Step-by-step considerations for designing wide-bandwidth multichannel systems

Next-generation aerospace and defense and test and measurement system bandwidths are moving from 10s to 100s of megahertz to multigigahertz of instantaneous bandwidths. Trends in phased-array radar, 5G wireless test systems, electronic warfare and digital oscilloscopes are pushing bandwidths higher and dramatically increasing the number of channels in a system.

These trends complicate signal-chain design, including the data converter, clocking and power supply. Selecting the appropriate data converter, synchronizing multiple channels, and optimizing your power supply are critical to achieving the necessary bandwidth across multiple channels.

Selecting your data converter
Every system architecture starts with the device that has the greatest influence on system performance; in wideband systems, that’s the data converter. Its selection depends on your answers to these questions:

  • Will you use a zero intermediate frequency (IF)/complex mixer architecture (Figure 1)?
    • Pros: the analog-to-digital converter (ADC) input bandwidth and sampling rates are lower than other architectures, and you can simplify or eliminate filtering.
    • Cons: you need two ADC channels per antenna element for the I and Q paths and the mixer image may result in lower system performance.
  • Will you use a heterodyne approach (Figure 2)?
    • Pros: you need only a single data-converter channel and the ADC input bandwidth is lower than radio frequency (RF) sampling.
    • Cons: it requires one or more mixers, the signal image and generated harmonics will complicate filtering, and it’s difficult to adjust the frequency of interest; you must move the Local Oscillator (LO).
  • Will you use direct RF sampling (Figure 3)?
    • Pros: you don’t need a mixer given the simplified signal chain, and it’s easy to adapt the frequency digitally using digital down converters (DDCs) and numerically controlled oscillators (NCOs).
    • Cons: the highest signal frequency needs to be within the ADC’s input bandwidth, and you’ll need to do frequency planning to achieve the highest performance.

Figure 1: Typical complex mixer architecture

Figure 2: Typical heterodyne architecture

Figure 3: Typical RF sampling architecture

  • What is the widest bandwidth signal that you need to measure?
    • At a minimum, the data converter will need a sampling rate of at least 2.5 times the instantaneous bandwidth of the signal for direct sampling, or 1.25 times for zero IF.
    • For the best performance, a sampling rate of ~10 times the instantaneous bandwidth will allow you to more easily avoid signal harmonics and spurs.

TI’s RF-sampling frequency planner, analog filter and DDC Excel calculator can help with frequency planning and filtering requirements and show you the effects of complex digital decimation of your signal.

As I mentioned earlier, wideband systems need high-sampling-rate converters. For example, an RF sampling system with a signal bandwidth of 1 GHz would benefit from a data converter with a ~10-GSPS conversion speed to avoid signal harmonics. Until recently, TI’s fastest converter was the ADC12DJ3200, which is a 12-bit ADC that runs at 3.2 GSPS per channel in dual-channel mode or 6.4 GSPS in single-channel mode. But even in single-channel mode, it does not meet the desired 10-GSPS speed. So to meet this requirement, the Flexible 3.2 GSPS Multichannel AFE Reference Design for DSOs, RADAR and 5G Wireless Test Systems, shown in Figure 4, combines two ADC12DJ3200s on one board.

Figure 4: Multichannel AFE reference design block diagram

This reference design offers system flexibility, since it can run in quad-channel, 3.2-GSPS mode, dual-channel, 6.4-GSPS mode or as a single channel running at up to 12.8 GSPS. Our 12.8-GSPS analog front end reference design for high-speed oscilloscope and wide-band digitizer illustrates onboard interleaving of the two ADCs.

Now, with the introduction of our new dual-channel, 5.2-GSPS ADC12DJ5200RF, you have even more performance and flexibility for your next generation designs. Since the ADC12DJ5200RF is pin compatible with the ADC12DJ3200, we were quickly able to modify our existing reference design and can now offer a Scalable 20.8 GSPS reference design for 12-bit digitizers. At 20.8 GSPS, the entire 8-GHz input bandwidth of the device can be digitized in a single capture.

Clocking your design
Now that you have selected your data converter, you have to design a clocking architecture. Clocking a single data converter is straightforward, but many systems, such as our interleaved design, need to synchronously clock multiple converters simultaneously. For example, large phased-array systems can have hundreds or thousands of channels. TI has several devices and reference designs to help with this design challenge.

Our Multichannel JESD204B 15-GHz Clocking Reference Design for DSO, Radar and 5G Wireless Testers is a full clock subsystem. The design, shown in Figure 5, includes multiple clock reference options such as the LMK61E2 programmable oscillator and the LMK04828, a clock distribution device with 14 independent clock outputs, as well as two LMX2594 phase-locked loops/synthesizers, which provide ultra-low phase-noise clocks up to 15 GHz, as shown in Figure 6. In addition, the LMX2594 can also generate a synchronous SYSREF clock for data converters with a JESD204B interface. The LMX2594 also gives you the ability to sync the phase of the clocks across multiple devices. In the reference design, you will find phase-noise plots at multiple frequencies, as seen in figure 6, and also measurements of channel-to-channel skew with skews less than 10 ps.

Figure 5: Multichannel JESD204B 15-GHz clocking block diagram

Figure 6: LMX2594 phase noise at 15 GHz

As configured, the board supports up to two data converters and two field-programmable gate arrays (FPGAs) but can easily be adapted to clock up to six converters and one FPGA. However, many systems require significantly more channels. For these situations, our High Channel Count JESD204B Clock Generation Reference Design for RADAR and 5G Wireless Testers and High Channel Count JESD204B Daisy Chain Clock Reference Design for RADAR and 5G Wireless Testers enable the operation of our clocks in either a tree structure, shown in Figure 7, or a daisy-chain configuration. Using these techniques, you can expand to thousands of channels with minimal degradation in system performance.

Figure 7: JESD204B clock generation reference design tree structure block diagram

Powering your design
Once your clocking architecture is settled, the next challenge is powering your design. Data converters and clocks can be sensitive to the switching noise of DC/DC converters, so most power designers will follow the DC/DC converter with a low-noise, low-dropout regulator (LDO). However, with careful layout and filtering, LDOs are often not necessary on many supplies.

The 3.2-GSPS multichannel analog front end reference design that I mentioned earlier has a full power supply, including both DC/DC regulators and LDOs, as shown in Figure 8. The LDOs on this design can be bypassed with filters to test which supplies are most sensitive to the switching noise. Our testing confirmed that there is no impact on the performance of the design while bypassing the LDOs, which also gives the added benefit of increased power-supply efficiency.

Figure 8: 3.2-GSPS multichannel AFE reference design power-supply block diagram

The board also includes a series of header pins along the top that enable you to bypass the onboard power solution with a new design, such as our Low noise power-supply reference design maximizing performance in 12.8 GSPS data acquisition systems. This reference design, shown in Figure 9, enables the synchronization of all DC/DC regulators to a master clock, making it easier to filter out converter switching noise. In addition, you can shift the phase of the clock to each converter so that all converters are not switching at the same time, lowering the total switching energy. Finally, the DC/DC converters on the reference design are more efficient, lowering the total power loss on the board. As with the original design, the LDOs can still be bypassed.

Figure 9: Low noise power supply reference design block diagram

You can see that selecting the correct data converter is just one of your challenges. Once you’ve made that selection, it’s critical to select the best clock and power-supply designs in order to not degrade the performance of your costly data converter.

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