AD6654BBCZ datasheet(34/88 Pages) AD

34 / 88 page

AD6654

Rev. 0 | Page 34 of 88

NUMERICALLY CONTROLLED OSCILLATOR (NCO)

Each channel consists of an independent complex NCO and a

complex mixer. This processing stage is comprised of a digital

tuner consisting of three multipliers and a 32-bit complex NCO.

The NCO serves as a quadrature local oscillator capable of

producing an NCO frequency of between –CLK/2 and +CLK/2

with a resolution of CLK/232 in the complex mode, where CLK

is the input clock frequency.

The frequency word used for generating the NCO is a 32-bit

word. This word is used to generate a 20-bit phase word. A

16-bit phase offset word is added to this phase word. 18 bits of

this phase word are used to generate the sine and cosine of the

required NCO frequency. The amplitude of the sine and cosine

are represented using 17 bits. The worst-case spurious signal

from the NCO is better than −100 dBc for all output

frequencies.

Because all the filtering in the AD6654 is low-pass filtering, the

carrier of interest is tuned down to dc (frequency = 0 Hz). This

is illustrated in Figure 47. Once the signal of interest is tuned

down to dc, the unwanted adjacent carriers can be rejected

using the low-pass filtering that follows.

NCO FREQUENCY

The NCO frequency value is given by the 32-bit twos comple-

ment number entered in the NCO frequency register.

Frequencies between −CLK/2 and +CLK/2 (with +CLK/2

excluded) are represented using this frequency word:

0x8000 0000 represents a frequency given by −CLK/2.

0x0000 0000 represents dc (frequency is 0 Hz).

0x7FFF FFFF represents CLK/2 − CLK/232.

The NCO frequency word can be calculated using following the

equation:

(

)

CLK

FREQ

NCO

mod

where:

NCO_FREQ

is the 32-bit twos complement number represent-

ing the NCO frequency register.

fCH

is the desired carrier frequency.

fCLK

is the clock rate for the channel under consideration.

mod( ) is a remainder function. For example, mod(110, 100) =

10 and, for negative numbers, mod(−32, 10) = −2.

Note that this equation applies to the aliasing of signals in the

digital domain (that is, aliasing introduced when digitizing

analog signals).

For example, if the carrier frequency is 100 MHz and the clock

frequency is 80 MHz,

(

)

mod

CLK

This, in turn, converts to 0x4000 0000 in the 32-bit twos

complement representation for NCO_FREQ.

NCO TUNES SIGNAL TO

SIGNAL OF INTEREST

AFTER FREQUENCY TRANSLATION

SIGNAL OF INTEREST

SIGNAL OF INTEREST IMAGE

–fs/2

–7fs/16

–3fs/8

–5fs/16

–fs/4

–3fs/16

–fs/8

–fs/16

fs/16

fs/8

3fs/16

fs/4

5fs/16

3fs/8

7fs/16

fs/2

–fs/2

–7fs/16

–3fs/8

–5fs/16

–fs/4

–3fs/16

–fs/8

–fs/16

fs/16

fs/8

3fs/16

fs/4

5fs/16

3fs/8

7fs/16

fs/2

FREQUENCY TRANSLATION (SINGLE 1MHz CHANNEL TUNED TO BASEBAND)

WIDEBAND INPUT SPECTRUM (30MHz FROM HIGH SPEED ADC)

WIDEBAND INPUT SPECTRUM (–fsample/2 TO +fsample/2)

Figure 47. Frequency Translation Principle Using the NCO and Mixer

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