Motorola Internal Use
4
V2288 – Circuit Description
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
TX_CP
is then fed from
Pin B1
of MAGIC to
Pin 6
of
the
TX VCO IC U301
, adjoining this line is the loop filter (See Loop Filter
document).
11)
The Loop filter comprises mainly of
U310 / Q310 / Q311
and it’s main function is to
‘smooth’ out any overshoots when the channel is changed, see Fig 11.1. If this
overshoot were fed to the TX VCO the resulting burst would not meet the world
standards for GSM with respect to bandwidth, see Fig 11.2.
Channel 24
TX_CP
CLK
BDX
Look
Up
ROM
Σ
AFC
Channel
Info
Digital
representation
of TX VCO
F/B
Digital
representation
of RX VCO
CLK
Overshoot
Channel 56
Fig 11.1
Acceptable
3dB
Bandwidth
Unacceptable
3dB
Bandwidth
Fig 11.2
