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 vs Phase Continuity
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.