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

3.3.2 Direct Digital Synthesis (DDS) Mode

Direct digital synthesis (or DDS) mode is a technique to generate an analog signal at a frequency specified by a digital code. The AFE74xx supports DDS mode where the DAC outputs a constant analog signal at a frequency specified by the 32-bit accumulator word of the active TX NCO. Each DAC can operate in DDS mode independent of all other DACs. For example, it is possible to transmit a modulated waveform on DAC A and DAC C, while DDS mode is enabled in DAC B and DAC D. DDS mode is enabled by navigating to the Configuration tab in the AFE74xx GUI and selecting on in the constant tone drop-down menu of the respective DAC, as shown in Figure 7.

enabling-DDS-mode-in-DAC-A-through-the-AFE74xx-GUI.gifFigure 7. Enabling DDS Mode in DAC A

Although each DAC contains two NCOs, TXNCO0 is the selected by default and is programmed to 1700 MHz. Therefore, when DDS mode is enabled, the DAC A output signal is 1700 MHz. Figure 8 shows the output of DAC A on a spectrum analyzer.

DAC-A-output-of-1700mhz-in-DDS-mode.gifFigure 8. DAC A Output of 1700 MHz in DDS Mode