CXD1968AR
AGC
There are two AGC outputs from the CXD1968AR – one for the RF tuner stages and one for the IF tuner stages.
These generate PWM outputs that need to be integrated off-chip to provide an analog control signal to drive
an external variable gain amplifier (analog VGA) in the tuner. Each integrator is implemented with a simple
external single pole RC filter, values selected to give a cutoff frequency of 400Hz, removing the high frequency
components.
RF AGC
The RF AGC outputs a PWM value via the RF AGC pin, which has been programed into the
RF_IFAGC_CTRL0 and RF_IFAGCQ_PWM registers (renamed to cover the new ZIF AGC modes).
The RF AGC pin is enabled by setting the RF_AGC_EN bit in the RF_AGC_CTRL0 register.
The RF AGC pin may alternatively be used as a logic I/O pin, see below.
IF AGC
The IF AGC outputs a PWM value via the IF AGC pin. The AGC setting can be generated automatically or
manually overridden.
Automatic Gain Control
The CXD1968AR contains an internal AGC block, which monitors the signal level at the output of the
ADC and provides a Pulse Width Modulated (PWM) control signal to implement an automatic gain
control loop. The AGC is capable of tracking out 60Hz AM interference with the specified time constant,
and large changes in input level. A 40dB input swing can be corrected in 20ms (irrespective of QAM/
QPSK modulation scheme).
The IF AGC automatic mode is enabled by resetting the AGC Set bit in the AGC_CTL register. The
sense of the output is controlled by the AGC Neg bit in the same register.
Manual Gain Control
The AGC loop can be set manually using the AGC Set bit in the AGC_CTL register and entering a value
of 0 to 1023 in the AGC Manual field in this register.
RF AGC GPO
The RF AGC pin may be configured to generate a fixed logic “0” or “1” output level (3.3V) where the RF AGC
variable output function is not required. A typical application is to switch a dual bandwidth SAW filter or
other circuit function in software.
If the RF AGC output pin is to be used for this kind of application, the integrating RC filter is not required.
This example generates a logic “1” or 3.3V level at this output.
Example: 0xD8 0xB2 0x07 ; set RF AGC output pin High
RF AGC GPI
The RF AGC pin may be configured as a logic input (3.3V logic but 5V tolerant) where the RF AGC variable
output function is not required. This may be used to sense operation of an external circuit.
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