Motorola Internal Use
5
A008 / A6188+ – Circuit Description
this data is already decided for the transmission burst, and therefore a frame synch is
not required.
6)
The SPI bus signals enters into the MAGIC at
Pin G7
(
BCLKX
) and
Pin J2
(
BDX
)
7)
The operation of the MAGIC is very complex and with respect to the transmit path,
integrates the functions of the Modem and its function of GMSK (Gaussian Minimum
Shift Keying) and also the functions of the TIC (Translational Integrated Circuit).
8)
A very basic block view of how the transmit path works within the MAGIC is
demonstrated in: Fig 8.1
Internal MAGIC Operation
Fig 8.1
9)
The data is transmitted from Whitecap to MAGIC on TX SPI bus
BDX
, within the
MAGIC each bit of data is clocked into a register. The clocked bit and the 3
preceding bits on the register are then clocked into the 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 SPI is then
added to this new digital word, this word is then 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, this is
digitally phase compared with the ‘modulation’ from the look up ROM. The
difference creates a DC error voltage
TX_CP
that forms part of the TX Phase locked
loop.
10)
The error correction voltage
CP_TX
is then fed from
Pin B1
of MAGIC to
Pin 8
of
the
TX VCO IC U250
, adjoining this line is the loop filter (See Loop Filter document
on the GSM Service Support Website).
TX_CP
CLK
BDX
Look
Up
ROM
Σ
AFC
Channel
Info
Digital
representation
of TX VCO
F/B
Digital
representation
of RX VCO
CLK