17-03-2014, 02:51 PM
Transmit Center and Symbol Clock Frequency Tolerance
Purpose:
To verify that the center frequency offset of the device under test (DUT) is within the
conformance limits.
Resource Requirements:
• Vector Signal Analyzer (VSA), Rohde & Schwarz FSQ26 or equivalent
• ERP access point/station with external antenna jack
• Associated ERP AP or station with external antenna jack (optional)
• Controlling Software (Matlab)
Last updated: April 2008
Discussion:
Reference [1] describes the Extended Rate Physical Medium Dependent (ERP-PMD) sublayer
for the 2.4 GHz band. Section 19.4.7 outlines the transmit function and parameters associated
with the PMD sublayer, and subclauses 19.4.7.2 and 19.4.7.3 specify the transmit center
frequency and symbol clock frequency tolerances.
Transmit frequency offset is the difference between the channel center frequency and the
transmitter center frequency. When the transmit center frequency is offset from the actual
channel frequency, the spectrum of the transmit signal falls in the adjacent channels. This causes
interference in the adjacent channels. This error is even more important for OFDM transmission
where many subcarriers are derived from the same clock. The center frequency tolerance limits
are ± 25-ppm maximum, which corresponds to a frequency drift of ± 60.3 kHz for channel 1 and
± 61.55 kHz for channel 11.
For ERP-DSSS data rates, reference [1] recommends keeping the transmit center frequency and
symbol clocks locked to the same local oscillator; conversely, for ERP-OFDM rates, clock lock
is a mandatory requirement. For this reason, both of these clock tolerances can generally be
tested by measuring either frequency error. The measurement is conducted using a VSA,
comparing its demodulation clock with the digitally demodulated data chipping from the DUT.
Procedure:
1. For DUTs that support continuous transmission, configure the DUT to transmit 100-byte
PSDU’s continuously on channel 6 at a bit rate of 11 Mbps at maximum output power.
Otherwise, configure the DUT to associate with the AP on channel 6 at a bit rate of 11
Mbps at maximum output power; configure the AP to repeatedly transmit 100-byte ICMP
echo request packets to the DUT.
2. Configure the VSA to capture an entire packet.
3. Compute the average center frequency and symbol clock frequency offsets over the entire
packet.
4. For enhanced accuracy, repeat steps 2-3 multiple times and average the deviations.
5. Repeat steps 1-4 for all supported data rates and channels.
Observable Results:
a. The transmit center frequency tolerance should not exceed +/- 25 ppm.
b. The transmit symbol clock tolerance should not exceed +/- 25 ppm