Nokia Mobile Phones NSE-5 Series Service Manual page 74

NSE–5
System Module
To ensure enough power gain in the GSM1800 TX chain the TX signal
then passes through the buffer (pre amplifier). The buffer is driven into
saturation to compensate for variations in CRFU3 output level and ripple
in the first TX interstage filter and to ensure constant input level at the
GSM1800 PA.
The next stage is the second TX interstage filter, which attenuates
unwanted frequency components from the buffer. The interstage filter also
attenuates wideband noise. Both interstage filters is the same type of
bandpass SAW-filter.
Between the second interstage filter and the GSM1800 PA the second
attenuator is placed. The attenuator ensures both stability of the PA
because of constant 50 on the PA input and the right input level.
After the second attenuator in the GSM1800 TX chain, the TX–signal is
fed into the input of the GSM1800 PA. The GSM1800 PA contains three
amplifier stages, interstage, input and output matchings. The PA has more
than 33 dB power gain and the maximum output power is approx. 33 dBm
at an input level of 0 dBm. The gain control range is over 35 dB to get the
desired power levels and power ramping up and down.
The GSM1800 transmitter has no duplexer, but a TX/RX switch instead.
This is due to space limitations. The TX/RX switch is set to transmit
position with BAND_SEL = low and VTX = high.
After the TX/RX switch the signal is fed to the diplexer. There is a
directional coupler connected between the PA output and the input of the
TX/RX switch to provide feedback for the power loop.
Transmitter power control for GSM900 and GSM1800
The power control circuit consists of the gain control stage of the PA, a
power detector at the PA output and an error amplifier in the SUMMA.
There is a directional coupler connected after the PA output in both
chains, but the power sensing line and detector are common for both
bands. The GSM900 feedback signal is attenuated to the same level as
the GSM1800 feedback signal. The combining of the two feedback signals
is achieved with a diplexer. A sample is taken from the forward going
power. This signal is rectified with a schottky-diode and after RC-filtering a
DC-voltage is available. The DC–voltage reflects the output power. This
power detector is linear on absolute scale, with the exception that it
saturates on very low and high power levels, i.e. it forms an S-shaped
curve.
The detected voltage is compared in the error-amplifier in the SUMMA to
the TX power control voltage (TXC), which is generated by the
DA-converter in the COBBA. The output of the error amplifier is fed to the
gain control input of the PA. Because the gain control characteristics in the
PA are linear in absolute scale, the control loop defines a voltage loop,
when closed. The closed loop tracks the TXC-voltage. The shape of the
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PAMS
Technical Documentation
Issue 1 07/99