Motorola 38C V100 Service Manual page 15

Level 3 Service Manual
6881036B20
All information about the transmission burst is formulated within the Whitecap IC
(timing of the burst, transmit channel, error correction protocol, and in which frame
the information will be carried to the base station). All this information is then
added to the digitized audio and is transferred to the Magic IC U200 along the TX
SPI bus. The bus is made up of BCLKX (base-band clock transmit) Pin B3 and BDX
(base-band data transmit) Pin B6. Since the timing for this data is already decided
for the transmission burst, a frame synch is not required.
The SPI comes into the Magic IC at Pin G7 BCLKX and Pin J2 BDX.
Magic IC operation is very complex. With respect to the transmit path, however, it
integrates Modem, GMSK (Gaussian minimum shift keying), and TIC (translation-
al integrated circuit) functionality.
A very basic block view of how the transmit path works within the Magic IC is shown
in Figure 3.
CLK
Figure 3. Magic IC Internal Transmit Path
Data is transmitted from Whitecap IC to Magic IC on TX SPI bus BDX. Within the
Magic IC, each bit of data is clocked into a register. The clocked bit, and the three
preceding bits on the register, are then clocked into a look-up ROM which looks at
the digital word and, from that information, downloads the appropriate GSMK
digital representation. Channel information and AFC information from Magic IC
SPI is then added to this new digital word. The resulting word is representative of
the TX IF frequency of GIFSYN products. As in the case of the TIC, the TX frequency
feedback and the RX VCO frequency are mixed to give a difference signal which is
digitally phase-compared with the "modulation" from the look-up ROM. The differ-
ence creates a DC error voltage CP_TX that forms part of the TX phase-locked loop.
The error correction voltage CP_TX is then fed from Pin B1 of Magic IC to Pin 4
of the TX VCO IC U350 through the TX loop filter.
The loop filter, made up of U360, Q360, Q361, and C367, smooths out overshoots
in CP_TX when the channel is changed (see Figure 4). If this overshoot were fed to
MOTOROLA CONFIDENTIAL - PROPRIETARY
November 20, 2000
BDX
AFC
Look
Up
ROM
Digital
representation
of RX VCO
CP_TX
CLK
Circuit Description
Channel
Info
Digital
representation
of TX VCO
F/B
000715-O
11