Advertisement
P7389i – Circuit Description
7) The amplified signal is now injected to the base of the dual transistor mixer Q450.
Both mixers are supported by
provided by
RX275_GSM
8) The
RX VCO U250
Whitecap using the
drives the RX VCO IC using the
RVCO_275 (SF_OUT
9) The generated RX VCO signal is then split, with a part going back to the
DM IC – U200.
Pin A3 to serve as the feedback for the RX VCO Phase lock loop.
The other part is firstly amplified through a
being used to mix with the received frequencies through the emitters of the dual
mixer transistor
Q450.
10) The mixer will produce sum and difference signals i.e. RX'ed frequency + RX VCO
frequency and RX'ed frequency - RX VCO frequency. It will be the difference signal
OF 400Mhz, that is now fed to the
filter is the same as was used in previous 400MHz products. The purpose of the SAW
filter is to provide comprehensive removal of harmonics created during the mixing
process.
11) The IF signal fed to the SAW filter will be 400Mhz. The reason for the change to
400Mhz from 215Mhz is to limit the span of the RX VCO e.g.
Description
EGSM L Channel
400Mhz
PCS H Channel
400Mhz
EGSM L Channel
215Mhz
PCS H Channel
215Mhz
As can be seen if the IF was kept at 215Mhz, the frequency span would have to be an
extra 370Mhz. This is turn assists in reducing the part count.
12) The 400Mhz IF signal is then passed to the Isolation Amplifier Q480
The purpose of an Isolation Amp is to couple an analogue signal to adjoining parts of
a circuit with 2 different grounds. Also to protect the base band signals from any stray
RF. The Isolation Amp is supported by
Isolation
Amp
Motorola Internal Use
RX275
(Q112).The tuned emitter biasing voltage is
(Q110) and
RX275_DPCS
is now an integrated circuit and is controlled firstly from the
MQ SPI
bus to program the MAGIC DM and then MAGIC DM
CP_RX
+
GPO4
through Q1102).
SAW Filter FL457
IF
Channel
975
810
975
810
SW_VCC
MAGIC DM
DM
(Q2102)
signal Pin A1. The power is supplied by
Tuned Transistor Amplifier Q252,
(Surface Acoustic Wave), this
Received
RX VCO
Frequency
Frequency
925.2Mhz
1325.2Mhz
1989.8Mhz
1589.8Mhz
925.2Mhz
1140.5Mhz
1989.8Mhz
1774.8Mhz
(MAGIC DM DM U200
101001010100
IC
U200
3
MAGIC
before
Difference
264.6Mhz
634.6Mhz
Pin C7)
Base-band
Signal
Advertisement
