MT Demodulate AM (SSB)
- Updated2023-02-17
- 6 minute(s) read
MT Demodulate AM (SSB)
Demodulates a single sideband (SSB) amplitude-modulated signal.
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Inputs/Outputs
AM modulated waveform
The modulated complex baseband time-domain data for demodulation.
t0
The trigger (start) time of the Y array.
Default value: 0.0
dt
Time interval between data points in the Y array.
Default value: 1.0
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.
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.
Default value: 1.0
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.
- taps: 81
- approx freq.: 0.01
- search (± % of Fsampl): 5
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 value: 81
approx freq.
The estimated frequency, in hertz (Hz), of the pilot tone in the AM modulated waveform parameter.
Default value: 0
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 value: 0.25
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.
Alpha
The parameter, in meters, as required by the Costas loop design.
Default value: 1 m
Beta
The parameter as required by the Costas loop design.
Default value: 20 u
error in
Error conditions that occur before this node runs.
The node responds to this input according to standard error behavior.
Default value: No error
suppressed carrier?
A Boolean that determines whether the carrier has been suppressed in the incoming modulated waveform.
Default value: FALSE
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 value: TRUE
AM demodulated waveform
The recovered message signal.
measurements
The pilot tone location, carrier offset, and phase offset.
extracted pilot tone
The location of the pilot tone in the AM modulated waveform frequency spectrum.
frequency offset
The measured carrier frequency offset, Hz, of the specified AM modulated waveform.
error out
Error information.
The node produces this output according to standard error behavior.
AM-SSB Demodulation
The AM-SSB demodulation procedure uses the transmitted pilot tone
as a reference for performing coherent demodulation. The following diagram illustrates the AM-SSB demodulation procedure.
During the process of AM-SSB demodulation, the received AM-SSB signal (assumed to be sampled at the sample frequency
) is first resampled to a new sample frequency equaling 4. The resampled AM-SSB modulated baseband waveform is then upconverted to an intermediate frequency (IF) passband waveform at a carrier frequency of . 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-SSB 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 .