17
SECTION 6
DIAGRAMS
6.1 Radio Frequency (RF) Block Diagram
6.1.1 General
The radio part of the CMD-J6/J26 handles the air interface according to the GSM specifications.
It is the main contributor for the dualband functionality for E-GSM 900 and DCS 1800.
Receiver
The receiver consists of two separated RF paths for GSM and DCS 1800 each.
On the transmission line between the antenna switch of the RF part and the antenna there is the RF connector
including a mechanical switch to connect an external antenna for accessory use. The integrated mechanical
switch switches between the helix antenna and the external RF antenna connection. As long as a plug is
connected the antenna is deactivated and all RF signals go across the connector.
The GMSK (Gaussian-filtered minimum shift keying) modulated 900 MHz / 1800 MHz signal received by the
antenna is filtered, gained and downconverted into the baseband via an intermediate frequency of 440 MHz.
The 440 MHz filter is followed by a tuneable amplifier and IQ demodulator. Behind the demodulator there are
additional base band amplifiers and low pass filters.
The In Phase (I) and Quadrature (Q) signal outputs are balanced each.
A 13 MHz voltage-controlled-temperature-compensated crystal oscillator (VCTCXO) generates the clock
frequency for the PLL and baseband part.
Transmitter
In the transmit direction the digitally GMSK-modulated baseband signal is upconverted to the transmit frequency
in the 900 MHz / 1800 MHz band via an intermediate frequency of 195 MHz (GSM) / 325 MHz (DCS 1800) by
means of an IQ modulator and the subsequenting sum loop.
All RF and IQ signals are balanced signals in order to suppress crosstalk effects. According to GSM
recommendations transmitter and receiver are never active at the same time.
(TDMA system Time Division Multiple Access).
2
1
ϕ
3/5
1
ϕ
M
1
ϕ
N
1
N
1
LO2 (IF) PLL
1 MHz step
N
N+1
1
LO1 (RF) PLL
200 kHz step
PLL
Antenna
RF Connector with
mechanical switch
13 MHz
Transceiver
90°
0
°
∑
2
1
VCTCXO
LO1 (RF) PLL:
- RX -
GSM 1365..1400 MHz
DCS 1365..1440 MHz
- TX -
GSM 1370..1440 MHz
DCS 1385..1460 MHz
LO1
VCO
RX
TX
GSM
880..915 MHz
DCS1800
1710..1785 MHz
Loop
Filter
DCS
GSM
Ceramic Filter
1805..1880 MHz
SAW Filter
925..960 MHz
LNA
LNA
SAW Filter
1805..1880 MHz
IF
SAW Filter
440 MHz
LO2 (IF) PLL
RX 880 MHz
TX 975 MHz
GSM :5
DCS :3
GSM 195 MHz
DCS 325 MHz
RF
RF IF
IF
LO
LO
Q
I
90°
0
°
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