NEC DB2000, Service Manual

Page: 15 / 199

Part 1 : Receiver
The front end
The RF receive signal (GSM 935Mhz - 960Mhz, PCN 1805Mhz-1880Mhz) is
input via the antenna or coaxial connector S400. The coaxial connector has a
built in switch, which is used to switch between the antenna and auxiliary RF input.
See
Fig.1: The Receiver.
The Diplexor IC400 and D400/D401 diode packs are used to control the Tx and
Rx paths. These diodes are enabled by the control signals:- GSM_PRE_ON (Tx),
V_G_LNA (Rx), PCN_PRE_ON (Rx) and V_P_LNA (Tx) respectively. When combined
with the diplexor they provide sufficient protection for the receiver LNA’s from Tx
signals. The receiver front end requires Band Pass Filter’s (BPF) FL303 and FL300
to further protect the LNA’s from out-of-band signals and Tx signals. For the GSM
path the RF signal passes into the BPF FL303, through a matching circuit and into
IC300 #29.
At this stage the signal passes through a LNA within IC300 to improve the signal
to noise characteristics. This LNA is controlled by the enable supply:- V_G_LNA (
derived from IC27-#29) and the output from IC300 #26 is then fed to the input of
BPF-FL302. After this the RF signal is then passed to the Dual Balanced Balun-
L305 for filtering and then fed via balanced outputs to IC300 #13-14 for
conversion into an intermediate frequency (IF).
In the PCN path the RF signal passes through FL300 into a discrete LNA (TR300),
which is controlled by the enable supply:- V_P_LNA (derived from IC27- #30) and
then passed through BPF-FL301 to L305.
The interstage filters provide further out-of-band signal rejection and further image
rejection. The two balanced outputs from L305 are then passed into the
PMB2410 IC300 ready for conversion into an intermediate frequency.