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Linear FM Waveform

Linear FM (LFM) pulse waveform

Library

Waveforms

phasedwavlib

Description

The Linear FM Waveform block generates a linear FM pulse waveform with specified pulse width, pulse repetition frequency (PRF), and sweep bandwidth. The block outputs an integer number of pulses or samples.

Parameters

Sample rate

Specify the sample rate, in hertz, as a positive scalar. The ratio of the Sample rate parameter to the Pulse repetition frequency parameter must be an integer. This is equivalent to requiring that the pulse repetition interval be an integer multiple of the sample interval.

Method to specify pulse duration

Specify the method to set the pulse duration as Pulse width or Duty cycle. When you set this parameter to Pulse width, the pulse duration is set using the PulseWidth parameter. When you set this parameter to Duty cycle, the pulse duration is computed from the values of the Pulse repetition frequency (Hz) and Duty Cycle parameters.

Pulse width (s)

Specify the duration of each pulse as a positive scalar. The product of Pulse width and Pulse repetition frequency must be less than or equal to one. Units are in seconds.

Duty Cycle

Specify the waveform duty cycle as a scalar between 0 and 1, inclusive. This parameter appears when you set the Method to specify pulse duration parameter to Duty cycle

Pulse repetition frequency (Hz)

Pulse repetition frequency, PRF, specified as a scalar or a row vector. Units are in Hz. The pulse repetition interval PRI is the inverse of the PRF. The value of Pulse repetition frequency (Hz) must satisfy these constraints:

  • The product of Pulse width and Pulse repetition frequency (Hz) must be less than or equal to one. This is equivalent to the requirement that the pulse width is less than a pulse repetition interval. For the phase-coded waveform, the pulse width is the product of the chip width and number of chips.

  • The ratio of sample rate to any element of Pulse repetition frequency must be an integer. This is equivalent to requiring that the number of samples in one pulse repetition interval be an integer.

You can select the value of PRF by using block parameter settings or an input port.

  • When the Enable PRF selection input check box is not selected, you set the PRF using block parameters.

    • To implement a constant PRF, specify Pulse repetition frequency (Hz) as a positive scalar.

    • To implement a staggered PRF, specify Pulse repetition frequency (Hz) as a row vector with all strictly positive values. After the waveform reaches the last element of the vector, the process continues cyclically with the first element of the vector. When PRF is staggered, the time between successive output pulses is determined sequentially by the successive values of the PRF vector.

  • When the Enable PRF selection input check box is selected, you can implement a selectable PRF by specifying Pulse repetition frequency (Hz) as a row vector with positive real-valued entries. But this time, when you execute block, select a PRF by passing an index into the PRF vector into the PRFIdx port.

In all cases, when you use a varying PRF and choose Output signal format as Pulses, the number of samples per pulse can vary as well. When you choose Samples, the number of samples per pulse is fixed.

Enable PRF selection input

Check this box to select which predefined PRF to use during the simulation via input. Uncheck this box to use the Pulse repetition frequency parameter to define the PRF sequence used in the simulation.

Sweep bandwidth

Specify the bandwidth of the linear FM sweep, in hertz, as a positive scalar.

Sweep direction

Specify the direction of the linear FM sweep as Up or Down.

Sweep interval

If you set this parameter to Positive, the waveform sweeps in the interval between 0 and B, where B is the value of the Sweep bandwidth parameter. If you set this parameter value to Symmetric, the waveform sweeps in the interval between –B/2 and B/2.

Envelope function

Specify the envelope function as Rectangular or Gaussian.

Output signal format

Specify the format of the output signal as Pulses or Samples.

If you set this parameter to Samples, the output of the block is in the form of multiple samples. The number of samples is the value of the Number of samples in output parameter.

If you set this parameter to Pulses, the output of the block is in the form of multiple pulses. The number of pulses is the value of the Number of pulses in output parameter.

The value of Output signal format must be set to Samples when the Pulse repetition frequency (Hz) parameter is a row vector.

Number of samples in output

Number of samples in the block output, specified as a positive integer. This parameter appears only when you set Output signal format to Samples.

Number of pulses in output

Specify the number of pulses in the block output as a positive integer. This parameter appears only when you set Output signal format to Pulses.

Simulate using

Block simulation method, specified as Interpreted Execution or Code Generation. If you want your block to use the MATLAB® interpreter, choose Interpreted Execution. If you want your block to run as compiled code, choose Code Generation. Compiled code requires time to compile but usually runs faster.

Interpreted execution is useful when you are developing and tuning a model. The block runs the underlying System object™ in MATLAB. You can change and execute your model quickly. When you are satisfied with your results, you can then run the block using Code Generation. Long simulations run faster than they would in interpreted execution. You can run repeated executions without recompiling. However, if you change any block parameters, then the block automatically recompiles before execution.

When setting this parameter, you must take into account the overall model simulation mode. The table shows how the Simulate using parameter interacts with the overall simulation mode.

When the Simulink® model is in Accelerator mode, the block mode specified using Simulate using overrides the simulation mode.

Acceleration Modes

Block SimulationSimulation Behavior
NormalAcceleratorRapid Accelerator
Interpreted ExecutionThe block executes using the MATLAB interpreter.The block executes using the MATLAB interpreter.Creates a standalone executable from the model.
Code GenerationThe block is compiled.All blocks in the model are compiled.
For more information, see Choosing a Simulation Mode (Simulink) from the Simulink documentation.

Ports

Note

The block input and output ports correspond to the input and output parameters described in the step method of the underlying System object. See link at the bottom of this page.

PortDescriptionSupported Data Types
PRFIdxPRF selection index.Double-precision floating point
OutOutput waveform.Double-precision floating point

Introduced in R2014b

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