AD6636PCB datasheet(26/72 Pages) AD

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AD6636

Rev. 0 | Page 26 of 72

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.

When amplitude offset correction circuit is disabled, the value

in the amplitude offset correction register multiplied with the

Q path data and added to Q path data continuously. This

method can be used to manually set the amplitude offset

instead of using the automatic amplitude offset correction

circuit.

Because all the filtering in the AD6636 is low-pass filtering, the

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

is illustrated in Figure 30. Once the signal of interest is tuned

down to dc, the unwanted adjacent carriers can be rejected

using the low-pass filtering that follows.

INPUT CROSSBAR MATRIX

The AD6636 has four ADC input ports and six channels. Two

input ports can be paired to support complex input ports.

Crossbar mux selection allows each channel to select its input

signal from the following sources: four real input ports, two

complex input ports, and internally generated pseudorandom

sequence (referred to as a PN sequence, which can be either real

or complex). Each channel has an input crossbar matrix to

select from the above-listed input signal choices.

NCO Frequency

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

complement number entered in the NCO frequency register.

Frequencies between −CLK/2 and CLK/2 (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).

The selection of the input signal for a particular channel is

made using a 3-bit crossbar mux select word and a 1-bit

complex data input bit selection in the ADC input control

control. Table 12 lists the valid combinations of the crossbar

mux select word, the complex data input bit values, and the

corresponding input signal selections.

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

The NCO frequency word can be calculated using following the

equation:

(

)

clk

FREQ

NCO

mod

NUMERICALLY CONTROLLED OSCILLATOR (NCO)

where:

Each channel consists of an independent complex NCO and a

complex mixer. This processing stage comprises a digital tuner

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

NCO serves as a quadrature local oscillator capable of produc-

ing an NCO frequency of between −CLK/2 and +CLK/2 with a

resolution of CLK/232 in complex mode, where CLK is the input

clock frequency.

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.

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.

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

digital domain (that is, aliasing introduced when digitizing

analog signals).

Table 12. Crossbar Mux Selection for Channel Input Signal

Complex Input Bit

Crossbar Mux Select Bit

Input Signal Selection

000

Input Port A magnitude and exponent pins drive the channel.

001

Input Port B magnitude and exponent pins drive the channel.

010

Input Port C magnitude and exponent pins drive the channel.

011

Input Port D magnitude and exponent pins drive the channel.

100

Internal PN sequence’s magnitude and exponent bits drive the channel.

000

Input Ports A and B form a pair to drive I and Q paths of the channel, respectively. Input

Port A exponent pins drive the channel exponent bits.

001

Input Ports C and D form a pair to drive I and Q paths of the channel, respectively. Input

Port C exponent pins drive the channel exponent bits.

010

Internal PN sequence’s magnitude and exponent bits drive the channel.

AD6636PCB Datasheet(PDF) 26 Page - Analog Devices

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