IQ Meas Time and IQ Ref Time (Custom IQ)
When
is enabled, the and trace data are the time data results of digital demodulation. is the data results for the measured input signal. trace data is the data results that would be derived from an ideal input signal (reference signal).Concepts
The Custom IQ 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.
Filtering
Different filters can be selected for the I/Q measured and I/Q reference signals. To do this, display the
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, 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.
compensates for bothTime-Domain Displays
The VSA's digital demodulator produces time-domain data. If
is selected, the VSA displays the I/Q measured signal in the time domain. Likewise, if , 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 Trace Formats).
, , , , and formats (seeIf 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 to increase the resolution; decrease it to decrease the resolution.
See Also