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Vector Modulators

Vector modulators allow the frequency, amplitude and/or phase of a carrier wave to be controlled by digital signals. This allows the transmitted bit rate to increase while keeping the baud rate within the frequency constraints of the transmission channel.

Vector modulators are used in all sorts of modems. They are used on telephone links, as well as in terrestrial and satellite microwave facilities. The modulator consists of a quadrature oscillator, some level shifters, multipliers and an adder. In some cased, baseband processing may be included in order to control the spectral content of the transmitted signal.

Here are some applications note which provide more details:

Digital Modulation

QPSK Transceiver Simulation

All high performance modems today use some form of phase modulation.

ASK Vector Modulator

The simplest modulator varies the carrier amplitude according to the data pattern. The second input to the modulator is set to zero.

FSK Vector Modulator

The data pattern is applied to one input while the other is used to control the frequency shift.

BPSK Modulator

This is the simplest type of PSK modulator since it has only two output phase states. It uses a multiplier, which can be an IC or ring type.

This can be easily modeled in SystemView:

The multiplier output has 2 phase states. The duration of each state corresponds to one signaling element or baud. The baud rate is therefore equal to the bit rate.

4-PSK

Quadrature modulation uses two data channels denoted I and Q displaced by 90o with respect to each other. These may originate from a single data stream that has passed through a serial to parallel converter. It may seem somewhat paradoxical, that although these two channels are combined prior to transmission, they do not interfere with each other.

 

This model can be considerably reduced by taking advantage of the SystemView library.

 

 

8-PSK

This process of encoding more bits into each output baud or phase state can be continued. Organizing binary bits into 3 bytes corresponds to 8 different conditions.

 

8_QAM

An 8 state device can have 2 different amplitude possibilities for each of 4 possible output phases.

 

Student Assignments

 

Generate the constellation and eye diagrams for the 8-QAM model.

Create a SystemView model of a 16-QAM modulator. (Ignore the serial to parallel converter.)

 

 

QPSK Transceiver Simulation

Read SystemView Applications Note 121b and examine the following SystemView models:

QPSK Tx and Rx

QPSK Tx and Rx 2

QPSK Tx and Rx 3

QTxRx 4

QTxRx 5

 

Be prepared to explain the purpose of every token.

 




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