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Spectrum due to switching

Purpose of measurement—what it proves

GSM transmitters ramp RF power rapidly. The “transmitted RF carrier power

versus time” measurement is used to ensure that this process happens at the

correct times and happens fast enough. However, if RF power is ramped

too quickly, undesirable spectral components exist in the transmission. This

measurement is used to ensure that these components are below the acceptable

level.

If a transmitter ramps power too quickly, users operating on different

frequencies, especially those close to the channel of interest, will experience

significant interference.

Failures with this measurement often point to faults in a transmitter’s output

power amplifier or leveling loop.

Theory in pictures

Measurements of spectrum due to switching are performed in a similar fashion

to the measurement of spectrum due to modulation and wideband noise. The

analyzer is tuned to and measures at multiple offset frequencies in zero-span

mode. In this case no time gating is used, so power from both the ramping and

modulation processes affect the measurement. The effect of ramping dominates

the spectrum due to switching measurements.

Again, the specifications are relative so the first step in the process is to establish

a reference. This reference is once again not the same as “mean transmitted RF

carrier power” in the way that it is measured (resolution bandwidth = 300 kHz).

Measure carrier power

in pre-defined bandwidth

Tune to offset

Repeat through offset list

Amplitude

Frequency

Time

Measure power at offset

in pre-defined bandwidth

Subtract offset power from carrier power

Report relative (dBc) result

50%

90%

Figure 15. Theory of spectrum due to switching

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Agilent Technologies Switch/Measure Manual de usuario Pagina 18