Editor's note: I am happy to bring you, my friend and former long-time colleague at Texas Instruments, Matthias Feulner, one of the best high-speed communications experts in the industry
The latest direct radio frequency (RF)-sampling transceivers – including the Texas Instruments AFE7444 and AFE7422 devices that support four and two antenna channels, respectively – provide a number of powerful capabilities that enable advanced system features like multiband and multimode operation, as well as frequency translation and fast frequency hopping. These capabilities have become increasingly popular in system concepts such as multifunctional arrays, where different subarrays of a large phased-array antenna would be configurable to perform multiple functions depending on the situation or mission needs; these Include radar, communications or electronic warfare (EW) functionality, as shown in Figure 1.
In addition, these systems frequently require implementation of fast frequency hopping to step their operating frequency through repeating or arbitrary sequences, as illustrated in Figure 2. This is done to avoid jamming, to prevent signal detection or to implement spoofing techniques that alter the signature of a reflected radar signal.
Frequency-agile operation across multiple Nyquist zones
In order to explore these capabilities further, let’s first look at the functional blocks of an integrated RF-sampling transceiver, as shown in Figure 3.
Functional blocks of AFE7444/AFE7422 RF-sampling transceivers
When combined, these functional blocks deliver enhanced functionality in the following ways:
- Operation across a very wide RF-frequency range from a few MHz up to 6 GHz, handling a very wide instantaneous bandwidth up to 1.5 GHz.
- A digital signal processing block enables the aggregation and disaggregation of multiple sub-bands or waveforms that can each be processed as individual digital data streams on the receive and transmit sides.
Multiband or multimode signal processing
Let’s now consider the use case of processing a multiband or multimode signal, leveraging wideband sampling and synthesis as well as digital processing features, as shown in Figure 4.
This setup generates a multiband signal with three distinct sub-bands spanning a total bandwidth of 2.75 GHz. The receiver samples the entire band across multiple Nyquist zones, then feeds the sample data to a digital down-conversion block with multiple parallel stages, selecting the individual sub-bands and converting them to baseband signals through individual numerically controlled oscillators (NCOs) and digital mixers. Applying decimation then reduces the output rate according to the individual signal’s bandwidth and suppresses out-of-band impairments.
Conversely, on the transmit side, individual digital input streams can be supplied to multiple parallel up-conversion stages, up-converting the baseband signals to their respective target frequencies. Data is then up-sampled to the RF digital-to-analog converter (DAC) sampling rate and a combined wideband signal ranging from 700 MHz to 3.45 GHz is synthesized by the RF DAC in the final stage.
Frequency conversion and frequency hopping
You can extend the previous case by picking just one individual band, using internal digital loopback and then applying a frequency shift to the selected sub-band before retransmitting the signal, as shown in Figure 5.
This setup captures the multiband signal as described earlier. The digital down-conversion block selects one individual sub-band, converts it to a baseband signal and passes it through a digital filter, which cleans up out-of-band impairments like harmonics or mixing products. A digital loopback path within the chip allows to directly feed the digital receiver’s digital output data to the transmitter path without having to go off-chip, and without having to hook up any additional processing devices.
Simply up-convert the filtered signal back to the originally received frequency and you will have built an on-chip digital repeater. To implement a frequency-hopping transmitter, program the NCO of the transmit section to the desired new frequency and re-transmit the frequency-shifted signal. This is illustrated in the yellow trace in Figure 5’s spectrum analyzer plot, comparing it to the originally received multiband spectrum, which is the green trace.
Now that I have illustrated the basic concept, a similar approach could be used to support additional use cases, including:
- Multiband frequency conversion. Since multiple parallel digital down- and up-converter blocks exist, you could receive and disaggregate a multiband signal into individual sub-bands, then apply an individual frequency shift to each one, loop back internally within the chip and retransmit each sub-band in a new frequency location.
- Fast frequency hopping. Since it’s possible to reprogram NCO frequency updates on a time scale of a few microseconds or alternate the multiple NCOs available per signal path in a ping-pong fashion, it is then also possible to receive and transmit frequency-agile signals that follow either repeating or arbitrary sequences. The transition between two frequencies is shown in Figure 6.
- Ramp generation/direct digital synthesis mode. A built-in sine-wave tone generator for each transmitter enables the generation of frequency ramps and frequency-modulated continuous waveforms (FMCW) frequently used in radar systems.
- Simultaneous wideband scanning and narrowband observation. Because each receiver front-end sampling stage can connect to multiple digital processing stages, there is an option to configure one receive path for wideband mode, output full Nyquist-band sample data and observe an instantaneous bandwidth of up to 1.5 GHz to scan for the presence of any signals. In parallel, you could configure a second path for a narrowband decimation mode to zoom in and exactly analyze any signal that was detected in wideband mode.
As you can see based on some typical applications, demanding and complex new use cases can be implemented with integrated RF-sampling transceivers, which provide a level of integration and flexibility previously not available.