
Motorola Internal Use
3
P7389i – Circuit Description
7)
The amplified signal is now injected to the base of the dual transistor mixer
Q450
.
Both mixers are supported by
RX275
(
Q112
).The tuned emitter biasing voltage is
provided by
RX275_GSM
(
Q110
) and
RX275_DPCS
(
Q2102
)
8)
The
RX VCO U250
is now an integrated circuit and is controlled firstly from the
Whitecap using the
MQ SPI
bus to program the MAGIC DM and then MAGIC DM
drives the RX VCO IC using the
CP_RX
signal
Pin A1
. The power is supplied by
RVCO_275
(
SF_OUT
+
GPO4
through
Q1102
).
9)
The generated RX VCO signal is then split, with a part going back to the
MAGIC
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
Tuned Transistor Amplifier Q252,
before
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 fre RX VCO
frequency and RX’ed frequency - RX VCO frequency. It will be the difference signal
OF 400Mhz, that is now fed to the
SAW Filter FL457
(Surface Acoustic Wave), this
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
IF
Channel
Received
Frequency
RX VCO
Frequency
Difference
EGSM L Channel
400Mhz
975
925.2Mhz
1325.2Mhz
264.6Mhz
PCS H Channel
400Mhz
810
1989.8Mhz
1589.8Mhz
EGSM L Channel
215Mhz
975
925.2Mhz
1140.5Mhz
634.6Mhz
PCS H Channel
215Mhz
810
1989.8Mhz
1774.8Mhz
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
SW_VCC
(
MAGIC DM DM U200
Pin C7
)
Isolation
Amp
MAGIC DM
DM
IC
U200
101001010100
Base-band
Signal