Back in my April blog post this year, Analog Devices Design Tools: ADISimADC Frequency Folding Tool , we took some time to look at the Frequency Folding Tool that is available on the Analog Devices web site. I would like to revisit this tool as there have been a few handy improvements that have been made based on some great feedback. As you may recall, this tool can be accessed from the ADI Data Converter Tools web page shown below. There are several useful data converter tools available from this webpage and I encourage you to take a look each one that is available. For example, this is a useful place to download Visual Analog and SPIController which are used to evaluate many different high speed ADC evaluation boards available from Analog Devices.
In the April blog we looked at how the tool could be used to show the aliasing effects of sampling in an ADC. We looked at single tone as well as wideband (modulated) carriers. Let’s look at some similar examples here, but with a few extra added bells and whistles that we can use with the updated tool. First, notice that the Nyquist zone boundary labels have been added to the tool. In this example, the Sampling Frequency is set to 1250 MHz. The number of Nyquist Zones has been set to “4.” The input frequency is set to 1000 MHz. The Nyquist boundaries are labeled under these conditions at 625 MHz, 1250 MHz, 1875 MHz, and 2500 MHz. From the tool, it is evident that the input tone is in the second Nyquist zone. The fundamental input tone at 1000 MHz aliases into the first Nyquist zone at 250 MHz. The second harmonic of the fundamental input tone is at 2000 MHz and aliases into the first Nyquist zone at 500 MHz.
Next, by checking the boxes beside the 4th , 5th , and 6th harmonic boxes, these harmonics can be enabled and analyzed in the Frequency Folding Tool in addition to the second and third harmonics. Using the same conditions from the example above, we can now see the fifth harmonic appear in the analysis. In most applications, the analyzing the harmonic levels up to the sixth harmonic is sufficient as most of the energy from the harmonics of the ADC will reside in these harmonics. Generally, the signal level of the harmonics of higher order are low enough that they can be ignored in most applications.
Let’s take a closer look here though. In the spirit of Halloween, I think we have some ghostly harmonics lurking in the shadows in here. When a signal is input into the second Nyquist zone or higher they will alias into the first Nyquist zone. The harmonics of this input signal will also alias into the first Nyquist zone. If the sampling frequency and input frequency line up in the right fashion, the harmonics can alias into the same locations in what is commonly referred to as “stacking.” In addition, harmonics can alias into the same location as the alias of the fundamental frequency. Let’s move the fundamental frequency slightly to help observe the aliasing in the previous example. In this case, we will set the fundamental frequency to 1010 MHz which is just enough offset to help us see the aliasing effects from the frequency plan in the previous example.
By observing the locations of the harmonics in the figure above (and reviewing the listed harmonic locations in the previous figure), it can be deduced that the fourth and sixth harmonics were aliasing onto the same location as the alias of the fundamental frequency (250 MHz) when the fundamental frequency was set to 1000 MHz. In addition, the second and third harmonics aliased to the same location as well (500 MHz). Generally frequency plans are created to avoid stacking of the harmonics especially when the harmonics alias into the same location as the alias of the fundamental frequency. When the harmonics alias into the same location as the fundamental, the quality of the fundamental signal can degrade and possibly result in signal degradation such as poor EVM (error vector magnitude). I’d encourage you to try out the online tool and observe the aliasing effects of an ADC. Make sure to frequency plan correctly so you can avoid being attacked by ghostly harmonics!