Digitizer overview

The AD9081 or AD9082 (“MxFE”) are ideal digitizers for this wideband RF front end due to their high level of integration, performance, and RF sampling rates. The MxFE’s high instantaneous and analog input bandwidth enable RF sampling up to 8 GHz, providing architectural and frequency planning flexibility at the system level. They also have highly configurable, on-chip DSP capabilities (DDC/DUC channelizers, NCOs, and programmable FIR filters) as well as four DAC channels and two-to-four ADCs for use in multi-channel systems.

Part number	Nb. of ADCs/DACs	ADC max. sample rate	DAC max. sample rate
AD9081	4 ADCs/4 DACs	4 GSPS	12 GSPS
AD9082	2 ADCs/2 DACs	6 GSPS	12 GSPS

The ADI Frequency Planning Utility tool was used to determine the optimal AD9082 ADC sample rate, IF sampling frequency, and IF bandwidth. A 6 GSPS ADC rate allows for maximum instantaneous bandwidth (IBW) and complete avoidance of HD2 aliasing in-band for a 1 GHz signal bandwidth centered at 4.5 GHz (2nd Nyquist). The figure below illustrates this.

Figure 1 AD9082 frequency folding at a 6 GSPS ADC sample rate

Important

The receiver and transmitter front ends described in this reference design use a frequency plan designed using the AD9082, specifically with 6 GSPS and 12 GSPS ADC and DAC sample rates, respectively. However, the RF components themselves are wideband enough to allow for flexibility in the frequency plan and filtering, which can be adjusted to work with the lower ADC sample rates.

The ADF4377 integrated PLL/VCO is an ideal choice for generating the MxFE sample clock. The ADF4377 can generate a clean, low-jitter clock from 800 MHz up to 12.8 GHz, and it’s differential outputs are capable of directly driving the MxFE clock pins.