AD6636BBCZ1 datasheet(27/72 Pages) AD

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AD6636

Rev. 0 | Page 27 of 72

NCO TUNES SIGNAL TO

SIGNAL OF INTEREST

AFTER FREQUENCY TRANSLATION

SIGNAL OF INTEREST

SIGNAL OF INTEREST IMAGE

–fs/2

–7fs/8

–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/8

fs/2

–fs/2

–7fs/8

–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/8

fs/2

FREQUENCY TRANSLATION (SINGLE 1MHz CHANNEL TUNED TO BASEBAND)

WIDEBAND INPUT SPECTRUM (30MHz FROM HIGH SPEED ADC)

WIDEBAND INPUT SPECTRUM (–fsamp/2 TO fsamp/2)

Figure 30. Frequency Translation Principle Using the NCO and Mixer

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.

If the carrier frequency is 50 MHz and the clock frequency is

80 MHz,

(

)

125

mod

clk

This, in turn, converts to 0xE000 0000 in the twos complement

32-bit representation.

Mixer

The NCO is accompanied by a mixer. Its operation is similar to

an analog mixer. It does the down-conversion of input signals

(real or complex) by using the NCO frequency as a local

oscillator. For real input signals, this mixer performs a real

mixer operation (with two multipliers). For complex input

signals, the mixer performs a complex mixer operation (with

four multipliers). The mixer adjusts its operation based on the

input signal (real or complex) provided to each individual

channel.

Bypass

The NCO and the mixer can be bypassed individually in each

channel by writing Logic 1 in the NCO bypass bit in the NCO

control register of the channel under consideration. When

bypassed, down-conversion is not performed and the AD6636

channel functions simply as a real filter on complex data. This is

useful for baseband sampling applications, in which the input

Port A (or C) is connected to the I signal path within the filter

and the Input Port B (or D) is connected to the Q signal path.

This might be desired, if the digitized signal has already been

converted to baseband in prior analog stages or by other digital

preprocessing.

Clear Phase Accumulator on Hop

When clear NCO accumulator bit of NCO control register is set

(Logic 1), the NCO phase accumulator is cleared prior to a

frequency hop. Refer to the Chip Synchronization section for

details on frequency hopping. This ensures a consistent phase of

the NCO on each hop. The NCO phase offset is unaffected by

this setting and is still in effect. If phase-continuous hopping is

needed, this bit should be cleared (NCO accumulator is not

cleared). The last phase in the NCO phase register is the

initiating point for the new frequency.

Phase Dither

The AD6636 provides a phase dither option for improving the

spurious performance of the NCO. Writing Logic 1 in the phase

dither enable bit of NCO control register of individual channels

enables phase dither. When phase dither is enabled, random

phase is added to LSBs of the phase accumulator of the NCO.

When phase dither is enabled, spurs due to phase truncation in

the NCO are randomized.

The energy from these spurs is spread into the noise floor and

the spurious free dynamic range is increased at the expense of a

very slight decrease in the SNR. The choice of whether to use

phase dither in a system is ultimately decided by the system

goals. If lower spurs are desired at the expense of a slightly

raised noise floor, phase dither should be employed. If a low

noise floor is desired and the higher spurs can be tolerated or

filtered by subsequent stages, then phase dither is not needed.

AD6636BBCZ1 Datasheet(PDF) 27 Page - Analog Devices

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