IQ Ref Spec (802.11b/g DSSS/CCK/PBCC)

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, IQ Ref Spec is the frequency spectrum of the IQ Ref Time trace data. The VSA produces the spectrum by windowing and FFTing the I/Q reference data.

The VSA's demodulator removes carrier-frequency error. Therefore, in IQ Ref Spec displays, you must add the carrier-frequency error to marker readouts to obtain the input signals frequency information. The carrier-frequency error (Freq Err) is shown in the symbol table.

By default, the VSA displays the data in log(dB) magnitude format. The format can be changed in the Trace Properties dialog box (click Trace > Format > Format select box).

The span for the displayed spectrum is a function of the chip rate:

$image\DSSS_spanDemodEq_wmf.jpg$

where: Span-Demod is the span displayed when you select IQ reference spectrum.

It is possible to set the chip rate such that Span-Demod exceeds the information bandwidth of the VSA. The information bandwidth of the VSA is set with the span frequency property

The following illustration shows the case where Span-Demod exceeds span.

$image\span_demod_wmf.jpg$

The illustration shows an "x" in the region where Span-Demod exceeds span. Because the VSA essentially filters out all information outside of span, the VSA will not show information that may exist in this region.

Aliasing

It is important to know that the VSA uses one sample rate to acquire data and another sample rate (Sample-Rate-Demod) to display the demodulated data:

Sample Rate = 1.28 x [span]
Sample_rate_demod = chip rate

As shown, the frequency span property determines the sample rate used to acquire data, whereas the chip rate determines the sample rate used to display demodulated data.