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AD7008JP50 Datasheet(PDF) 8 Page - Analog Devices |
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AD7008JP50 Datasheet(HTML) 8 Page - Analog Devices |
8 / 16 page REV. B –8– 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: Numerically Controlled Oscillator (NCO) + Phase Modulator SINE and COSINE Look-Up Tables In Phase and Quadrature Modulators Digital-to-Analog Converter 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 ( ωt) or a(t) = cos (ωt). 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: ω = 2 πf. +1 0 –1 2 π 0 MAGNITUDE PHASE Figure 9. 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. ∆Phase =ωdt Solving for w: ω= ∆ Phase dt = 2 πf Solving for f and substituting the reference clock frequency for the reference period: 1 f CLOCK = dt : f = ∆Phase × f CLOCK 2 π 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 π. 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 π = 232 . Likewise, the ∆Phase term is scaled into this range of numbers 0 ≤ ∆Phase ≤ 232 – 1. Making these substitutions into the equation above: f = ∆ Phase × f CLOCK 2 32 where 0 ≤∆Phase < 2 32 With a clock signal of 50 MHz and a phase word of 051EB852 hex: f = 51EB852 × 50 MHz 2 32 = 1.000000000931 MHz 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. |
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