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MT Demodulate AM (VSB) (G Dataflow)

Last Modified: January 9, 2017

Demodulates a vestigial sideband (VSB) amplitude-modulated signal.

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AM modulated waveform

The modulated complex baseband time-domain data for demodulation.

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t0

The trigger (start) time of the Y array.

Default: 0.0

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dt

Time interval between data points in the Y array.

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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modulation index

The expected modulation index of the AM demodulated waveform parameter. This value is used to scale the AM demodulated waveform parameter.

  • Set this value to the estimated modulation index of the incoming AM modulated waveform signal to scale the AM demodulated waveform parameter by this value. The resulting scaled AM demodulated waveform can be used to quantify error between the actual and expected modulation index.
  • Set this value to 1.0 to return an AM demodulated waveform with no scaling. When you set suppressed carrier? to FALSE, the peak amplitude value of the unscaled AM demodulated waveform represents the true modulation index of the incoming AM modulated waveform.
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Note  

The node ignores the modulation index parameter if you set the suppressed carrier? to TRUE.

Default: 1.0

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pilot extraction filter

Parameters used to locate the pilot tone and extract it from the specified AM modulated signal.

The node designs a bandpass filter for locating the pilot tone, and a band-reject filter for removing the pilot tone from the recovered signal, in a normalized frequency range:

[dt *approx freq. - 0.01 * search (± % of Fsampl)

dt * approx freq.+ 0.01 * search (± % of Fsampl)]

where Fsampl is the sampling rate of the AM modulated waveform parameter.

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Note  

Carrier frequency offset can change the location of the pilot tone in the AM modulated waveform parameter. In such a case, specify a value for search (+/-% of Fsampl) large enough for the demodulator to locate the pilot tone in the presence of carrier frequency offset. For example, assuming a sample rate of 1.00 S/s, a pilot tone at 0.01 Hz, and a carrier frequency offset of 0.03 Hz, specify the following parameters for the pilot extraction filter cluster:

  • taps: 81
  • approx freq.: 0.01
  • search (± % of Fsampl): 5
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taps

The number of taps used to design the pilot tone extraction filter.

The number of taps you specify in the pilot extraction filter cluster is also used to set the number of taps used in the pilot removal or rejection filter before recovering the demodulated message signal. If the pilot tone is near the message bandwidth, the default number of taps specified in the pilot extraction tap cluster might be insufficient to provide enough pilot rejection. As a result, the demodulated message signal might be distorted, even though the pilot was successfully extracted. In such cases, NI recommends experimenting by increasing the number of filter taps, although the increase affects the execution speed of the demodulation node.

Default: 81

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approx freq.

The estimated frequency, in hertz (Hz), of the pilot tone in the AM modulated waveform parameter.

Default: 0

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search (+/- % of Fsampl)

The search window size as a percentage of the input sample rate (1/dt). To avoid erroneous detection of the pilot tone, ensure that this search window does not include any portion of the message spectrum.

Default: 0.25

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costas loop parameters

Optional parameters controlling the Costas loop. Usually Beta is chosen to be less than 0.02 * Alpha. These parameters are used in tracking carrier phase offset in the specified AM modulated waveform. The default settings are typically unchanged except in the presence of severely time-varying carrier phase offset.

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Alpha

The parameter, in meters, as required by the Costas loop design.

Default: 1 m

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Beta

The parameter as required by the Costas loop design.

Default: 20 u

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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.

Default: no error

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suppressed carrier?

A Boolean that determines whether the carrier has been suppressed in the incoming modulated waveform.

Default: FALSE

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reset?

A Boolean that determines whether the node continues demodulation using internal states from previous iterations.

TRUE The node redesigns the pilot extraction filter and clears the internal filter states on each call.
FALSE The node does not change the pilot extraction filter design across multiple calls, and sets the internal filter state to ensure continuous filter operation.

Default: TRUE

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AM demodulated waveform

The recovered message signal.

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Note  

Wire the AM demodulated waveform parameter to any LabVIEW waveform measurement node for further analysis. If the information signal is a single tone (normalized) and modulation index is set to 1.0 with suppressed carrier? set to FALSE, the peak amplitude value of the AM-demodulated waveform represents the true modulation index of the incoming AM-modulated waveform.

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measurements

The pilot tone location, carrier offset, and phase offset.

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extracted pilot tone

The location of the pilot tone in the AM modulated waveform frequency spectrum.

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frequency offset

The measured carrier frequency offset, Hz, of the specified AM modulated waveform.

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error out

Error information. The node produces this output according to standard error behavior.

AM-VSB Demodulation

The AM-VSB demodulation procedure uses the transmitted pilot tone e j ω p i l o t t as a reference for performing coherent demodulation. The following diagram illustrates the AM-VSB demodulation procedure.

During the process of AM-VSB demodulation, the received AM-VSB signal (assumed to be sampled at the sample frequency f s ) is first resampled to a new sample frequency equaling 4 f s . The resampled AM-VSB modulated baseband waveform is then upconverted to an intermediate frequency (IF) passband waveform at a carrier frequency of f s . The IF passband waveform is passed through a real IF pilot tone extraction filter and a real IF pilot tone rejection filter. The extracted pilot tone at the output of the IF pilot tone extraction filter is passed through a carrier phase and frequency offset estimator to estimate the channel impairments. The estimated carrier phase and frequency offset corrections are applied to the extracted AM-VSB modulated signal-only component at the output of the pilot tone rejection filter. The resulting recovered message signal is obtained after downconversion and resampling to the input sample rate f s .

Where This Node Can Run:

Desktop OS: Windows

FPGA: Not supported


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