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SBAS888A December 2020 – August 2021

PRODUCTION DATA

8.3.4.4 Numerically Controlled Oscillator (NCO) and Digital Mixer

The decimation block is equipped with a 32-bit NCO and a digital mixer to fine tune the frequency placement prior to the digital filtering. The oscillator generates a complex exponential sequence of:

e^jωn (default) or e^–jωn

where: frequency (ω) is specified as a signed number by the 32-bit register setting

The complex exponential sequence is multiplied with the real input from the ADC to mix the desired carrier to a frequency equal to f_IN + f_NCO. The NCO frequency can be tuned from –F_S/2 to +F_S/2 and is processed as a signed, 2s complement number. After programming a new NCO frequency, the MIXER RESTART register bit or SYNC pin has to be toggled for the new frequency to get active. Additionally the ADC3664 provides the option via SPI to invert the mixer phase.

The NCO frequency setting is set by the 32-bit register value given and calculated as:

NCO frequency = 0 to + F_S/2: NCO = f_NCO × 2³² / F_S

NCO frequency = -F_S/2 to 0: NCO = (f_NCO + F_S) × 2³² / F_S

where:

NCO = NCO register setting (decimal value)
f_NCO = Desired NCO frequency (MHz)
F_S = ADC sampling rate (MSPS)

The NCO programming is further illustrated with this example:

ADC sampling rate F_S = 125 MSPS
Input signal f_IN = 10 MHz
Desired output frequency f_OUT = 0 MHz

For this example there are actually four ways to program the NCO and achieve the desired output frequency as shown in Table 8-1.

Table 8-1 NCO value calculations example

Alias or negative image	f_NCO	NCO Value	Mixer Phase	Frequency translation for f_OUT
f_IN = –10 MHz	f_NCO = 10 MHz	343597384	as is	f_OUT = f_IN + f_NCO = –10 MHz +10 MHz = 0 MHz
f_IN = 10 MHz	f_NCO = –10 MHz	4638564680	as is	f_OUT = f_IN + f_NCO = 10 MHz + (–10 MHz) = 0 MHz
f_IN = 10 MHz	f_NCO = 10 MHz	343597384	inverted	f_OUT = f_IN– f_NCO = 10 MHz – 10 MHz = 0 MHz
f_IN = –10 MHz	f_NCO = –10 MHz	4638564680	inverted	f_OUT = f_IN– f_NCO = –10 MHz – (–10 MHz) = 0 MHz