There are 2 separate NCOs per channel - one for each band (that is, NCO1 = band 1) and 4 different frequencies can be programmed per NCO as shown in Figure 7-37. The NCO frequencies are located in the DDCAB/CD pages (0x05 0x08 for channel A/B and 0x05 0x10 for channel C/D) in registers 0x100 to 0x17D. Depending on # of bands used, the frequencies for each NCO are selected in registers 0x3B and 0x41 (DIGITAL page) as shown in Table 7-21. If the NCO frequencies are the same for channel A/B and channel C/D, the frequencies can be written to both DDCAB and DDCCD pages simultaneously by selecting both pages (0x05 0x18).

Figure 7-37 Multi-Band NCO

Single band DDC uses the frequencies of both NCO1 and NCO2 for a combined 8 different frequencies for NCO1 using 3 bit control (NCO2 CHx [1] and NCO1 CHx [1:0]). The NCO2 selection bit (D3) decides if frequencies from NCO1 or NCO2 are being used. In dual and quad band DDC operating mode, there are 4 frequencies per NCO available and selected using 2 register bits (NCOx CHx [1:0]).

To select a different frequency for the NCO, two registers (0x3B and 0x41) in the DIGITAL page must be updated. Assuming a SPI clock frequency of 10 MHz (100 ns period), programming two registers (2x (16 bit address and 8 bit data) = 48 bit) means that the NCO frequency would be updated in ~ 5 us.

When updating the currently being used NCO frequency to a new frequency, the following command has to be written to load the new frequency into the NCO - 0x181 0x00/0x30 in each of the DDCAB/CD pages.

The NCO phase accumulators can be reset using the external SYSREF signal. A SYSREF mask can be setup such the SYSREF signal only goes to the NCO and the remaining device remains unaffected. The following register writes configure the SYSREF mask to only affect the NCO. After completion, the SYSREF mask should be set back to default.