Nokia 636, 638 Service Manual
After Sales
Technical Documentation
System Module
NHA–2
Original 06/97
Page 4–22
TX VCO
The VCO is a Clapp type oscillator. The oscillator’s resonance frequency is de-
termined by the circuitry L651, C659, V659, C663, C630, C631 and C675. The
center frequency of the resonance circuit is not adjustable (By increasing or de-
creasing the capacitor C659 the resonance frequency can be changed). The
VCO signal is amplified by V657 and fed to the prescaler and to the mixer.
termined by the circuitry L651, C659, V659, C663, C630, C631 and C675. The
center frequency of the resonance circuit is not adjustable (By increasing or de-
creasing the capacitor C659 the resonance frequency can be changed). The
VCO signal is amplified by V657 and fed to the prescaler and to the mixer.
TX Loop Filter
Output pulses from the phase detector N650 pin 17 have to be supplied to the
loop filter. The integrator, which is constituted of R674, C680 and C683, con-
verts positive and negative pulses to d.c. voltage. The remaining ripple is fil-
tered in the low–pass filter accomplished by R672 and C665.
loop filter. The integrator, which is constituted of R674, C680 and C683, con-
verts positive and negative pulses to d.c. voltage. The remaining ripple is fil-
tered in the low–pass filter accomplished by R672 and C665.
Transmitter
The transmitter is realized with four discrete transistors. The modulated RF sig-
nal from the TX synthesizer is applied to the input transistor V660 of the trans-
mitter. V660 amplifies the signal for the power control stage V803, which is
biased to B–class. The power level is controlled by the collector voltage of
V803. Transistors V804 and V805 are biased to C–class and amplify the RF
signal to the desired output power level. The amplified RF signal is fed through
a low–pass filter to the duplex filter. The harmonics of the transmitter are re-
duced by the duplex filter. A voltage proportional to the output power is rectified
from a directional coupler by d.c. biased Schottky diode V801. This rectified
voltage is fed to a differential amplifier which consists of double transistor V809.
There is a negative feedback from the rectified voltage to the bias voltage of
diode V801 consisting of transistor V810 and associated components. The pur-
pose of arranging the feedback is to suppress the rectified voltage to a range
below 3 volts. The reference voltage is filtered from the PWM signal TXC by
R814 and C826. The differential amplifier adjusts the collector voltage of the
transistor V803 so that the reference voltage and the voltage proportional to the
output power are equal. The transmitter is switched on when TXE goes high
(logic 1), which enables the transmitter power control circuit by transistor V808
and the first stage of the transmitter. When the transmitter is inactive (TXE low)
the RF level from the transmitter is reduced below –60 dBm.
nal from the TX synthesizer is applied to the input transistor V660 of the trans-
mitter. V660 amplifies the signal for the power control stage V803, which is
biased to B–class. The power level is controlled by the collector voltage of
V803. Transistors V804 and V805 are biased to C–class and amplify the RF
signal to the desired output power level. The amplified RF signal is fed through
a low–pass filter to the duplex filter. The harmonics of the transmitter are re-
duced by the duplex filter. A voltage proportional to the output power is rectified
from a directional coupler by d.c. biased Schottky diode V801. This rectified
voltage is fed to a differential amplifier which consists of double transistor V809.
There is a negative feedback from the rectified voltage to the bias voltage of
diode V801 consisting of transistor V810 and associated components. The pur-
pose of arranging the feedback is to suppress the rectified voltage to a range
below 3 volts. The reference voltage is filtered from the PWM signal TXC by
R814 and C826. The differential amplifier adjusts the collector voltage of the
transistor V803 so that the reference voltage and the voltage proportional to the
output power are equal. The transmitter is switched on when TXE goes high
(logic 1), which enables the transmitter power control circuit by transistor V808
and the first stage of the transmitter. When the transmitter is inactive (TXE low)
the RF level from the transmitter is reduced below –60 dBm.
The rectified voltage, which is proportional to the power output signal, is fed
along TXI line through R820 to the BB–unit. This TXI line (TX power on Indica-
tor) is used to avoid false transmission.
along TXI line through R820 to the BB–unit. This TXI line (TX power on Indica-
tor) is used to avoid false transmission.