U3600BM_05 Datasheet, PDF (5/44 Pages) ATMEL Corporation – Single-chip Cordless Telephone IC

U3600BM
3. System Description
Radio frequency IC for analog cordless telephone application in 26/50 MHz band (CTO stan-
dard). The IC performs full duplex communication. The transmitting and receiving frequency are
depending on whether the IC is used in the handset or in the base station.
Frequency converter comprise an FM transmitter with switchable output power and first receiver
mixer in the same unit. A two-wire bus interface can be used for the frequency control as well as
for switching the transmitter power amplifier and the receiver. Fine frequency adjust of reference
quartz oscillator is programmable.
The receive part is designed for a double conversion architecture. The incoming radio frequency
signal will be filtered and amplified before reaching the first mixer. At this stage the RF signal will
be converted down to the first intermediate frequency (10.7 MHz) by using a crystal oscillator
(LO1).
The transmit part contains two PLL controlled VCOs. The frequency modulation is accomplished
by super-posing the incoming audio signal on the PLL control voltage. Final frequency is a prod-
uct of mixing VCO1 with first local oscillator of receiver part (VCO3). The FM modulated carrier
is amplified by externals power amplifier before entering the output filter and the antenna
connector.
3.1 Adjustments for VCO1 and VCO2
To be able to use a wide frequency range for the VCOs (i.e., VCO2 26.3 MHz to 49.9 MHz) the
two internal VCOs (VCO1 and VCO2, i.e., the VCOs of the transmit part) have a rough adjust
and a fine adjust to increase the frequency range given by the phase comparator.
The rough adjusts for these VCOs are correlated with the country setting. For every country
there are two sets of VCO rough adjust settings, one for the base and one for the handset. See
tables at channels frequencies and dividers.
To compensate the variation in production there is a fine adjust for each of the VCOs. The fine
adjusts of the internal VCOs could be set manually (for test purposes) or set by the automatic
mode. Theoretically the sign of the changing (increase/ decrease when the voltage of the phase
comparator is to high) is selectable, but we need value 1 () in all cases.
Setting VCO1 (VCO2) under normal conditions:
EAFA1 (EAFA2) = 1, automatic fine adjust VCO1(VCO2) enabled
SAFA1 (SAFA2) = 1, sign of auto fine adjustment of VCO1 (VCO2) = 1.
3.2 Adjustment for VCO3
In order to increase the adjustment range of VCO3 with fixed external tank elements and/or for
“band switching”, especially for US frequencies, VCO3 has programmable capacitors inside.
These capacitors can be added by serial bus (FA3 [4:0]) between LO1 and LO2. There are 31
steps available, every step adding a capacitor of 0.5 pF.
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