Aerospace, Military and Defence
Figure 3. Single-mixer frequency conversion used in high IF digital receivers.
Figure 4. Today’s spectral scanning approach, with wideband single-mixer tuning into the MxFE sampling at 6 GSPS ADC. Mixer low sideband flips into direct sample band and sweeps with LO.
realistic high performance RF filters to meet the image rejection requirement. These RF filters are high in the system SWaP-C Pareto. The RF preselector filtering (Figure 2, yellow) is required to mitigate multiblocker induced IMD2 spurs (that is, F2 − F1 and F2 + F1). The requirement for IMD2 mitigation is independent from the image problem, but the front-end filtering often works to address both.
Spectral sensing today (MxFE) Today’s wideband spectral sensing approach improves on days of yore. Thanks to Analog Devices’ mixed-signal front end (MxFE), the ADC sample rate is high enough that you can direct sample the intermediate high IF following that first mixer mentioned previously. Thus, in today’s wideband receivers employing MxFE, the RF tuner often doesn’t require dual-mixer stages. The second Nyquist IF direct sampling is high enough in frequency to allow adequate frequency spacing of the desired input RF band and image band so that an attainable RF filter can do the job. Figure 3 shows today’s single-mixer approach, and the frequency plan is shown in Figure 4. Today’s biggest SWaP-C savings come from eliminating an entire frequency translation stage with mixer, RF amps, filters, and other components. Another SWaP-C benefit of today’s higher IF capability is that the direct sample now covers most of LF to 5.5 GHz. So, you don’t always need an RF tuner covering all the way down to 2 GHz. In a lot of cases, you could get away with a 5 GHz to 18 GHz RF tuner. Shifting the low limit of the tuner
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from 2 GHz to 5.5 GHz seems minor but is quite significant as it eases filtering, frequency planning, and required LO range. The caveats are you still need to figure out how to cover the gap between the first and second Nyquist, which in the 6 GSPS ADC is roughly 2.7 GHz to 3.3 GHz. Another consideration is the need for switched or tunable ADC antialiasing RF filters that let you toggle between the first and second Nyquist operations.
The RF filters remain high in the system SWaP-C Pareto because they’re: ● High performance, requiring low IL, flat pass band, and steep rejection skirts ● Large, using distributed planar geometries on high Q ceramics like alumina ● Lots of them are still required The suboctave RF preselector is still required, but the requirements may ease allowing less aggressive filtering. The benefit comes from the direct signal chain not using an RF mixer, which should improve IP2.
Spectral sensing in the near future Looking ahead, even higher sample rate digital data converters get us over the tipping point to fully software-defined wideband radio at smallest SWaP-C. Today, plenty of companies already market high speed data converters at many 10s of GHz, but buyer beware: pay close attention to the multiblocker dynamic range. In order for high RF direct sample data converters to transform radar and EW, the excellent dynamic range of their narrow-band predecessors must be maintained. As sample rates and iBW push higher, maintaining excellent noise and linearity (that is, the dynamic range) is difficult and relies on countless architectural considerations. This is where ADI differentiates from the competition. Next-generation higher sample rate data converters will allow many architecture improvements over today’s MxFE scheme that is mentioned previously. We see the following three factors as the most significant: ● Direct RF sample higher IF, separating the desired and image band far enough that lower Q tunable MMIC filters are adequate. The
MxFE ability to direct sample in the second Nyquist maxes out around 6 GHz. ADI’s next- generation high speed digital data converters will significantly extend this coverage, and the resulting benefits are enormous. > Now, you’ve finally eliminated planar high Q ceramic filters, which is a big SWaP-C saving. > The RF filters go from fixed (every use case has a custom set of filters) to tunable. This means a single-wideband hardware configuration can be software programmed to optimize the right performance trade for many customer frequency schemes across many use cases. ● Direct RF sample from low frequency up toward millimeter wave (mmW), except the Nyquist gap. Across this direct sample zone, you’re digitally tuning while steering an RF tunable filter to knock down IMD2 inducing blockers. Noncontinuous multiband systems, common in radar, can likely eliminate the RF mixer and avoid the gap between Nyquist zones. In this case, the block diagram simplifies further to what is shown in Figure 5,
Figure 5. Direct RF sample digital receiver. Components in Electronics March 2023 17
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