Sony Ericsson K800 Electrical Repair Manual page 4

Antenna
The internal antenna is common for all GSM bands and the WCDMA band. There is an antenna
switch between the antenna and the RF electronics allowing usage of an external antenna.
GSM 900/1800/1900
One synthesizer is used for both RX and TX operation. Direct conversion is used for RX and TX.
In TX mode, the PLL works directly on the transmitted frequency, whereas the RX VCOs operates
at the double received frequency. The LO is then divided by two just before entering the RX mixer.
The main component for the frequency synthesis and up-conversion is the GSM RF ASIC. A
direct modulation concept is used and the base-band chip has, together with the GSM RF ASIC,
all the required functions for direct modulation. The use of direct modulation means that no
intermediate frequency (IF) in the transmitter chain is needed. The receiver is of direct conversion
type, i.e. a homodyne receiver (the RX VCOs operates at two times the received frequency).
Thus, there is no need for any other LOs than a "main loop" in this radio.
The modulated transmitter signal is amplifi ed by a power amplifi er before being routed to the front
end module and the antenna.
The front end module contains all necessary functions for separating GSM and WCDMA, and
routing WCDMA signals to and from the WCDMA RF module. It also contains a TX/RX switch for
GSM for routing the received GSM RF signal to the correct GSM receiver input, and routing the
GSM TX signal to the antenna. Band selection fi ltering for GSM RX is also done in the front end
module.
All the GSM parts including the front end module are housed in a separate module.
WCDMA
The WCDMA receiver ASIC contains the receiver chain, the RF synthesizer and the circuitry
needed for the13 MHz crystal reference oscillator, plus the 2.3GHz RF synthesizer. The
transmitter ASIC is also used to generate the IF signal (4*190MHz) needed in Rx receiver.
2.2.6 GSM/DCS/PCS Receiver
The fi rst stage of the receiver is a band selection fi lter, which suppresses unwanted frequencies.
The receiver is a homodyne receiver. The local oscillator frequency is generated by a frequency
synthesizer, which allows the receiver to be set at frequencies in intervals of 200 kHz. The
synthesizer is controlled from the logic/audio part. The receiver chip also contains A/D converters,
and thus I and Q samples are sent to the logic/audio parts as serial data.
2.2.7 WCDMA Receiver
The receiver is of classical super-heterodyne type, with intermediate frequency at 190MHz.
The signal picked up from the antenna is sent, through the antenna switch (which is used to switch
among GSM TX/RX, DCS TX/RX, PCS TX/RX and UMTS) to the WCDMA duplexer.
The main purpose of the WCDMA duplexer is to route the received signal from the antenna
towards the receiver, while simultaneously allowing the transmitter signal from the PA to go to the
antenna and limiting the leakage from the PA to the LNA inside the receiver ASIC.
FUNCTION OVERVIEW Description
The receive signal from the duplexer reaches then the LNA (integrated into the WCDMA RX ASIC)
and the amplifi ed signal is fi ltered by means of an external inter stage SAW fi lter. The fi ltered
signal returns into the WCDMA RX ASIC to be down converted to the IF frequency, 190MHz, by
means of a mixer. The fi rst local oscillator signal is generated in the WCDMA RX ASIC and is
190MHz higher than the input signal, so it ranges between 2300 and 2360MHz.
The IF signal is fi ltered by an external IF SAW fi lter, then amplifi ed and demodulated. After
demodulation, a chain of amplifi ers and low pass fi lters will render the analog I and Q signals.
High pass fi ltering (needed to remove DC offset) is provided externally using series capacitors.
The analog I and Q signals are then processed by the WCDMA signal processor, that contains the
4-bit, analog to digital converters. The sampling rate is 15.36MHz, which is four times the chip rate
(3.84MHz).
2.2.8
The I and Q signals generated from the baseband is modulating the output frequency directly, this
is done in the dedicated GSM/DCS/PCS RF ASIC.
An output power control loop, controlled by the logic, is regulating the output power of the power
amplifi er during the burst.
The transmitter has a dedicated GSM/GPRS output power control block with a logarithmic 8 bit
DAC in a mixed signal ASIC, and one dual band power amplifi er. The DAC is controlled by a serial
bus from the GSM/GPRS baseband controller ASIC.
The PA control block applies a transfer function (2
the DAC. This new value becomes an internal reference value. The regulator in the PA control
block measures I
the difference between the I
(PI-regulator) and then adjusts the PA regulator voltage to cancel the error. The PAREG signal is
fi ltered by a low-pass to give a smooth ramping signal.
2.2.9
Figure 2. WCDMA transmitter.
The picture above shows in a generalized the way how the WCDMA transmitter works. A TX
ASIC is able to generate a dynamic range of more than 81dB while the power amplifi er work as an
amplifi er with fi xed amplifi cation.
The VGA, QVGA and "HGBufferbias" plus "Buffgain" are all inside the TX ASIC.
GSM/DCS/PCS Transmitter
nd
(feed current to the power amplifi er) and calculates the error signal by taking
PA
and the internal reference signal. This error signal is integrated
PA
WCDMA Transmitter
HGBUFFERBIAS
3 bit Power control DAC,
VGA QVGA
Buff gain,
1 bit
Module
PA-DAC
6 bit 2 bit
8 bit
DC/DC for PA
SEMC Electrical Repair Manual
K800 - K810
order low pass) to the reference value from
8 bit
PA -
1202-3085 rev. 2 4 (121)