RF Image Filtering

Co-location Deployment Considerations for Direct RF Sampling Transceivers (WP541)

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An advantage of ZIF receivers is the lower requirements on RF image filtering due to the inclusion of a low pass filter between the IQ demodulator and the ADC. While this is still the case for 5G NR receiver design, the advantage is largely diminished due to the availability of low-cost miniature ceramic bandpass filters. Several companies now provide such parts as standard catalog filters for each NR band, including Johanson and Murata. The Murata equivalent of the Johanson C-band ceramic filter used in the analysis is the Murata LFB213G60CGUE234 [REF 3].

Although it is often claimed that the anti-alias RF filter in Figure 2 is not required in a ZIF receiver, it is generally used in practice. The main goal of the AAF there is to remove out-of-band HD2 and HD3 products generated by the amplifier stages to avoid any MxN mixing products in the demodulator that will eventually fall into the baseband and on top of the wanted C-band. For instance, 2xLO mixing with HD2 and 3xLO mixing with HD3 will all land at the same location as the wanted band.

The following figure illustrates the problem with the ACS blocker scenarios. In 5GNR, the ACS blocker is as wide as 20 MHz and sitting directly adjacent to the wanted band. The HD2 of the blocker is 40 MHz wide, and its HD3 is 60 MHz wide. As a result, the mixed down version of the HD2 and HD3 is wide enough to spill onto the wanted band even though the blocker itself does not (as shown in the figure). For this reason, it is important to have the RF AAF to clean up the RF spectrum prior to the mixer stage. However, this does not help with the HD2/HD3 generated by the active mixer in the ZIF demodulator.

Figure 1. Adjacent Channel Selectivity Blocker with MxN Mixing Products