This application note describes the NCO-based frequency hopping capability of the RF-sampling AFE7422 and AFE7444 (AFE74xx). The AFE7422 and AFE7444 are quad-channel, wideband, RF-sampling analog front ends (AFE) based on 14-bit, 9-GSPS DACs, and 14-bit, 3-GSPS ADCs. With operation at an RF of up to 6 GHz, these devices enable direct RF sampling into the C-band frequency range without the need for additional frequency conversions stages. The AFE74xx improvement in density and flexibility enables high-channel-count, multimission systems, and makes these devices a very attractive option for wideband, frequency-hopping applications.
All other trademarks are the property of their respective owners.
Frequency hopping describes a method in which communication systems rapidly change the operating frequency for a specific application. Applications such as radar, electronic warfare (EW), and communications use frequency hopping in order to avoid interference, avoid detection, or find signals that are attempting to remain undetected. The faster these systems can change frequencies, or frequency hop, the more agile these systems become, thus increasing the chance to avoid interference and detection. In a traditional frequency hopping system, where an analog mixer and PLL or VCO is used as a local oscillator, changing frequencies can take quite a long time. As RF sampling has become more prevalent, frequency hopping is moving toward a NCO-based hopping technique.
Phase coherency, or phase memory, defines the ability for a synthesizer to maintain phase so that when switching to another source, the original frequency source runs continuously in the background and maintains phase, even when not selected. Therefore, upon returning to the original frequency, the original phase is unaltered. Phase coherency is especially useful in systems where multiple frequency sources use a single reference clock. The overall system may switch sources to reflect the desired frequency source on the RF output, while all other synthesizers run continuously in the background while maintaining phases relative to the reference. Phase-coherent radar systems eliminate the need for recalibration when switching between multiple frequencies because the phase relationship relative to the reference is maintained. Figure 1 shows an example of phase-coherent frequency hopping between NCO0 and NCO1 (programmed to frequencies f1 and f2, respectively), and run continuously, even when not selected.
Phase continuity, however, refers to a smooth and continuous transition from one frequency to another on the RF output when the selected source changes frequencies. Noncontinuous or abrupt transitions in the output frequency may lead to unwanted spurious content during fast Fourier transform (FFT) analysis. Figure 2 shows an example of continuous frequency hopping, where the selected source switches from NCO0 (programmed to F1) to NCO1 (programmed to F2). As seen in Figure 2, when frequency hopping from frequency 1 to frequency 2, there is a continuous transition between frequency 1 and frequency 2.