Linear FM Waveform

Linear FM (LFM) pulse waveform

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  • Library:

  • Phased Array System Toolbox / Waveforms

  • Linear FM Waveform block

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.

Ports

Input

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Index to select the pulse repetition frequency (PRF), specified as a positive integer. The index selects the PRF from the predefined vector of values specified by the Pulse repetition frequency (Hz) parameter.

Example: 4

Dependencies

To enable this port, select Enable PRF selection input.

Data Types: double

Frequency offset in Hz, specified as a scalar.

Example: 2e3

Dependencies

To enable this port, set Source of Frequency Offset to Input port.

Data Types: double

Output

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Pulse waveform samples, returned as a complex-valued vector.

Data Types: double

Pulse repetition frequency of current pulse, returned as a positive scalar.

Dependencies

To enable this port, set the Output signal format parameter to Pulses and then select the Enable PRF output parameter.

Data Types: double

Matched filter coefficients, returned as a vector or matrix.

Dependencies

To enable this port, select Enable Matched Filter Coefficients Output.

Data Types: double
Complex Number Support: Yes

Parameters

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Sample rate of the output waveform, specified as a positive scalar. The ratio of Sample rate (Hz) to each element in the Pulse repetition frequency (Hz) vector must be an integer. This restriction is equivalent to requiring that the pulse repetition interval is an integral multiple of the sample interval.

Method to set the pulse duration, specified as Pulse width or Duty cycle. When you set this parameter to Pulse width, the pulse duration is set using the Pulse width (s) 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.

The duration of each pulse, specified as a positive scalar. Set the product of Pulse width (s) and Pulse repetition frequency to be less than or equal to one. This restriction ensures that the pulse width is smaller than the pulse repetition interval. Units are in seconds.

Example: 300e-6

Dependencies

To enable this parameter, set the Method to specify pulse duration parameter to Pulse width.

Waveform duty cycle, specified as a scalar in the range [0,1].

Example: 0.7

Dependencies

To enable this parameter, set the Method to specify pulse duration parameter to Duty cycle.

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 pulse repetition frequency, 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 condition expresses the requirement that the pulse width is less than one 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 condition expresses the requirement that the number of samples in one pulse repetition interval is an integer.

You can select the value of PRF by using block parameter settings alone or in conjunction with the input port, PRFIdx.

  • When the Enable PRF selection input parameter is not selected, 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 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 cycles through the successive values of the PRF vector.

  • When the Enable PRF selection input parameter 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 the block, select a PRF by passing an index into the PRF vector into the PRFIdx port.

In all cases, the number of output samples is fixed when you set the Output signal format to Samples. When you use a varying PRF and set Output signal format to Pulses, the number of output samples can vary.

Select this parameter to enable the PRFIdx port.

  • When enabled, pass in an index into a vector of predefined PRFs. Set predefined PRFs using the Pulse repetition frequency (Hz) parameter.

  • When not enabled, the block cycles through the vector of PRFs specified by the Pulse repetition frequency (Hz) parameter. If Pulse repetition frequency (Hz) is a scalar, the PRF is constant.

Bandwidth of the linear FM sweep in Hz, specified as a positive scalar.

Example: 1e3

Specify the direction of the linear FM sweep as Up (increasing frequency) or Down (decreasing frequency).

FM frequency sweep interval, specified as Positive or Symmetric. If you set this parameter to Positive, the waveform sweeps the frequency 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 the interval between –B/2 and B/2.

FM signal amplitude envelope, specified as Rectangular or Gaussian.

Source of frequency offset, specified as Property or Input port.

  • When set to Property, the offset is determined by the value of the Frequency Offset parameter.

  • When set to Input port, the offset is determined by the value of the FreqOffset port.

Frequency offset in Hz, specified as a scalar.

Example: 2e3

Dependencies

To enable this parameter set the Source of Frequency Offset parameter to Input port.

Source of simulation sample time, specified as Derive from waveform parameters or Inherit from Simulink engine. When set to Derive from waveform parameters, the block runs at a variable rate determined by the PRF of the selected waveform. The elapsed time is variable. When set to Inherit from Simulink engine, the block runs at a fixed rate so the elapsed time is a constant.

Dependencies

To enable this parameter, select the Enable PRF selection input parameter.

The format of the output signal, specified as Pulses or Samples.

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

Number of samples in the block output, specified as a positive integer.

Example: 1000

Dependencies

To enable this parameter, set the Output signal format parameter to Samples.

Data Types: double

Number of pulses in the block output, specified as a positive integer.

Example: 2

Dependencies

To enable this parameter, set the Output signal format parameter to Pulses.

Data Types: double

Select this parameter to enable the PRF output port.

Dependencies

To enable this parameter, set Output signal format to Pulses.

Select this parameter to enable the Coeff output port.

Block simulation, 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 with generated code than in interpreted execution. You can run repeated executions without recompiling, but if you change any block parameters, then the block automatically recompiles before execution.

This table shows how the Simulate using parameter affects the overall simulation behavior.

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).

See Also

Introduced in R2014b

Phased Array System Toolbox Documentation

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