Integrated RF sampling transceiver supports fast frequency hopping, multi-band and multi-mode operation
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By Matthias Feulner, applications manager for high-speed data converters at Texas Instruments The latest direct-wireless radio frequency (RF)-sampling transceivers – including Texas Instruments’ AFE7444 and AFE7422 devices, which support four and two antenna channels, respectively – offer a range of powerful capabilities that enable advanced system features such as multiband and multimode operation, as well as frequency conversion and fast frequency hopping. These functions are becoming increasingly prevalent from a system concept perspective, such as multifunction arrays, where different subarrays of a large phased array antenna can be configured to perform multiple functions depending on the situation or mission requirements; this can include radar, communications, or electronic warfare (EW) functions, as shown in Figure 1. 85)]![]() [/font ]Multifunctional phased array system In addition, these systems often need to implement fast frequency hopping so that they can gradually adjust to the operating frequency through repeated or arbitrary sequences, as shown in Figure 2. This can be done to avoid jamming, prevent signal detection, or facilitate the implementation of anti-electronic spoofing techniques (electronic spoofing: tampering with the electronic signature of radar reflections). Figure 2 Frequency Agility Operation Across Multiple Nyquist Zones To further understand these functions, let’s first examine the functional blocks of an integrated RF sampling transceiver, as shown in Figure 3. Figure 3 AFE7444/AFE7422 Functional Blocks of the RFSampling Transceiver When the receiver is combined with the transmitter, these functional blocks provide enhanced capabilities in the following ways: - Operate across an extremely wide RF frequency range from a few MHz up to6 GHz and handle very wide non-instantaneous bandwidths up to1.5 GHz.
- Digital signal processing module, supporting aggregation and deaggregation of multiple sub-bands or waveforms, each of which can be processed as an independent digital data stream on the receiving or transmitting side.85)]Multi-Band or Multi-Mode Signal Processing
Let us now consider the use case of processing multi-band or multi-mode signals by leveraging wideband sampling, synthesis, and digital processing capabilities. This is shown in Figure 4. Figure 4 Multiband transmit and receive configuration using the AFE7422 and the AFE7444[size] This setup generates a multiband signal consisting of three distinct subbands with a total bandwidth of 2.75 GHz. The receiver samples across the entire band spanning multiple Nyquist zones and then feeds the sampled data to a digital downconversion block with multiple parallel stages. This is done by selecting multiple sub-bands and converting them to baseband signals through independent numerically controlled oscillators (NCOs) and digital mixers. Decimation is applied and then the output sampling rate is reduced based on the bandwidth of the individual signals, while suppressing out-of-band impairments. Conversely, on the transmit side, the individual digital input streams are fed into multiple parallel digital up-conversion stages, which convert the baseband signals to their respective target frequencies. The data is then upsampled to the RF digital-to-analog converter (DAC) output sample rate, and a combined wideband signal (ranging from 700 MHz to 3.45 GHz) is synthesized by an RF DAC in the final stage. Frequency Conversion and Frequency Hopping You can extend the previous case by selecting only a single frequency band and, taking advantage of the internal digital loopback, applying a frequency shift to the selected subband before retransmitting the signal. This is shown in Figure 5. Picture 5 [color =#aa6666]![]() Frequency Conversion or Frequency Hopping UsingAFE7444/AFE7422[size] This setup can capture the multi-band signals described earlier. The digital down-conversion block selects an individual sub-band, converts it to a baseband signal and passes it through a digital filter. The digital filter removes out-of-band impairments such as harmonics or mixing products. An on-chip digital loopback path allows the digital output data from the digital receiver to be fed directly into the transmitter path without leaving the chip and without having to connect any additional processing equipment. Simply up-converting the filtered signal back to the originally received frequency creates an on-chip digital repeater. To implement a frequency-hopping transmitter, the transmitter section’s NCO is programmed to output the desired new frequency and then the frequency-shifted signal is retransmitted. This is shown as the yellow trace in the spectrum analyzer plot in Figure 5 and compared to the originally received multiband spectrum (green trace). 6 ![]() Frequency jump on the oscillator [color =rgb(85, 85, 85)] So far, I have illustrated the basic concepts, and similar approaches can be used to support other use cases, including:[ /align] - Multi-band frequency conversion. Using multiple parallel digital down-conversion and up-conversion modules, you can receive and demultiplex multi-band signals into multiple independent sub-bands. The sub-band signal is then subjected to independent frequency shifting and fed back to the transmitter path via an internal digital loop on chip, where the sub-band signal is retransmitted at the new frequency. 3] Fast frequency hopping. Since we can reprogram the NCO to get an updated frequency in a few milliseconds, or rotate the multiple NCOs available on each signal path in ping-pong mode, we can receive and transmit in repetitive or arbitrary sequences. Frequency agile signal. The transition between these two frequencies is shown in Figure 6.
- Ramp generation/direct digital synthesis mode. The built-in sine wave audio generator for each transmitter supports the generation of frequency ramps and frequency modulated continuous waves (FMCW) commonly used in radar systems.
- Synchronous wideband sweep and narrowband Because each receiver front-end sampling stage can be connected to multiple digital processing stages, you can choose to configure a receive path for wideband mode. Output sampled data spanning the full Nyquist band and observe up to 1.5 GHz non-instantaneous bandwidth to scan for any signal. At the same time, you can configure a second path in narrowband decimation mode to zoom in and accurately analyze all signals detected in wideband mode. size]
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