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8 / 27 page LMS7002M 8 LMS7002M – FPRF MIMO Transceiver IC tan( α/2) tan( α/2) Qout Iout Iin Qin Figure 14: Implementation of IQ phase correction TX DC Correction DC offset correction in the TXTSP path is achieved by using the following equation: Q DC Qin Qout I DC Iin Iout _ _ . Here, DC_I and DC_Q are programmable DC offset correction parameters which the BB modem should adjust to minimize the TX DC and TX LO leakage feed-through. The hardware implementation is given below. DC_I DC_Q Iin Qin Qout Iout Figure 15: TX DC offset correction module RX DC Correction As mentioned previously, there are multiple reasons for DC to appear at the RX output. The most difficult to correct, in a static manner, is the second order distortion (IP2) component which changes with the RX input level as well as the RX gain set up. Hence, a compensation loop running in real time is required to track and correct the DC at the RX output. A simple digital implementation of such a loop is given in Figure 16. The averaging (COMB) filter calculates the DC of the corrector input and subtracts it to cancel out the offset. The loop is running all the time so any change of the RX DC due to the signal level change, RX gain change or temperature will be tracked and cancelled automatically. The only programmable parameter in the loop is DCAVG which defines the averaging window size. - Iin Qin Qout Iout DCAVG - Figure 16: RX DC offset correction module Inverse SINC Filter The inverse sinc filter compensates for sinx/x amplitude roll off imposed by the DAC. The filter is designed to compensate from DC to 0.35fs where fs is the DAC sampling frequency. Impulse and amplitude responses are shown in Figure 17 and Figure 18.a respectively. Figure 18.b plots the equivalent DAC amplitude response with the inverse sinc function compensation applied. The in band (0 – 0.35f s ) amplitude ripple is less than +/- 0.04 dB. H( 0) = 0.0101318 = h( 4) h( 1) = -0.0616455 = h( 3) h( 2) = 0.855469 Figure 17: INVSINC impulse response -4 -3.5 -3 -2.5 -2 -1.5 -1 -0.5 0 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 Frequency [f/fs] (a) (b) Figure 18: INVSINC (a) and equivalent DAC (b) amplitude response Complex Mixer The complex mixer used in the RXTSP and TXTSP is designed to implement the following set of equations: t Q t I I c in c in out sin cos , t Q t I Q c in c in out cos sin , where Iin and Qin are provided from the IQ pre-processing stages while cosine and sine signals are generated by the NCO. An option to choose the sign in the mixing equations is implemented which in fact gives the ability to do up-mixing or down-mixing in both TX and RX chains. The hardware implementation is shown in Figure 19. IIN QIN + -/+ + +/- IOUT QOUT INCO QNCO Figure 19: Complex mixer Numerically Controlled Oscillator The quadrature carrier signal, required to implement low digital IF, is generated by the local NCO. The internal NCO design is based on a DDFS (Direct Digital Frequency Synthesis) algorithm with a 32-bit |
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