http://cxema.ru
5.2
Functional Description
The main building block for the receiver is the IC U201. The receiver is a double superhet type with the first IF at
201MHz; this is then converted down to zero IF.
The Rx IC contains the following stages:
1.
LNA
2.
RF mixer
3.
Gain controlled 5-stage IF amplifier
4.
I,Q quadrature down converter
5.
Baseband Op Amps for further amplification and some filtering of the baseband I,Q signals
Figure 1:
Receiver IC U201
600-0501
RF input to the receiver is either via the antenna or via the I/O connector for test purposes and handsfree
operation. The input signal from the antenna or the I/O connector is fed into the LNA through the ANT – Rx path of
the ceramic duplexer (FL104). The duplexer provides a low-pass filter function for the Tx signals between its Tx
and ANT ports and an additional notch in the Rx band to minimize the Tx noise within the Rx band. The response
between the ANT and Rx ports is a bandpass filter response providing the roofing filter function for the Rx front
end.
The LNA gain can be controlled via a three-wire bus between a typical value of 17dB and approximately -3dB. The
LNA gain reduction is required for operation under strong signal conditions where input power levels are greater
than about -40dBm.
The output from the LNA goes through a differential BP SAW filter and is differentially fed into the 1st
down-converter mixer. The LO for the mixer is generated by a PLL (U304) employing a modular VCO (U305). The
output from the VCO is buffered by an RF MMIC amplifier (U306). The LO frequency range is 1136.2 to
1160.8MHz.
The IF output at 201MHz from the mixer is filtered by the differential IF SAW filter (FL203) before it is fed into the
gain-controlled IF amplifiers. The use of differential filters eliminates the need for baluns and provides some space
advantage.
The IF amplifier is a five stage cascaded section. The gain is controllable by a three-wire bus from -10 to +70dB in
2dB 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 its baseband. The IF
LO is generated at 402MHz by an external discrete VCO. An on-chip divider on the Rx IC divides this by two and
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 budget requirements.
The DC level at the output of the Op Amps is 0.95V and a resistive DC adder is used to increase this DC levels to
1.425V as required by the baseband IC VEGA (see Section 8).
Issue 1
Section 5
MCUK980101G8
Revision 0
5 - 2
Technical Guide
RECEIVER
