Agilent Technologies Switch/Measure Manual de usuario descargar pdf (Pagina 7)

Base station transmitter measurements in GSM

Phase error and mean frequency error

Note: For each measurement, process and limits vary between device type.

Purpose of measurement—what it proves

Phase error is the fundamental parameter used in GSM to characterize

modulation accuracy. These measurements reveal much about a transmitter’s

modulator performance. Poor phase error indicates a problem with the I/Q

baseband generator, filters or modulator in the transmitter circuitry. The

output amplifier in the transmitter can also create distortion that causes

unacceptably high phase error. In a real system, poor phase error will reduce

the ability of a receiver to correctly demodulate, especially in marginal signal

conditions. This ultimately affects range.

Frequency error measurements indicate poor synthesizer/phase lock loop

performance. This is especially important in a BTS with frequency hopping

active. Poor frequency error measurements can show, for example, that a

synthesizer is failing to settle quickly enough as it shifts frequency between

transmissions. In a real system poor frequency error can cause many problems,

for example, the target receiver might be unable to gain lock and the

transmitter might cause interference with other users. If the latter is the case,

other measurements can determine this with certainty.

Theory in pictures

Phase and frequency error measurements are complex, however modern test

equipment can perform all of the necessary signal processing and mathematics

automatically. Figure 3 shows how the measurement works. The test receiver

or analyzer samples the transmitter output in order to capture the actual phase

trajectory. This is then demodulated and mathematically the ideal phase

trajectory is derived. Subtracting one from the other results in an error signal.

The mean gradient of this signal (phase/time) gives frequency error. The

variation of this signal is defined as phase error and is expressed in terms of

root mean squared (RMS) and peak.

Time (bits)

Phase (deg)

Time (bits)

Phase (deg)

Time (bits)

Phase (deg)

010100011011100... ...01110001101011111

Bit 0 Bit 147

RMS phase error = 1.3°

Peak phase error = 14.7°

Mean freq error = 67 Hz

Sample actual phase trajectory

Demodulate

ompute perfect phase trajectory

Subtract perfect from actual

Derive numerical results

Figure 3. Theory of phase error and mean frequency error

1 2 3 4 5 6 7 8 9 10 11 12 ... 35 36

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