IQ Meas Time (802.11b/g DSSS/CCK/PBCC)

Menu Path: Trace > Data > IQ Meas Time where X is the selected input channel.

When 802.11b/g DSSS Direct sequence spread spectrum. The data transmission scheme (sometimes referred to as a "'modulation" scheme) used in 802.11b WLANs. DSSS uses a radio transmitter operating at a fixed centre frequency, but using a relatively broad range of frequencies, to spread data transmissions over a fixed range of the frequency band. 802.11a and 802.11g (when not operating in 802.11b mode) use Orthogonal Frequency Division Multiplexing (OFDM)./CCK complementary code keying/PBCC packet binary convolutional code Demodulation is enabled, the IQ Meas Time trace is the demodulated time data results sampled at the chip times. This topic provides the following information:

• I-Q Meas Time trace data description

• Compensation information

• Time-Domain Trace Data Display information

• Parameter Interaction information

 

I-Q Meas Time

The measured time data sampled at exactly the chip times with IQ origin offset, system gain normalization and carrier locking applied to the input signal.

Compensation

I/Q origin offset and burst amplitude-droop

Demodulation is performed with carrier lock and chip lock. In addition, the VSA compensates for I/Q origin offset and burst amplitude-droop.

Phase Drift

Circular arcs in the constellation of the IQ Meas Time trace display is an indicator of phase drift. The Track Phase demodulation parameter can be used to remove the phase drift, which will improve the IQ constellation trace and reduce the EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format. data results (MeasSetup > IEEE 802.11b/g Demod Properties > Advanced tab > Track Phase check box).

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. The data can be displayed in many formats including IQ Polar diagrams, Constellation diagrams, Eye diagrams and more (see Selecting a Trace Format)

If normalization is OFF, the VSA displays the instantaneous magnitude. If normalization is ON, the VSA scales the IQ measured trace to map it onto the unit circle, which yields a unitless y-axis.

Parameter Interaction

The following parameters affect how the VSA displays the I/Q measured signal:

• The Result Length parameters (Automatic and Manual) determine the measurement acquisition time, which in turn, determines the length of the displayed trace data.

• The Measurement Offset/Interval parameters limit the displayed trace data.

See Also

Available Trace Data (802.11b/g DSSS/CCK/PBCC)

I/Q Ref Time (802.11b/g DSSS/CCK/PBCC)