Table Of Contents

MT Demodulate FM (G Dataflow)

Version:
    Last Modified: February 7, 2018

    Performs frequency demodulation on the incoming I/Q signal.

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    FM modulated waveform

    The baseband (downconverted) time-domain data for demodulation.

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    t0

    The trigger (start) time of the acquired signal.

    Default: 0.0

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    dt

    Time interval between data points in the acquired signal.

    Default: 1.0

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    Y

    The complex-valued time-domain data array. The real and imaginary parts of this complex data array correspond to the in-phase (I) and quadrature-phase (Q) data, respectively.

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    FM deviation

    A value that indicates whether to scale the FM demodulated waveform. You can set this value to 1.0 or to the expected frequency deviation of the incoming FM signal for demodulation.

    • Set this value to 1.0 to return an FM demodulated waveform parameter with no scaling. In this case, the maximum amplitude, in hertz (Hz), of the FM demodulated waveform parameter is equal to the actual frequency deviation of the incoming FM signal.
    • Set this value equal to the expected frequency deviation of the incoming FM signal to divide the FM demodulated waveform parameter value by this value. You can use the resulting scaled FM demodulated waveform to quantify the degree of error between the expected frequency deviation and the actual frequency deviation.
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    carrier correction?

    A Boolean that determines whether to enable carrier correction.

    TRUE Determines a linear correction to the carrier frequency estimate and compensates for it in the FM demodulated waveform.
    FALSE Carrier correction is not enabled. Use this option when performing continuous acquisition.

    Default: FALSE

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    error in

    Error conditions that occur before this node runs.

    The node responds to this input according to standard error behavior.

    Standard Error Behavior

    Many nodes provide an error in input and an error out output so that the node can respond to and communicate errors that occur while code is running. The value of error in specifies whether an error occurred before the node runs. Most nodes respond to values of error in in a standard, predictable way.

    error in does not contain an error error in contains an error
    If no error occurred before the node runs, the node begins execution normally.

    If no error occurs while the node runs, it returns no error. If an error does occur while the node runs, it returns that error information as error out.

    If an error occurred before the node runs, the node does not execute. Instead, it returns the error in value as error out.

    Default: No error

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    reset?

    A Boolean that determines initialization of internal node states. The differentiation step of the demodulation process uses a three-tap FIR filter and involves a transient response.

    TRUE Resets the initial state of the filter to zero at each call to this node.
    FALSE The differentiation begins with the filter in its final state from the previous call to this node.

    Default: FALSE

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    remove transient when reset?

    A Boolean that determines whether to truncate the first several samples of the FM demodulated waveform parameter when the node has been reset (for example, on first call or when reset? is set to TRUE.)

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    Note  

    On first call to this node or when reset? is set to TRUE, set the remove transient when reset? parameter to TRUE to remove the transient of the FIR filter applied by the FM demodulator.

    Default: FALSE

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    FM demodulated waveform

    The frequency-demodulated information signal.

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    carrier offset

    The offset, in hertz (Hz), between the incoming modulated carrier frequency and the estimated carrier frequency. The offset is returned whether carrier correction? is TRUE or FALSE. If the carrier frequency drifts as a function of time, this node calculates the offset by performing a weighted linear fit on the phase information in the I/Q signal, and then taking the slope of this linear fit. You can use this slope to monitor drift in the carrier frequency.

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    error out

    Error information.

    The node produces this output according to standard error behavior.

    Standard Error Behavior

    Many nodes provide an error in input and an error out output so that the node can respond to and communicate errors that occur while code is running. The value of error in specifies whether an error occurred before the node runs. Most nodes respond to values of error in in a standard, predictable way.

    error in does not contain an error error in contains an error
    If no error occurred before the node runs, the node begins execution normally.

    If no error occurs while the node runs, it returns no error. If an error does occur while the node runs, it returns that error information as error out.

    If an error occurred before the node runs, the node does not execute. Instead, it returns the error in value as error out.

    FM Demodulation

    This node computes the information signal as a result of a demodulation process, which takes place in two steps internally. First the incoming I/Q data is phase-demodulated, and then the result of this phase demodulation is differentiated using a software FIR with three taps. The node returns the information signal in the FM demodulated waveform.

    Where This Node Can Run:

    Desktop OS: Windows

    FPGA: Not supported

    Web Server: Not supported in VIs that run in a web application


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