AD7008JP50 Analog Devices Inc, AD7008JP50 Datasheet - Page 8

• Manufacturer Part Number: AD7008JP50
• Description: IC CMOS DDS MODULATOR 44-PLCC
• Manufacturer: Analog Devices Inc

Specifications of AD7008JP50

• Rohs Status: RoHS non-compliant
• Noise Floor: *
• Voltage - Supply: 5V
• Package / Case: 44-LCC

AD7008
CIRCUIT DESCRIPTION
The AD7008 provides an exciting new level of integration for
the RF/Communications system designer. The AD7008 com-
bines the numerically controlled oscillator (NCO), SINE/CO-
SINE look-up tables, frequency, phase and IQ modulators, and
a digital-to-analog converter on a single integrated circuit.
The internal circuitry of the AD7008 consists of four main sec-
tions. These are:
The AD7008 is a fully integrated Direct Digital Synthesis
(DDS) chip. The chip requires one reference clock, two low-
precision resistors and six decoupling capacitors to provide
digitally created sine waves up to 25 MHz. In addition to the
generation of this RF signal, the chip is fully capable of a broad
range of simple and complex modulation schemes. These
modulation schemes are fully implemented in the digital domain
allowing accurate and simple realization of complex modulation
algorithms using DSP techniques.
THEORY OF OPERATION
Sine waves are typically thought of in terms of their amplitude
form: a(t) = sin ( t) or a(t) = cos ( t). However, these are non-
linear and not easy to generate except through piece wise con-
struction. On the other hand, the angular information is linear
in nature. That is, the phase angle rotates though a fixed angle
for each unit of time. The angular rate depends on the fre-
quency of the signal by the traditional rate of:
Knowing that the phase of a sine wave is linear and given a ref-
erence interval (clock period), the phase rotation for that period
can be determined.
Solving for w:
Solving for f and substituting the reference clock frequency for
the reference period:
Numerically Controlled Oscillator (NCO) + Phase Modulator
SINE and COSINE Look-Up Tables
In Phase and Quadrature Modulators
Digital-to-Analog Converter
+1
–1
2
0
0
f
f
CLOCK
1
Phase
Figure 9.
Phase
MAGNITUDE
Phase
dt
PHASE
2
dt :
f
CLOCK
dt
2 f
= 2 f.
–8–
The AD7008 builds the output based on this simple equation.
A simple DDS chip will implement this equation with 3 major
subcircuits. The AD7008 has an extra section for I and Q
modulation.
Numerically Controlled Oscillator + Phase Modulator
This consists of two frequency select registers, a phase accumu-
lator and a phase offset register. The main component of the
NCO is a 32-bit phase accumulator which assembles the phase
component of the output signal. Continuous time signals have a
phase range 0 to 2 . Outside this range of numbers, the sinu-
soidal functions repeat themselves in a periodic manner. The
digital implementation is no different. The accumulator simply
scales the range of phase numbers into a multibit digital word.
The phase accumulator in the AD7008 is implemented with 32
bits. Therefore in the AD7008, 2 = 2
term is scaled into this range of numbers 0
Making these substitutions into the equation above:
With a clock signal of 50 MHz and a phase word of 051EB852
hex:
The input to the phase accumulator (i.e., the phase step) can be
selected either from the FREQ0 Register or FREQ1 Register,
and this is controlled by the FSELECT pin. The phase accu-
mulator in the AD7008 inherently generates a continuous 32-
bit phase signal, thus avoiding any output discontinuity when
switching between frequencies. This facilitates complex fre-
quency modulation schemes, such as GMSK.
Following the NCO, a phase offset can be added to perform
phase modulation using the 12-bit PHASE Register. The con-
tents of this register are added to the most significant bits of the
NCO.
Sine and Cosine Look-Up Tables
To make the output useful, the signal must be converted from
phase information into a sinusoidal value. Since phase informa-
tion maps directly into amplitude, a ROM look up table con-
verts the phase information into amplitude. To do this the
digital phase information is used to address a Sine/Cosine ROM
LUT. Only the most significant 12 bits are used for this pur-
pose. The remaining 20 bits provide frequency resolution and
minimize the effects of quantization of the phase to amplitude
conversion.
In Phase and Quadrature Modulators
Two 10-bit amplitude multipliers are provided allowing the easy
implementation of either Quadrature Amplitude Modulation
(QAM) or Amplitude Modulation (AM). The 20-bit IQMOD
Register is used to control the amplitude of the I (cos) and Q
(sin) signals. IQMOD [9:0] controls the I amplitude and
IQMOD [19:10] controls the Q amplitude.
The user should ensure that when summing the I and Q signals
the sum should not exceed the value that a 10-bit accumulator
can hold. The AD7008 does not clip the digital output; the
output will roll over instead of clip.
f
f
51EB852 50 MHz
Phase
2
32
2
32
f
CLOCK
where 0
1.000000000931 MHz
32
.
Likewise, the Phase
Phase
Phase
2
32
2
32
REV. B
– 1.