Analog Front End (Afe) - Analog Devices ADRV9001 User Manual

Reference Manual ADRV9001
TRANSMITTER SIGNAL CHAIN
FM/FSK Modulation
The ADRV9001 provides a frequency modulation (FM)/frequency shift keying (FSK) for standards that use constant-envelope frequency
modulation schemes, such as digital mobile radio (DMR), Analog FM, P25 Phase 1, and Phase 2. It also has the option to perform symbol
mapping and interpolation operations on the transmit data received from a baseband processor for FM/FSK modulation. This capability
provides more flexibility when preparing the data for transmission. The ADRV9001 also has an option to allow user send either pre-mapped and
pre-interpolated transmit data by enabling this functionality, or send post-mapped and post-interpolated data by bypassing this functionality. For
example, for the DMR standard, which uses 4.8 ksps symbol data, the baseband processor sends the symbol data directly to the ADRV9001
and lets it map the symbol data and then interpolate the data to generate frequency deviation data. Note that this functionality is currently not
enabled in the datapath.
Currently, to use the FM/FSK modulation capability of the ADRV9001, perform symbol mapping, interpolating, and pulse shaping filtering in the
baseband processor to generate frequency deviation data before sending it to the ADRV9001. Two different options for FM/FSK modulation are
deployed in the ADRV9001: Direct FM/FSK and IQ FM/FSK, as shown in Figure 127. The following subsections briefly discuss these.
Direct FM/FSK
Implement the frequency modulation by modulating the transmitter RF PLL directly in the Direct FM/FSK option. Figure 134 shows the
transmitter datapath with the Direct FM/FSK.
Figure 134. Direct FM/FSK Datapath Block Diagram
Figure 134 shows that the baseband processor TX_DATA can optionally go through the symbol mapping/interpolation and programmable
FIR, and after interpolation and frequency deviation mapping, the Frac-N PLL implements the FM/FSK modulation at the desired RF output
frequency. Finally, the PLL output is attenuated before feeding to the transmitter RF interface. The programmable 128-tap FIR works as the
pulse shaping filter. In this scenario, optionally load the filter coefficients according to the standard requirement through API commands. In
Direct FM/FSK modulation, the DAC and LPF can both be powered down, and some digital blocks such as the common interpolators, power
amplifier protection, and transmitter attenuation control can all be bypassed. Therefore, it can save power.
IQ FM/FSK
Implement IQ FM/FSK modulation by modulating the digital NCO, as shown in Figure 127. The modulated IQ data goes through the
interpolator, DAC, LPF, and then is upconverted to RF frequency by the mixer. The processing stages before the digital FM/FSK modulator are
like the Direct FM/FSK option, which also includes optional symbol mapping/interpolation and pulse shaping functions. The selection between
the direct FM/FSK and IQ FM/FSK is determined by profile.

ANALOG FRONT END (AFE)

Digital-to-Analog Converter (DAC)
The ADRV9001 integrates a 16-bit DAC, which can be operated at a standard rate of 184.32 MHz, 368.64MHz, or 552.96 MHz (note: with an
arbitrary sample rate, DAC can operate at other different rates as well.) The selected profile sets the sampling rate. The DAC is auto-tuned
to remove mismatches in the DAC units, which improves the linearity of the DAC. Boost the nominal full-scale current of the DAC by 3 dB
through the API command "adi_ADRV9001_Tx_OutputPowerBoost_Set()". The increased signal swing throughout the entire analog signal
chain results in better AM noise performance. By default, the 3 dB boost is disabled.
Low Pass Filter (LPF)
The analog LPF is used to attenuate the sampling images of the DAC. The frequency response has a second-order Butterworth shape. The
corner frequency is auto-tuned to compensate for process and temperature variations. The API profile sets the operating corner frequency. The
ADRV9001 allows to configure LPF at three different power consumption levels to achieve a system power saving target.
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