U3600BM Datasheet, PDF (5/43 Pages) ATMEL Corporation – SINGLE CHIP CORDLESS TELEPHONE IC

U3600BM
System Description
Adjustments for VCO1
and VCO2
Adjustment for VCO3
Speed-up of the Loop
Filter of PLL1
(“Modulator PLL”)
Radio frequency IC for analog cordless telephone application in 26/50 MHz band (CTO
standard). 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 product 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.
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.
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 0.5pF.
To have a fast locking time for the modulator loop there is a precharge and a speed-up
mode for the external loop filter.
During receive mode (VCO3 enabled, VCO1 disabled) the modulator loop filter is
precharged to about half of the internally regulated 2.5 V charge-pump voltage.
During the first 30 ms after enabling VCO1 the modulator phase comparator is in speed-
up mode. In this mode the current of the pase comparator which charges the loop filter
is much larger than in normal mode. Additionally to the automatically switched 30 ms
speed-up mode, the speed-up can be activated for any time by setting the bit SU1.
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