User`s guide

2-7

GPIB Tutorial and Examples

Example C - Making Measurements

Tips:

For help, look at the “PERFORM FAST POWER MEASUREMENT” section of example

program 1.

DSP Measurements

Modulation accuracy for GSM is determined by measuring the phase and frequency error.

The Agilent 8922M/S uses its DSP analyser for making this measurement. When

triggered, the DSP analyser samples a single GSM TDMA burst and performs several

measurements in parallel namely: peak phase error, rms. phase error, frequency error,

power versus time, data bit display, burst timing and TX peak power. The DSP analyser

shares measurement hardware with the Agilent 8922M/S real-time demodulator. The

hardware can be switched from one mode to another, either making measurements or

demodulating. The demodulator is used to decode the trafﬁc channel and control channel

data being transmitted by the mobile on the up-link. This control information includes the

FACCH and SACCH used for changing ARFCN and TX Level. For this reason, different

techniques are used for ARFCN and TX Level changing while performing DSP

measurements.

TX Level changes are signalled on the down-link SACCH. When the level change is

complete, the mobile signals the new TX Level on the up-link SACCH. The

Agilent 8922M/S does not need to decode this message for the level change to operate

correctly. For TX Level changes, the DSP analyser can remain conﬁgured for

measurements and does not need to re-conﬁgured for demodulation. However, the up-link

SACCH reports, used in our program to conﬁrm the TX Level change, are not being

demodulated and so are not available to read.

ARFCN changes are signalled using the FACCH. In order for the channel assignment to

work correctly, a two-way exchange of messages has to take place between the mobile and

the Agilent 8922M/S. This requires the DSP analyser to be conﬁgured for demodulation.

When an ARFCN change is requested during a DSP measurement, the DSP analyser is

automatically re-conﬁgured for demodulation, the channel assignment is performed, once

complete, the DSP analyser is conﬁgured for measurements once more.

Single trigger mode is most suitable for DSP measurements over GPIB. ARFCN changes

can be accomplished simply by programming the new ARFCN and triggering the DSP

measurement. The measurement will begin only once the channel change has been

completed. TX Level changes are accomplished by programming the new TX Level then

triggering the DSP measurement. Since the DSP measurement takes 200 to 300

milliseconds to be initialised and sample a TDMA burst, there is a reasonable probability

that in many cases the mobile will have changed level in time for the ﬁrst measurement.

Checking the measured TX power is close to the expected power provides conﬁrmation

that the TX Level change has been performed. If the measured power is different from the

expected power, the DSP measurement can be performed up to two additional times. The

time taken for three DSP measurements exceeds the time allowed for a mobile to make a

TX Level change.