Motorola T2290, Service Manual, Page: 88 / 129

TDMA T2290/T2297: ZIF_SYNTH (U110)
B6
Description
U110(ZIF_SYNTH) can be divi ded int o two functional sub systems: The ZIF (Zero-Intermediate-Frequency) which p rovides all of the functions of the back-end of a
receiver, and the SYN (SYNthesizer) which c ontains pha se-locked loops and modulators to produce the Local Oscillator (LO) and modulated transmit carri er.
The ZIF impleme nts the back end of the receiver. The incomin g signal is attenuated, amplif ied and mixed d own to an extremely low frequency(Baseband)-not quite DC.
The first amplifier a nd an Automatic Gain Control(AGC) circuit adjusts the amplifier gain to maintain a c onstant level in the baseband filter. C109 is used to add stability
to the AGC circuit.
The first mixing that occurs in th e ZIF requires a 2nd Local Oscill ator that is running at 221.184MH z. The 2nd LO is divided down the n phased shifted before being
mixed with the IF signal., produci ng the baseband s ignal
The baseband si gnal is low-pass filte red using a programmable low-pass fil ter. In TDMA mode, the baseband sign al is routed to U1800(DCI) via the RX_I and RX_Q
lines. In analog mode, the baseband signa l is up-converted and then FM-demodulated, producing the DEMOD signal which is rou ted to U1800(DCI) for filtering and de-
emphasis.
The RSSI(Receive Sign al Strength Indicator) is a voltage that increases with respect to the received sign al strength.
The SPI Bus is a serial i nterface used to program the internal filters and frequency di viders for U110, allowing selectivity of cellular channels.
RF 19.44Mhz input is used to p rovide a frequency reference for U110.
IF
2nd LO
C109
Shifter
SPI BUS
8/16/96 BIT
MODES
U1000
SPI BUS
DATA
CLK
CEX
RSSI
AGC_RSSI
MAIN
LOOP
Offset
LOOP
LP_SWITCH
VCO
Offset VCO
Cap
Filters
Ref Osc
19.44MHz
RX_Q
RX_I
Demod
UP
Converter
DEMOD
U110
Step
Atten.