MT Demodulate FM VI
Performs frequency demodulation on the incoming I/Q signal. The recovered information signal is returned in the FM demodulated waveform parameter.
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FM modulated waveform specifies the baseband (downconverted) time-domain data for demodulation.
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FM deviation specifies 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.
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carrier correction? toggles carrier correction on and off.
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reset? controls initialization of internal VI states. The differentiation step of the demodulation process uses a three-tap FIR filter and involves a transient response.
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remove transient when reset? specifies whether to truncate the first several samples of the FM demodulated waveform when the VI has been reset (for example, on first call or when reset? is set to TRUE). On first call to this VI or when reset? is set to TRUE, set this parameter to TRUE to remove the transient of the FIR filter applied by the FM demodulator. The default is FALSE. |
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error in (no error) accepts error information wired from previously called VIs. Use this information to decide if any functionality should be bypassed in the event of errors from other VIs. Right-click the front panel error in control and select Explain Error or Explain Warning from the shortcut menu for more information about the error displayed.
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FM demodulated waveform returns the frequency-demodulated information signal.
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carrier offset returns the offset between the incoming modulated carrier frequency and the estimated carrier frequency specified in the MT Configure Downconversion Settings VI. If the carrier frequency drifts as a function of time, this VI 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 the drift in the carrier frequency. When carrier correction? is set to FALSE, this parameter returns 0. |
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error out passes error or warning information out of a VI to be used by other VIs. Right-click the front panel error out indicator and select Explain Error or Explain Warning from the shortcut menu for more information about the error displayed.
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Details
The information signal is computed 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.
This VI calculates any residual carrier frequency left in the FM modulated waveform. If you use this VI with NI hardware, the residual frequency is the offset from the expected carrier frequency specified in the MT Configure Downconversion Settings VI. If the actual carrier frequency of the acquired signal is different from the center frequency of the receiver, a DC offset is introduced into the generated FM demodulated waveform. If the carrier correction? parameter is set to TRUE, this offset is removed. This correction is made only in software; the VI does not attempt to establish a lock with the unit under test (UUT).
You can wire the FM demodulated waveform parameter to any of the LabVIEW waveform measurement VIs for more measurements. If the information signal is a single tone, you can wire the FM demodulated waveform parameter to the Extract Single Tone Information VI to measure the amplitude,frequency, and phase of the information signal.