AD6654/PCB Analog Devices Inc, AD6654/PCB Datasheet - Page 34

• Manufacturer Part Number: AD6654/PCB
• Description: BOARD EVALUATION FOR AD6654
• Manufacturer: Analog Devices Inc

Specifications of AD6654/PCB

• Module/board Type: Evaluation Board
• For Use With/related Products: AD6554
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

AD6654
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/2
–fs/2
–fs/2
SIGNAL OF INTEREST IMAGE
–7fs/16
–7fs/16
–3fs/8
–3fs/8
–5fs/16
–5fs/16
–fs/4
–fs/4
FREQUENCY TRANSLATION (SINGLE 1MHz CHANNEL TUNED TO BASEBAND)
AFTER FREQUENCY TRANSLATION
32
Figure 47. Frequency Translation Principle Using the NCO and Mixer
.
WIDEBAND INPUT SPECTRUM (–fsample/2
WIDEBAND INPUT SPECTRUM (30MHz FROM HIGH SPEED ADC)
–3fs/16
–3fs/16
SIGNAL OF INTEREST
–fs/8
–fs/8
–fs/16
–fs/16
Rev. 0 | Page 34 of 88
DC
DC
fs/16
fs/16
The NCO frequency word can be calculated using following the
equation:
where:
NCO_FREQ is the 32-bit twos complement number represent-
ing the NCO frequency register.
f
f
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,
This, in turn, converts to 0x4000 0000 in the 32-bit twos
complement representation for NCO_FREQ .
CH
CLK
NCO TUNES SIGNAL TO
SIGNAL OF INTEREST IMAGE
is the desired carrier frequency.
is the clock rate for the channel under consideration.
NCO
mod
TO +
fs/8
fs/8
(
fsample/2)
_
f
f
CH
CLK
FREQ
3fs/16
3fs/16
,
f
DK
=
2
)
fs/4
fs/4
32
=
20
80
mod
5fs/16
5fs/16
=
. 0
(
f
25
SIGNAL OF INTEREST
f
CH
CLK
3fs/8
3fs/8
,
f
CLK
7fs/16
7fs/16
)
fs/2
fs/2