phased.SteppedFMWaveform

Stepped FM pulse waveform

Description

The SteppedFMWaveform object creates a stepped FM pulse waveform.

To obtain waveform samples:

1. Define and set up your stepped FM pulse waveform. See Construction.

2. Call step to generate the stepped FM pulse waveform samples according to the properties of phased.SteppedFMWaveform. The behavior of step is specific to each object in the toolbox.

Note

Starting in R2016b, instead of using the step method to perform the operation defined by the System object™, you can call the object with arguments, as if it were a function. For example, y = step(obj,x) and y = obj(x) perform equivalent operations. When the only argument to the step method is the System object itself, replace y = step(obj) by y = obj().

Construction

sSFM = phased.SteppedFMWaveform creates a stepped FM pulse waveform System object, sSFM. The object generates samples of a linearly stepped FM pulse waveform.

sSFM = phased.SteppedFMWaveform(Name,Value) creates a stepped FM pulse waveform object, sSFM, with each specified property Name set to the specified Value. You can specify additional name-value pair arguments in any order as (Name1,Value1,...,NameN,ValueN).

Properties

SampleRate	Sample rate Signal sample rate, specified as a positive scalar. Units are Hertz. The ratio of sample rate to pulse repetition frequency (PRF) must be a positive integer — each pulse must contain an integer number of samples. Default: 1e6
DurationSpecification	Method to set pulse duration Method to set pulse duration (pulse width), specified as 'Pulse width' or 'Duty cycle'. This property determines how you set the pulse duration. When you set this property to 'Pulse width', then you set the pulse duration directly using the PulseWidth property. When you set this property to 'Duty cycle', you set the pulse duration from the values of the PRF and DutyCycle properties. The pulse width is equal to the duty cycle divided by the PRF. Default: 'Pulse width'
PulseWidth	Pulse width Specify the length of each pulse (in seconds) as a positive scalar. The value must satisfy PulseWidth <= 1./PRF. Default: 50e-6
DutyCycle	Waveform duty cycle Waveform duty cycle, specified as a scalar from 0 to 1, exclusive. This property applies when you set the DurationSpecification property to 'Duty cycle'. The pulse width is the value of the DutyCycle property divided by the value of the PRF property. Default: 0.5
PRF	Pulse repetition frequency 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. ThePRF must satisfy these restrictions: The product of PRF and PulseWidth 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 PRF 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 using property settings alone or using property settings in conjunction with the prfidx input argument of the step method. When PRFSelectionInputPort is false, you set the PRF using properties only. You can implement a constant PRF by specifying PRF as a positive real-valued scalar. implement a staggered PRF by specifying PRF as a row vector with positive real-valued entries. Then, each call to the step method uses successive elements of this vector for the PRF. If the last element of the vector is reached, the process continues cyclically with the first element of the vector. When PRFSelectionInputPort is true, you can implement a selectable PRF by specifying PRF as a row vector with positive real-valued entries. But this time, when you execute the step method, select a PRF by passing an argument specifying an index into the PRF vector. In all cases, the number of output samples is fixed when you set the OutputFormat property to 'Samples'. When you use a varying PRF and set the OutputFormat property to 'Pulses', the number of samples can vary. Default: 10e3
PRFSelectionInputPort	Enable PRF selection input Enable the PRF selection input, specified as true or false. When you set this property to false, the step method uses the values set in the PRF property. When you set this property to true, you pass an index argument into the step method to select a value from the PRF vector. Default: false
FrequencyStep	Linear frequency step size Specify the linear frequency step size (in hertz) as a positive scalar. The default value of this property corresponds to 20 kHz. Default: 20e3
NumSteps	Specify the number of frequency steps as a positive integer. When NumSteps is 1, the stepped FM waveform reduces to a rectangular waveform. Default: 5
FrequencyOffsetSource	Source of frequency offset Source of frequency offset for the waveform, specified as 'Property' or 'Input port'. When you set this property to 'Property', the offset is determined by the value of the FrequencyOffset property. When you set this property to 'Input port', the FrequencyOffset is determined by the freqoffset input argument. Default: 'Property'
FrequencyOffset	Frequency offset Frequency offset in Hz, specified as a scalar. Dependencies This property applies when you set the FrequencyOffsetSource property to 'Input port'. Default: 0 Hz
OutputFormat	Output signal format Specify the format of the output signal as 'Pulses' or 'Samples'. When you set the OutputFormat property to 'Pulses', the output of the step method takes the form of multiple pulses specified by the value of the NumPulses property. The number of samples per pulse can vary if you change the pulse repetition frequency during the simulation. When you set the OutputFormat property to 'Samples', the output of the step method is in the form of multiple samples. In this case, the number of output signal samples is the value of the NumSamples property and is fixed. Default: 'Pulses'
NumSamples	Number of samples in output Specify the number of samples in the output of the step method as a positive integer. This property applies only when you set the OutputFormat property to 'Samples'. Default: 100
NumPulses	Number of pulses in output Specify the number of pulses in the output of the step method as a positive integer. This property applies only when you set the OutputFormat property to 'Pulses'. Default: 1
PRFOutputPort	Set this property to true to output the PRF for the current pulse using a step method argument. Dependencies This property can be used only when the OutputFormat property is set to 'Pulses'. Default: false
CoefficientsOutputPort	Enable matched filter coefficients output port Enable the matched filter coefficients output port, specified as false or true. When you set this property to false, the object does not provide the matched filter coefficients used during the simulation as an output. When you set this property to true, the object provides the matched filter coefficients used during the simulation as an output. Default: false

Methods

bandwidth	Bandwidth of stepped FM pulse waveform
getMatchedFilter	Matched filter coefficients for waveform
plot	Plot stepped FM pulse waveform
reset	Reset state of stepped FM pulse waveform object
step	Samples of stepped FM pulse waveform

Common to All System Objects
release	Allow System object property value changes

Examples

Plot Stepped-FM Waveform and Spectrum

Create a stepped frequency pulse waveform object. Assume the default value, 1 MHz, for the sample rate. Then, plot the waveform.

Create the SteppedFMWaveform System object™ with 20 kHz frequency step size.

sSFM = phased.SteppedFMWaveform('NumSteps',3,'FrequencyStep',20e3);
fs = sSFM.SampleRate;

Plot the third pulse of the wave using the phased.SteppedFMWaveform.plot method. Pass in the pulse number using the 'PulseIdx' name-value pair.

plot(sSFM,'PulseIdx',3);

Alternatively, call the step method three times to obtain three pulses. Collect the three pulses in a single time series. Then plot the waveform using the plot function. You can see the full duty cycles of the pulses.

wavfull = [];
wav = step(sSFM);
wavfull = [wavfull;wav];
wav = step(sSFM);
wavfull = [wavfull;wav];
wav = step(sSFM);
wavfull = [wavfull;wav];
nsamps = size(wavfull,1);
t = [0:(nsamps-1)]/fs*1e6;
plot(t,real(wavfull))
xlabel('Time (\mu sec)')
ylabel('Amplitude')
grid

Plot the spectrum using the spectrogram function. Assume an fft of 64 samples and a 50% overlap. Window the signal with a hamming function.

nfft1 = 64;
nov = floor(0.5*nfft1);
spectrogram(wavfull,hamming(nfft1),nov,nfft1,fs,'centered','yaxis')

Apply Frequency Offset to Stepped FM Waveform

Apply a frequency offset to a stepped FM (SFM) pulse waveform. Plot the frequency spectrum of the waveform with and without a frequency offset applied.

Create an SFM waveform object which is configured to set the frequency offset from an input when the object is executed.

fs = 1e6;
sSFM = phased.SteppedFMWaveform('SampleRate',fs,'NumSteps',2, ...
    'FrequencyStep',20e3,'NumPulses',2,'FrequencyOffsetSource','Input port');

Execute the object two times. First set the frequency offset set to 0 Hz, and then to 2e4 Hz.

sfmwav = sSFM(0);
sfmwav_foffset = sSFM(2e4);

Plot the frequency spectrum of the complex signals. The frequency offset signal is shifted to the right.

[Pxx,f] = pwelch(sfmwav,[],[],[],fs,'centered');
[Pxx_offset,foffset] = pwelch(sfmwav_foffset,[],[],[],fs,'centered');
plot(f/1000,Pxx,foffset/1000,Pxx_offset)
ylabel('PSD');
xlabel('Frequency (kHz)');
legend({'No offset','Offset applied'},'Location','northwest');
grid on;

More About

Stepped FM Waveform

In a stepped FM waveform, a group of pulses together sweep a certain bandwidth. Each pulse in this group occupies a given center frequency and these center frequencies are uniformly located within the total bandwidth.

References

[1] Richards, M. A. Fundamentals of Radar Signal Processing. New York: McGraw-Hill, 2005.

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

• The plot method is not supported.

• See System Objects in MATLAB Code Generation (MATLAB Coder).

Version History

Introduced in R2011a