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AD9913 データシート(PDF) 12 Page - Analog Devices |
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AD9913 データシート(HTML) 12 Page - Analog Devices |
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12 / 32 page  AD9913 Rev. 0 | Page 12 of 32 THEORY OF OPERATION DDS CORE The DDS block generates a reference signal (sine or cosine based on the selected DDS sine output bit). The parameters of the reference signal (frequency and phase), are applied to the DDS at its frequency and phase offset control inputs, as shown in Figure 19. SYSTEM CLOCK 32 15 FREQUENCY CONTROL ANGLE TO AMPLITUDE CONVERSION (SINE OR COSINE) PHASE OFFSET CONTROL TO DAC MSBs DQ R ACCUMULATOR RESET 32 MSB ALIGNED DDS SIGNAL CONTROL PARAMETERS 10 15 32 32 14 32-BIT ACCUMULATOR Figure 19. DDS Block Diagram The output frequency (fOUT) of the AD9913 is controlled by the frequency tuning word (FTW) at the frequency control input to the DDS. In all modes except for programmable modulus, the relationship between fOUT, FTW, and fSYSCLK is: SYSCLK OUT f FTW f ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = 32 2 (1) where FTW is a 32-bit integer ranging in value from 0 to 2,147,483,647 (231 − 1), which represents the lower half of the full 32-bit range. This range constitutes frequencies from dc to Nyquist (that is, ½ fSYSCLK). The FTW required to generate a desired value of fOUT is found by solving Equation 1 for FTW as given in Equation 2 ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ = SYSCLK OUT f f FTW 32 2 round (2) where the round(x) function rounds the argument (the value of x) to the nearest integer. This is required because the FTW is constrained to be an integer value. For applications where rounding to the nearest available fre- quency is not acceptable, programmable modulus mode enables additional options. The relative phase of the DDS signal can be digitally controlled by means of a 14-bit phase offset word (POW). The phase offset is applied prior to the angle-to-amplitude conversion block internal to the DDS core. The relative phase offset (Δθ) is given by ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ π = θ Δ 14 14 2 360 2 2 POW POW where the upper quantity is for the phase offset expressed as radian units and the lower quantity as degrees. To find the POW value necessary to develop an arbitrary Δθ, solve the above equation for POW and round the result (in a manner similar to that described for finding an arbitrary FTW in Equation 1 and Equation 2). FTW POW 1 0 EXTERNAL INTERNAL REGISTER MAP AND TIMING CONTROL + DAC IOUT IOUT RSET DDS CORE I/O PORT CLOCK PORT PLL MULTIPLIER CLOCK SELECTION PROFILE SELECTIONS 1 0 1 0 Z–1 AUXILIARY ACCUMULATOR ANGLE TO AMPLITUDE PHASE ACCUMULATOR PHASE OFFSET 32 32 32 32 32 14 14 10 2 Figure 20. Detailed Block Diagram |
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