TCM4400 Datasheet, PDF (42/72 Pages) Texas Instruments – GSM/DCS BASEBAND AND VOICE A/D AND D/A RF INTERFACE CIRCUIT

4.3 Auxiliary RF Functions
The auxiliary RF functions include the following:
• Automatic frequency control
• Auxiliary analog converter (automatic gain control)
• RF power control
• Monitoring
Each of these functions is discussed in the following paragraphs.
4.3.1 Automatic Frequency Control (AFC)
The automatic frequency control function consists of a DAC optimized for high-resolution dc conversion.
The AFC digital interface includes two registers that can be written to using the serial interfaces. The content
of these registers controls a 13-bit DAC whose purpose is to correct frequency shifts of the oscillator to
maintain the master clock frequency in a 0.1 ppm range.
To optimize 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 10 LSBs) and AUXAFC2 (which
contains the three MSBs). The three MSBs are sent to the DAC through a shadow register whose content
is updated when LSBs are written in AUXAFC1. Proper operation of the AFC is ensured by writing the MSBs
first and then the LSBs.
The internal resistance and output voltage swing selection is controlled with bit AFZ of the AUXCTL 2
register. Power down is controlled with two bits: the first bit, AFCPN of the AUXCTL1 register, determines
whether the AFC can be powered down from the external PWRDN terminal; the second bit, AFCPD of the
AUXCTL1 register, controls the activation of the the AFC function. For more information about power-down
control, see Section 4.8 in this document.
The auxiliary analog functions of the GSM baseband A/D and D/A conversions are independently powered
from the AV_DD3/5 external terminal. The AFC output voltage swing is programmable to provide the largest
possible voltage range. This configuration is programmed with bit AFCZ of the AUXCTRL 2 register.
4.3.2 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 DAC that provides a control signal to set the gain of the RF section receive amplifier.
The 10-bit DAC is accessed through the internal register AUXAGC.
Power down is controlled with two bits. The first bit (AGCW of the AUXCTL 2 register) determines whether
the AGC can be powered down with the external GSM receive window activation (BDLON). The second bit
(AGCPD of the AUXCTL 2 register) controls the activation of AGC function. For more information about
power-down control, see Section 4.8 in this document.
The auxiliary analog functions of the GSM baseband A/D and D/A conversions are independently powered
from the AV_DD3/5 external terminal. The AGC output voltage swing is programmable to provide the largest
possible voltage range. This configuration is programmed with bit AGCSWG of the AUXCTL 2 register.
4–6