Digital Down Conversion (DDC) Theory
The Digital Down Converter or DDC A fundamental part of many communications systems is Digital Down Conversion (DDC). Digital radio receivers often have fast ADC converters delivering vast amounts of data; but in many cases, the signal of interest represents a small proportion of that bandwidth. A DDC allows the rest of that data to be discarded, allowing more intensive processing to be performed on the signal of interest. As an example, consider a radio signal lying in the range 39-40MHz. The signal bandwidth is 1MHz. However, it is often digitised with a sampling rate over 100MHZ, representing in the region of 200Mbyte/second. The DDC allows us to select the 39-40MHz band, and to shift its frequency down to baseband. Once this is complete, the sampling rate can be reduced – with a 1MHz bandwidth, a sampling rate of 2.5MHz would be fine - giving a data rate of only 5Mbyte/second. This is shown in Figure 1. How It Works A DDC works by first shifting the bandwidth of interes...