TCM4400E Datasheet, PDF (30/64 Pages) Texas Instruments – GSM/DCS BASEBAND AND VOICE A/D AND D/A RF INTERFACE CIRCUIT

TCM4400E
GSM/DCS BASEBAND AND VOICE A/D
AND D/A RF INTERFACE CIRCUIT
SLWS082A – JULY 1999 – REVISED MARCH 2000
PRINCIPLES OF OPERATION
auxiliary RF functions
The auxiliary RF functions include the following:
D Automatic frequency control
D Auxiliary analog converter (Automatic gain control)
D RF power control
D Monitoring
Each of these functions is discussed in the following paragraphs.
automatic frequency control (AFC)
The automatic frequency control function consists of a DAC converter optimized for high-resolution dc
conversion. The AFC digital interface includes two registers that can be written using the serial interfaces. The
content of these registers controls a 13-bit DAC, whose purpose is to correct frequency shifts of the oscillator
maintaining the master clock frequency in a 0.1 ppm range.
Optimizing the AFC function depends on the type of oscillator used and whether its sampling frequency is
programmable. This means that the lower the selected frequency the lower the resolution and power
consumption. Using a high-quality resonance oscillator filter permits the AFC circuit to operate at low frequency.
Thus, a low-cost oscillator permits operation at a higher internal frequency to ensure 13-bit resolution.
The AFC value is programmed with registers AUXAFC1, which contains the ten LSBs, and AUXAFC2, which
contains the three MSBs. The three MSBs are sent to the DAC through a shadow register the contents of which
is updated when LSBs are written in AUXAFC1, so proper operation of the AFC is ensured by writing the MSB’s
first and then the LSBs.
Internal resistance and output voltage swing selection are controlled with bit AFZ of AUXCTL2 register. Power
down is controlled with two bits: the first bit, AFCPN of AUXCTL1 register, determines whether the AFC can be
powered down from the external PWRDN terminal; the second bit, AFCPD of AUXCTL1 register, controls the
activation of the the AFC function. See the topic, power-down functional description for more details about
power-down control.
The auxiliary analog functions of the GSM baseband A/D and D/A conversions are independently powered from
the AV_DD5 external terminal.
auxiliary analog converter (automatic gain control (AGC))
The auxiliary analog converter control function includes a register which can be written to using the serial
interfaces and a 10-bit D/A converter that provides a control signal to set the gain of the RF section receive
amplifier. The 10-bit D/A converter is accessed through the internal register AUXAGC.
Power down is controlled with two bits. The first bit (AGCW of AUXCTL2 register) determines whether the AGC
can be powered down with the external GSM receive window activation (BDLON). The second bit (AGCPD of
AUXCTL2 register) controls the activation of AGC function. See the topic, power-down functional description
for more details about power-down control.
The auxiliary analog functions of the GSM baseband A/D and D/A conversions are independently powered from
the AV_DD5 external terminal.
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