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 Numerically Controlled Oscillator (NCO)

The complex digital mixers in the AFE7444 DDC and DUC include digital quadrature modulator (DQM) blocks with independent NCOs. The NCOs converts the complex input signal to a real output signal with flexible frequency placement between 0 and fDAC / 2, where fDAC is the DAC sampling clock frequency. The NCOs have a 32-bit frequency accumulator value that generates the sine and cosine terms for the complex mixing. The NCO block diagram is shown in Figure 5.

afe74xx-nco-block-diagram.gifFigure 5. NCO Block Diagram

The NCOs in the AFE74xx receivers and transmitters run continuously, whether currently selected or not, maintaining phase coherency over time. The numerically controlled oscillators in the DDC and DUC generate a complex exponential sequence, as shown in Equation 1:

Equation 1. x n = e j ω n

A 32-bit register setting, also known as the accumulator word, determines the frequency (ω) in Equation 1. The complex exponential sequence is multiplied by the real input from the ADC or DAC to mix the desired carrier to a frequency equal to:

Equation 2. fIN + fNCO

    where

  • fIN is the analog input frequency after aliasing (in undersampling systems)
  • fNCO is the programmed NCO frequency