SLOA313 May   2021 AFE7070 , AFE7071

 

  1.   Trademarks
  2. 1Introduction
  3. 2CF and SBS Characterization
    1. 2.1 Variation Over Frequency
    2. 2.2 Variation Over Power Supply
    3. 2.3 Variation Over LO Drive Level
    4. 2.4 Variation Over Temperature
    5. 2.5 Compensation Algorithm
    6. 2.6 Algorithm Calculations
    7. 2.7 Composite Results
    8. 2.8 Mitigate Frequency Variation With the NCO
    9. 2.9 Composite Results With the NCO
  4. 3Conclusion

1 Introduction

The AFE7070 device integrates a 12-bit, 65-MSPS dual digital-to-analog converter (DAC), baseband low-pass filters, and a quadrature modulator. The AFE7070 is a convenient low power digital-to-RF conversion device suitable for a variety of transmitter applications from 100 MHz to 2.7 GHz. The integrated baseband filters have a bandwidth of 10 MHz which support RF signals bandwidths up to 20 MHz. The DAC incorporates a Numerically Controlled Oscillator (NCO) which moves the complex signal within the 20 MHz RF band window. The DAC also incorporates digital quadrature modulator correction (QMC) to suppress the spurious performance of the modulator.

The integrated quadrature modulator behaves similarly to its discrete counterparts. In an ideal modulator, the carrier (that is, LO) and the image frequency are completely suppressed. In a real modulator, DC offset imbalances within the baseband inputs manifest as carrier feedthrough (CF). Amplitude and phase imbalance in the baseband inputs degrade the sideband suppression (SBS). The QMC correction in the DAC adjusts for the modulator imperfections to suppress those components to the noise floor; however, the corrections only hold for a single frequency point and for a single set of conditions. Once frequency, LO power, or temperature change, the corrections are no longer optimized and the carrier feedthrough and sideband suppression degrades.

It is impractical to calibrate and store optimized QMC values across all frequencies and conditions. The goal is it achieve the best suppression across all conditions with three room-temperature calibrations points: the low, mid, and high frequency within the band of interest. A simple algorithm approach modifies the QMC calibration points based on current operating conditions.

The use case focuses on operation in the VHF band with the following specifications:

  • Frequency: 136–174 MHz
  • Carrier Feedthrough: < –55 dBm over all use cases
  • Sideband Suppression: < –55 dBc over all use cases
  • Temperature Range: –40 to 55°C
  • Calibration points: 3

Initially it is important to understand the inherent performance of the device within the band of interest with respect to variation of the following parameters:

  • Variation over frequency
  • Variation over power supply
  • Variation over LO power
  • Variation over NCO
  • Variation over temperature

Although this use case focuses on the VHF band, the techniques and approach are suitable for any band.