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RECEIVER
Issue A
Section 5
MCUK990801G8
Revision 0
– 22 –
Technical Guide
5.2 Functional Description
The main building block for the Dual Band Receiver is the Hitachi Bright2 IC. The receiver is a double superhet type with the
1st IF at 225 MHz and 2nd IF at 45 MHz. The intermediate frequencies are common to both frequency bands.
The Rx IC contains the following stages:
1.
Gain controlled 1st mixer for GSM 900 band.
2.
Gain controlled 1st mixer for GSM 1800 band.
3.
Gain controlled 2nd mixer.
4.
Gain controlled IF amplifier
5.
I,Q quadrature down converter.
6.
Baseband Op Amps for further amplification and some filtering of the baseband I,Q signals.
5.2.1 Dual Band Receiver
Figure 5.1: Dual Band Receiver
RF input to the receiver is either via the antenna or via the H/F RF connector for test purposes. The input signal from the
antenna or the H/F RF connector is fed into the GSM 900/GSM 1800 dual band LNA MMIC via the Diplexer port, Tx/Rx
switches to the dual band Rx SAW filter. The diplexer band splits the two GSM frequency bands whilst the pin switches route
the signal flow from the receiver and the transmitter as required. The receiver 1st SAW filter provides the roofing filter for the
Rx front end.
The dual band LNA gain is constant, typically 18 dB for GSM 900 and 17dB for GSM 1800. Noise factor is typically 1.5dB for
both bands. Operation of the dual band LNA is controlled directly by the PCNnGSM band switch signal.
The output from the LNA passes through a dual BP SAW filter and is then fed into the 1st mixer for each band. Both mixers
are controlled by a 3-wire bus, typically providing b9.5dB and -2dB gain for GSM900, and b8.5dB and -3.5dB
for GSM 1800.
The IF output at 225 MHz from the 1st mixer is filtered by the differential IF SAW filter before it is fed into the 2nd mixer. The
use of differential filters eliminates the need for baluns and provides some cost and space advantage. The 2nd mixer is also
gain controlled from a 3-wire bus, typically b13dB and -3.0dB. Gain switching is synchronised with the 1st mixer.
Output from the 2nd mixer at 45 MHz is filtered by an L-C network before it is fed to the gain controlled IF amplifiers.The IF
amplifier is gain controlled by a three-wire bus from -29 to +69 dB in 2 dB steps. This function is used for AGC purposes.
The output from IF amplifiers is fed into two quadrature mixers where it is converted down to baseband. The IF LO is generated
at 540 MHz by an external discrete VCO module. An on-chip divider on the Rx IC divides this six and then by two. It also
produces two outputs in quadrature to generate the baseband I and Q signals. The outputs from the mixers are connected to
external pins through a pair of buffers. Two on-chip Op Amps are used to amplify the AC signal from the mixers to meet the
overall signal performance requirements.
The DC level at the output of the Op Amps is 1.35 V with a 200 mV p-p single ended AC swing controlled by the AGC loop to
maintain optimum receiver performance.
GSM 900
DUAL
LNA
U203
PGC
GSM 1800
1800
DUAL RX
SAW
FL203
DUAL BAND RECEIVER
(PART)
U101
FL200
IF
SAW
1st
Mixer
2nd
Mixer
900
1800
900
LPF
Rx
Tx
Rx
Tx
Ext
Ant.
HPF
10085-1
S101
DUAL Rx
SAW
FL201
DIPLEXER
I
Q
Demodulator
