IQ Meas Time and IQ Ref Time (Digital Demod)

Menu Path: Trace > Data > IQ Meas Time or IQ Ref Time

When Digital Demod is enabled, the IQ Meas Time and IQ Ref Time trace data are the time data results of digital demodulation. IQ Meas Time is the data results for the measured input signal. IQ Ref Time trace data is the data results that would be derived from an ideal input signal (reference signal).

Concepts

For all demodulation formats except FSK and CPM, the digital demodulator produces two signals: I/Q Measured and I/Q Reference.

I-Q measured signal

The IQ measured signal is the result of resampling the data to an integer number of points per symbol and applying carrier/symbol locking, IQ origin offset and amplitude droop compensation, system gain normalization, and filtering to the input signal. The filtering is user-selectable.

I-Q reference signal

A powerful analysis technique involves comparing a demodulated signal with an ideal signal generated from detected bits. The VSA detects bits from the measured IQ signal and reconstructs a sequence of ideal I and Q states. These are then treated as ideal impulses and are baseband filtered according to a reference filter selected by the user. The resultant IQ reference can be overlaid or compared side-by-side with the IQ measured signal.

The reference filter can be selected independently from the measured signal (although the alpha/BT Bandwidth Time Product is the same for both filters). Therefore, different filters can be applied to the measured and reference signals to accommodate special test requirements.

Special considerations for FSK demodulation

Separate I and Q signals do not exist for FSK demodulated signals. There is no phase information (the imaginary data is zero) because FSK demodulation performs FM Frequency Modulation demodulation on the input signal. Consequently, the FSK Frequency Shift Keying: A form of modulation using multiple carrier frequencies to carry the digital information. The most common is the two frequency FSK system using the two frequencies to carry the binary ones and zeros. demodulator produces real rather than complex data. The display result is a baseband, single-sided spectrum and a time display representing frequency versus time.

Filtering

Different filters can be selected for the I/Q measured and I/Q reference signals. To do this, display the Digital Demodulation Properties dialog box and choose a filter under Measurement Filter for the I/Q measured signal; choose a filter under Reference Filter for the I/Q reference signal.

Compensation

All demodulation schemes are performed with carrier lock, symbol lock, and filtering (if filtering is selected). In addition, all schemes (except those listed in the following Note) compensate for I/Q origin offset and burst amplitude-droop.

Compensation removes the effects of an error on the I/Q measured signal. For example, BPSK compensates for both I/Q origin offset and burst amplitude-droop, which means I/Q origin offset and burst amplitude-droop do not affect the I/Q measured signal.

Time-Domain Displays

The VSA's digital demodulator produces time-domain data. If IQ Meas Time is selected, the VSA displays the I/Q measured signal in the time domain. Likewise, if IQ Ref Time, the VSA displays the I/Q reference signal in the time domain.

The data can be displayed in a variety of trace data formats, including I-Q, Constellation, Q-Eye, I-Eye, and Trellis-Eye formats (see Trace Formats).

If normalization is OFF, the magnitude is shown in units of Volts. If normalization is ON, the I/Q reference signal and I/Q measured signal are normalized. For further details about trace normalization, see Normalize IQ Traces.

The following parameters affect how the VSA displays the I/Q measured and I/Q reference signals:

• Result Length determines the number of symbols, which determines the length of the displayed trace.
• Points / Symbol determines the number of points between symbols. Increase Points / Symbol to increase the resolution; decrease it to decrease the resolution.

See Also

IQ Meas Spec and IQ Ref Spec (Digital Demod)