Closed Loop Gain Control (Clgc) - Analog Devices ADRV9005 Reference Manual

Reference Manual
DIGITAL PREDISTORTION (DPD)
Figure 214 shows an example of the ACPR performance before and after DPD. The blue curve represents the ACPR performance before DPD,
which shows spectral regrowth. The black curve represents the ACPR performance after DPD. The ACPR performance significantly improves.
Satisfactory DPD performance also depends on the successful completion of some operations inside the device, such as the ADC and DAC
calibrations. Most of these operations are guaranteed internally in the device, but the following operations are user configurable. Therefore,
perform all these optional operations is important to achieve the optimal DPD performance.
Time alignment between transmit x(t) and loopback y(t) for data capture
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Transmit closed loop gain control (CLGC) tracking calibration
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Transmit local oscillator leakage suppression (LOL) calibration
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Transmit and receive quadrature error correction (QEC) calibration
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CLOSED LOOP GAIN CONTROL (CLGC)

The ADRV9001 provides CLGC as one of the transmit tracking calibration algorithms. The sole purpose of CLGC is to maintain a fixed gain
between the PA output and transmit digital input amplitude. The primary source of gain variation comes from the changes in the gain of the
PA, which can vary due to transistor temperature change in response to power output in the short term, RF carrier frequency change within the
bandwidth of the amplifier, and slow hardware degradation in the long term. Therefore, it is important to maintain a fixed gain in the transmit
path to maintain a precise calibrated transmit power at the antenna port.
CLGC helps to achieve a fixed gain target by adjusting the transmit attenuation automatically by monitoring the gain variation of the PA. It can
work with or without the DPD algorithm. When DPD is not activated, CLGC is an optional feature. Ensure the CLGC is always activated, when
DPD is activated. Note: The ADRV9001 provides the option to enable DPD without CLGC, but it is not recommended. DPD compensates for
the instantaneous compression of the peak signal by expanding the input signal so that the peak signals are linearized. When linearization is
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Figure 213. Raw Transmit Signal Input vs. Linearized Power Amplifier Output
Figure 214. ACPR Performance Before and After DPD
ADRV9001
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