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MT Downconvert Passband (MT Downconvert Passband (Complex)) (G Dataflow)

Version:
    Last Modified: February 7, 2018

    Downconverts a complex baseband signal, which is centered around a non-zero center frequency, to a center frequency of zero. This node can be used in simulated as well as hardware-equipped applications.

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    complex waveform

    Signal for downconversion in passband form.

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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 array representing the signal for downconversion.

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    carrier frequency

    The center frequency of the passband, in hertz (Hz). This frequency is downconverted to 0 Hz. Enter the expected carrier frequency of the incoming signal for downconversion.

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    passband bandwidth

    The bandwidth, in Hz, of the passband signal data. The node ignores this parameter if you set the reset? parameter to FALSE.

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    advanced filter parameters

    Filter parameters.

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    passband ripple

    The ripple in the passband, in dB. The ripple is the ratio of the maximum deviation from the average passband amplitude to the average passband amplitude. The value must be greater than zero.

    Default: 0.01

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    stopband start

    The start of the stopband, Hz.

    Default: 0

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    compute filter stopband ?

    A Boolean that determines whether to compute the stopband start or use the value that you specify in the stopband start parameter.

    TRUE Computes the stopband start based on the carrier frequency and passband bandwidth parameters.
    FALSE Uses the value that you specify in the stopband start parameter.

    Default: TRUE

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    stopband gain

    The stopband gain, in dB. The gain is the negative of the minimum attenuation of the stopband with respect to the average amplitude of the passband.

    Default: -96

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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 whether to use values specified by the initial phase, passband bandwidth, enable filter, passband ripple, and stopband start parameters.

    TRUE The node uses these parameter values at each call.
    FALSE The node ignores these parameters and continues using values supplied in the previous call. Reusing previous input values is useful when sequential data blocks represent contiguous signal data.

    Default: FALSE

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    initial phase

    The initial phase, in degrees, of the software local oscillator used in the downconversion process. The node ignores this parameter if you set the reset? parameter to FALSE. Use the initial phase parameter to match the phase of the incoming modulated carrier and the local oscillator(s) of the downconversion process.

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    enable filter

    A Boolean that determines whether to perform software filtration on the downconverted data.

    TRUE The node filters the downconverted waveform parameter using a software FIR filter.
    FALSE Disables the downconversion filter and generates unfiltered data in the downconverted waveform parameter.

    Default: TRUE

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    downconverted waveform

    The downconverted signal in complex envelope format.

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    t0

    The trigger (start) time of the acquired signal.

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    dt

    Time interval between data points in the acquired signal.

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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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    ripple

    The deviation of the passband gain from the nominal gain of 0 dB.

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    filter length

    Number of taps in the filter.

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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.

    Downconversion Filter

    Using a downconversion filter with this node may lead to the apparent loss of samples in the output waveform. Refer to Filter Delay for more information about this effect. When you set enable filter to TRUE, the downconverted I/Q signal is filtered by a software lowpass FIR filter with design parameters that are dictated by the following considerations:
    • If carrier frequency is greater than passband bandwidth, the filter stopband begins at carrier frequency.
    • If carrier frequency is less than passband bandwidth, the filter stopband begins between carrier frequency and (2 * carrier frequency) - (passband bandwidth/2).

    Filter Delay

    Finite impulse response (FIR) filters are used for different operations such as pulse-shaping, matched filtering, and downconversion filtering. For such filters, the output signal is related to the input signal as shown by the following equation:
    y [ n ] = b 0 x [ n ] + b 1 x [ n 1 ] + ... + b P x [ n P ]

    where

    P is the filter order

    x[n] is the input signal

    y[n] is the output signal

    bi are the filter coefficients

    The initial state for all samples in an FIR filter is 0. The filter output until the first input sample reaches the middle tap (the first causal sample) is called the transient response, or filter delay. For an FIR filter that has N taps, the delay is (N-1)/2 samples. This relationship is illustrated in the following figure, where a sine wave is filtered by an FIR filter with 50 taps.

    Recovering Samples in Single-Shot Operations

    In single-shot operations for modulators and demodulators, the filter delay is truncated before the signal is generated because these samples are not valid. Some samples at the end of the block do not appear at the modulator or demodulator output, and hence appear to have been lost.

    You can recover these samples by sending extra samples to the modulator or demodulator. To determine how many extra samples you must add, use the following guidelines:
    • For modulation: Let L be the pulse-shaping filter length, m be the number of samples per symbol, and M be the modulation order. The number of bits to be added to the input bit stream is given by the following formula:
      N = ( L 1 ) log 2 M m
    • For demodulation: Demodulation use filters during matched filtering. Let L be the length of the matched filter. The number of samples to be added to the input signal prior to filtering is given by the following formula:
      N = L 1 2
      The N extra samples are obtained by repeating the last sample value of the input signal N times to ensure signal continuity.

    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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