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12 Transient Spectrum (Spectrum due to switching)
12.1 What is Transient spectrum
In the GSM-system is all communication between the base station and the phone done
switched, in shape of bursts.
The burst is a squared output power pulse with step up and down-ramping.
Every time the voltage of the squared pulse changes rapidly there will be formed a number of
over tones.
The over tones have got different frequencies and amplitudes.
The amount of over tones and what amplitude they have got depends on how steep the up-
and down-ramping are, the higher up- and down-ramping, the higher amplitude and frequency
of the over tone.
The over tones form a spectrum that is called transient spectrum or "Spectrum due to
switching".
To be able to get lower amplitudes for the over tones in the transient spectrum, the up and
down-ramping does not change momentarily from zero-to-max/max-to-zero.
Instead this is done with two help steps, these two help steps are called Low and High
Intermediate Power level.
The Fig. 12.1 below shows what the control signal POWLEV from N500 (DAC 2) looks like.
The times in the figure are approximates measured at an approved phone at full Power level.
The control voltage POWLEV then passes through an exponential amplifier and a Bessel low
pass filter in N700 where the transient disturbance is reduced.
This gives a control voltage without the straight, vertical edges and the sharp corners that pro-
duces the over tones. The Zero Power level is a voltage level from the DAC that assures no
output power at all. The amplified and filtrated control voltage is called PAREG and looks
like the Fig. 12.2 below.
4/00021-3/FEA 209 544/25 C
©
Ericsson Mobile Communications AB
Fig. 12.1
Trouble Shooting Guide, Advanced
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