4 Noncoherent Sampling

Noncoherent sampling is when your analog input frequency and clock frequency are not reference-locked together. If you are using a setup similar to that shown in Figure 2-1, there is no need to use the 10MHz reference connection to lock the two signal generators together. Instead, you can smear the analog input frequency. For example, if you want to sample a 100MHz analog input signal, smearing means that you can instead use something close to 100MHz that is a prime number: 99.1235MHz or 101.1235MHz. Smearing makes sure that the ADC does not sample on the exact sample point along the sine-wave input signal each time, but instead walks along the signal and samples many points.

In your FFT data-capture program, using a windowing option like Blackman-Harris, this can make sure that the FFT capture spreads out the samples appropriately on the edges and prevents discontinuity between FFT captures. See Section 5 section later in this article for more information.

Figure 4-1 illustrates a valid noncoherent-sampled FFT data capture.

Noncoherent sampling makes sure that all FFT points are smeared appropriately. The fundamental and harmonic energy reside in more than one bin or point in the FFT capture and are available for measurement calculations such as SNR and SFDR.

Figure 4-2 is a zoomed-in plot of the FFT around the fundamental bin. The fundamental level is different roughly -3.5 dBFS; versus the value listed in the parametric table to the left, roughly -1 dBFS. The harmonic levels can appear differently as well. Again, this is because the FFT capture includes data from multiple points or bins and is the collective power summation of those bins. This is the -1dBFS number reflected on the left of the parametric table and in Figure 4-2.