7 Real World Examples: Frequency planning with decimation

The use of decimation on an ADC simplifies the frequency planning process because reducing the sampling rate effectively narrows the bandwidth of interest. Think of decimation as selectively focusing in on a narrower part of your spectrum. By focusing on a narrower band, more of the unwanted harmonics or spurs end up falling outside of the passband of interest, and in turn are filtered out. The following examples use the ADC3669 to demonstrate the difference that decimation makes when frequency planning. Figure 5 shows a traditional spectrum capture when the ADC is not performing decimation using an FFT size of 16384 points. You can see that the unwanted harmonics are in band and are negatively affecting performance.

It could be that these harmonics are additive noise contributed by the ADC, or some external analog frequencies. Figure 6 shows an example when the ADC is in real decimation mode, with a decimation factor of 2. You can see that the unwanted harmonic spurs now fall out of band and get filtered out by the decimation filter. Note that there is an additional +3dB improvement due to process gain.

Additionally, the resolution bandwidth of the FFT actually reduces by a factor of two as well, since we maintain the same number of points for the FFT computation. This helps to resolve analog frequencies into closer bins. Up to this point, we have only talked about real decimation, which simply filters the data without any frequency shift. Real decimation is great if your signal of interest falls somewhere less than Fs/4 each time you decimate. But what if you want to decimate a signal that falls outside of this range? The signal of interest is often not centered at zero frequency (baseband), but rather at some intermediate frequency. This is where complex decimation comes in to play. ADCs with newer digital features, such as the ADC3669, incorporate an NCO mixer in the complex DDC stage. Mixing the signal of interest with an NCO frequency shifts the signal to baseband before decimation, enabling you to take advantage of the benefits of decimation for a signal anywhere within the device’s bandwidth.

Figure 7 shows a capture of the ADC3669 in complex decimation mode with a decimation factor of 64 resulting in an effective sampling bandwidth of 7.8125MHz when the FFT is calculated using 8192 points. The input frequency is 70MHz and the NCO frequency is 71MHz. When the signal gets mixed with the NCO frequency, the signal shifts to baseband, resulting in a tone at approximately -1MHz.

The ADC3669 can capture a narrow band with a decimation factor of up to 32768, which is beneficial for applications with high-density RF bands or tight channel spacing. Decimating by such a high factor allows you to zoom into your signal of interest, filtering out virtually everything else. The range of decimation factors offered by modern ADCs such as the ADC3669 enables more flexibility when frequency planning, as it is much easier to filter out unwanted spurs. Figure 8 shows a capture with a decimation factor of 16384 calculated using 8192 FFT points, resulting in a resolution bandwidth of 3.726Hz. Even if your spurs are within thousands of hertz of the fundamental, you can easily filter them out with a high decimation rate.

As the NCO frequency is 4kHz lower than the input signal, the down converted signal appears at a positive frequency offset. While operating in this decimation mode and at 500MSPS, this ADC can sample signals within a 30.517kHz range around the programmable NCO frequency.