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

4.1 Maintaining Phase Continuity

When updating the output frequency of single RX or TX NCO, phase continuity is maintained using SPI. The programmed frequency is reflected at the receiver or transmitter output only after the newly programmed NCO is reset. NCO reset can occur through SPI, SYSREF, or JESD SYNC signal. For the scope of this application report, both SPI and SYSREF are evaluated as methods for issuing the NCO reset.

In the following experiments, the hop time is characterized using the AFE74xx transmitter and transmitter NCOs. Phase continuous frequency hopping is implemented using a single TX NCO. The DAC is configured in DDS mode to output a single tone, and The TXNCO0 accumulator word is continually reprogrammed to effectively hop the constant tone output anywhere between 0 Hz and fS / 2, where fS is the sampling frequency. Figure 9 is a visual depiction that shows how phase continuous frequency hopping is implemented with the AFE74xx.

implementing-phase-continous-frequency-hopping-with-AFE74xx-EVM.gifFigure 9. Implementing Phase Continuous Frequency Hopping With the AFE74xx Evaluation Module (EVM)