Motorola Internal Use
6
P7389i – Circuit Description
9)
The data is transmitted from Whitecap to MAGIC DM on TX SPI bus
BDX
, within
the MAGIC DM 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 DM 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 DM to
Pin 4
of the
TX VCO IC U350
, adjoining this line is the loop filter (See Loop Filter
document).
11)
The Loop filter comprises mainly of
U360 / Q360 / Q361 and C367
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.
12)
The Loop filter basically acts then as a huge capacitor and resistor to give a long CR
time for smoothing. It uses a small capacitor and the very high input impedance
buffer Op-Amp. During the
TX_EN
(Whitecap) period when the transmitter is
preparing to operate the capacitor charges, then on receipt of
DM_CS
(Whitecap)
when the Transmitter actually fires; the capacitor discharges through the Op-Amp
giving a smooth tuning voltage, carrying modulation to the TX VCO. The support
voltage for the Loop filter is
V1_FILT
(
V2
from
GCAP II
through
Q913
, then creates
V1_SW
which creates
V1_FILT
).
13)
The
TX VCO IC
U350
now creates our required output frequency with the support
signals
TX_DCS
(
TX275 + *DCS_SEL
through
(U1101)
.
TX275_GSM
(
*G RX275)
(Q110)
TX275_DPCS (*G RX275)
(Q2102)
These signals configure the VCO for correct mode of operation i.e. GSM 900 / 1800 /
1900.
Channel 24
Overshoot
Channel 56
Fig 11.1
Acceptable
3dB
Bandwidth
Unacceptable
3dB
Bandwidth
Fig 11.2