Frequency Synthesizers; Transmitter - Nokia 9000i Service Manual

RAE/RAK–1N
RF

Frequency Synthesizers

The stable frequency source for the synthesizers and base band circuits is
discrete voltage controlled crystal oscillator (VCXO) in GSM and
PCN.The frequency of the oscillators is controlled by an AFC voltage,
which is generated by the base band circuits. The VCXO is always
running when the CMT is powered up. The nominal frequency is 26 MHz
in GSM and PCN.
The UHF PLL generates the down conversion signal for the receiver and
the up conversion signal for the transmitter.
The VHF PLL signal ( divided by 4 in CRFRT) is used as a local oscillator
for the last mixer. Directly it is used as a second local oscillator in PCN.
The VHF PLL signal (divided by 2 in CRFRT) is also used in the I/Q
modulator of the transmitter chain. The VHF VCO is made of discrete
components.

Transmitter

The TX intermediate frequency is modulated by an I/Q modulator
contained on transmitter section of CRFRT IC. The TX I and Q signals are
generated in the RFI interface circuit and they are fed differentially to the
modulator.
Modulated intermediate signal is amplified or attenuated in temperature
compensated controlled gain amplifier (TCGA). The output of the TCGA is
amplified and the output level is typically –10dBm.
The output signal from CRFRT is band–pass filtered (in PCN low–pass
filtered) to reduce harmonics and the final TX signal is achieved by mixing
the UHF VCO signal and the modulated TX intermediate signal with
passive mixer. After mixing the TX signal is amplified and filtered by two
amplifiers and filters except in GSM there is only one filter. These filters
are dielectric filters in both GSM and PCN. After these stages the level of
the signal is typically 1 mW (0 dBm) in GSM and 2 mW (+3 dBm.) in PCN.
The discrete power amplifier amplifies the TX signal to the desired power
level. The maximum output level is typically 1.5...2.0 W in GSM and
0.8...1.0 W in PCN in the antenna terminal of the duplex filter.
The power control loop controls the output level of the power amplifier.
The power detector consists of a directional coupler and a diode rectifier.
Transmitted power is controlled with TCGA on TX–path of CRFRT. Power
is controlled with TXC and TXP signals. The power control signal (TXC),
which has a raised cosine form, comes from the RF interface circuit (RFI),
which is located in the baseband section.
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Amendment 1 04/97