SLAA870 February   2019 AFE7422 , AFE7444

 

  1.   Evaluating the frequency hopping capability of the AFE74xx
    1.     Trademarks
    2. 1 Introduction
    3. 2 Phase Coherency vs Phase Continuity
    4. 3 AFE74xx Architecture
      1. 3.1 AFE74xx Receivers: Multiband DDC
      2. 3.2 AFE74xx Transmitters: Multiband DUC
      3. 3.3 Numerically Controlled Oscillator (NCO)
        1. 3.3.1 Programming the NCO frequency
          1. 3.3.1.1 Example: Programming NCO to 1700MHz
        2. 3.3.2 Direct Digital Synthesis (DDS) Mode
    5. 4 Frequency Hopping Methods
      1. 4.1 Maintaining Phase Continuity
        1. 4.1.1 Phase Continuous Hop Time
          1. 4.1.1.1 Serial Peripheral Interface (SPI)
          2. 4.1.1.2 Test Setup
          3. 4.1.1.3 Software Configuration
          4. 4.1.1.4 Test Results
      2. 4.2 Maintaining Phase Coherency
        1. 4.2.1 TX NCO Hopping Using SPI
          1. 4.2.1.1 TX NCO Switch Using SPI Hop Time
            1. 4.2.1.1.1 Software Configuration
            2. 4.2.1.1.2 Test Results
          2. 4.2.1.2 AFE74xx DAC Settling Time
            1. 4.2.1.2.1 Hardware Setup
            2. 4.2.1.2.2 Software Configuration
            3. 4.2.1.2.3 Test Results
        2. 4.2.2 RX NCO Hopping Using the GPIO Pins
          1. 4.2.2.1 Test Setup
          2. 4.2.2.2 Software Configuration
          3. 4.2.2.3 Test Results
    6. 5 NCO Frequency Resolution Versus Hop Time
    7. 6 Fast Frequency Hopping With the Load and Switch
    8. 7 Register Addresses
    9. 8 References

2 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. 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.

example-of-phase-coherent-frequency-hopping.gifFigure 1. Example of Phase Coherent Frequency Hopping

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.

example-of-phase-continuous-frequency-hopping.gifFigure 2. Example of Phase Continuous Frequency Hopping