Setting Up a Measurement (802.11b/g DSSS/CCK/PBCC)
When setting up a 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 measurement, there are several things to be considered. This topic tells how to connect your signal to an 89600 VSA and successfully demodulate digital signals. The information is provided in this order:
- Connecting Your Signal to the Measurement Hardware
- Center Frequency
- Frequency Span
- Input Range
- Chip Rate
- Search Length
- Maximum Result Length
Connecting Your Signal to the Measurement Hardware
The measurement hardware connects your carrier signal or, if the measurement hardware has two input channels: the raw I (In-phase) and Q (Quadrature-phase) signals.
For all receiver configurations except , connect your signal to Channel 1. In this case, the VSA assumes that your signal contains both the I and Q components (the carrier signal). If the measurement hardware has two input channels, the VSA ignores the second input channel.
If the measurement hardware has a second input channel, the raw I (In-phase) and Q (Quadrature- phase) signals can be connected. To do this, select . With this configuration, the VSA removes the quadrature mixer from the demodulator's signal flow, which connects the I component of your signal to Channel 1; the Q component to Channel 2.
Notes for I+jQ input:
- Selecting an configuration enables DC coupling.
- Use User Correction to correct for any offset or gain imbalance between the I and Q signals.
- Range is set independently for the I and Q inputs, but can be coupled by selecting the Couple Paired Channels parameter.
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Center Frequency
The VSA's center frequency should be set to within approximately ±500 kHz kiloHertz: A radio frequency measurement (one kilohertz = one thousand cycles per second). of the transmitted carrier frequency. The VSA can lock to the signal when the center frequency is off by up to +/- 2.5 MHz Megahertz: A unit of frequency equal to one million hertz or cycles per second., but 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. values may be higher if not within +/- 500 kHz.
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Frequency Span
When selecting a frequency span, select the narrowest span that includes all of your signal components (select a span that is slightly larger than the bandwidth of your signal). If a span that is too narrow is selected, the measurement may have excessive errors or the VSA may lose carrier lock. If the span is less than 19.8 MHz, the VSA displays to indicate that the results may not be accurate.
If the span is too large, spurs may be aliased into the region of interest. Selecting Show All Frequency Points () can help determine if spurs are being included in the result. If so, the span should be reduced to exclude them and provide a more accurate result.
Using the Preset to Standard configuration setting will select a suitable default span.
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Input Range
The input range must be set correctly to obtain accurate measurements. Input ranges that are too low overload the measurement hardware's ADC Analog-to-Digital Converter. Input ranges that are too high increase noise, which increases errors reported in error summary data, such as EVM.
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Chip Rate
Use the Preset to Standard default value. Experienced users may adjust this parameter to fine-tune the demodulator.
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Search Length
The search length must be long enough capture a complete burst. Using long search lengths will increase the measurement time, but make it more likely that a pulse will be found.
If a pulse is not found a "PULSE NOT FOUND" status message will be displayed.
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Maximum Result Length
Use the Preset to Standard default value. This will provide a good starting value for the result length.
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See Also
