2 Coherent Sampling

Coherent sampling typically means you can take the 10MHz reference output used for the sample clock on one signal generator and connect the reference output to the 10MHz reference input used for the analog input signal on the other signal generator. Simply connecting the two signal generators together does not mean that the signal generators are phase-locked automatically, so for the reference lock to actually lock, you also need to select, enable, or initiate the signal generator with the 10MHz reference input in the signal generator’s menu selection.

The signal generator can typically display a message such as EXTREF, indicating that the two signal generators are reference-locked together. See Figure 2-1.

Figure 2-1 Typical Test Measurement Setup for FFT Data Capture

Next, select No Windowing or Rectangular Windowing in the FFT capture, enter the ADC sampling rate, and set the number of FFT points in the data-capture program or graphical user interface (GUI). Make sure that the data-capture board hardware can handle the number of FFT points. Then determine and calculate the exact analog input frequency and enter that frequency in the signal generator; you can also have to enter the frequency in the data-capture software. Figure 2-2 shows an example of a valid coherent sampled FFT data capture using the Texas Instruments (TI) ADC12DJ5200RF.

Figure 2-2 Example FFT Plot of the ADC12DJ5200RF Using Coherent Sampling

Coherent sampling makes sure that all of the FFT points are bin centered – that the fundamental and harmonic energy only reside in one bin or point in the FFT and can be placed at that exact location for measurement calculations such as the signal-to-noise ratio (SNR) and spurious-free dynamic range (SFDR). Figure 2-3 is a zoomed-in plot of the FFT around the fundamental bin. Note that the fundamental level, -1 dBFS, is the same as what is represented in the FFT plot.

Figure 2-3 Example FFT Plot of the ADC12DJ5200RF Using Coherent Sampling Zoomed in Around the Fundamental Bin