serial transmission is not possible without time division multiplexing.
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.
3. What device is used to demodulate a time division multiplexed analog wave?
a) High pass filter
b) Low pass filter
c) Band stop 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.
4. Which of the following device was used in early TDM/PAM telemetry systems?
b) Linear switch
c) Logic gates
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.
5. What is the time allocated for each channel if the number of samples per frame is 4 and the frame rate is 100frames/sec?
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.
6. What is the purpose of one shot multivibrator?
a) Trigger all AND gates at clock frequency
b) Trigger all OR gates at clock frequency
c) Trigger all AND gates at signal frequency
d) Trigger all OR gates at signal 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.
7. The circuit used to regenerate clock pulses from the transmitted PAM signals is called ____
a) Clock demodulator circuits
b) Timer circuits
c) Clock receiving circuits
d) 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.
8. What are used to reduce or stop synchronization problems while receiving?
a) Clock recovery circuits
d) Band pass filter
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.
9. In a four channel system, all four signals transmitted contain information.
Explanation: 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.
10. Which of the following is not an advantage of time division multiplexing?
a) Signal interference is less
b) More flexible
c) Full channel can be used for every signal
d) Fast data transfer
Explanation: 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.
Sanfoundry Global Education & Learning Series – Avionics.
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