# Phase register in phase accumulator

In the following flow diagram:

Is the "phase register" a one-bit register? What does it basically do with that summation mark? What type of register is it? I basically do not know what the phase accumulator do as well but for that I need to understand what phase register is doing first.

It is an N bit register that serves in combaination with the adder at the input to integrate the frequency tuning word (Phase being the integral of frequency after all).

If you look at the diagram you can clearly see that on each clock the register will be loaded with the modulus 2^N sum of its old output and the FTW.

In real parts N is quite often larger then the address range of the ROM (32 or even 48 bits is not uncommon), and various phase dithering and Taylor series approximations are used to improve the spurious performance.

• "Phase being the integral of frequency " I dont understand that part. Isnt the phase increment "2*pi / N" where N is sample per period? What do you mean by phase here? – cm64 Nov 14 '18 at 12:53

The way the phase accumulator works in Direct Digital Synthesis is that every clock cycle on fclk, the frequency tuning word is added to the phase register.

For example, assume the phase register starts at 0 and the frequency word is 5. After the first fclk pulse the phase register become equal to 5. After the second clock pulse, the phase register becomes 10. This continues for every clock pulse

The size of the phase register and tuning word depends on the size of Sine ROM and the accuracy you need to achieve in your waveform. Larger register will have more accuracy in determining the frequency that you can run at.

Here is a link to a datasheet for a DDS integrated circuit. Page 11 and 12 go into the theory of how they work.

The phase-register is an n bit width register as can be seen on the in/out-puts. The DDS can outputs arbitrary periodic function. Your example outputs a sine wave, but you can replace the sine ROM to an arbitrary function. The phase-register stores the actual phase of the periodic function.

What type? The register can be implemented as an n-bit width D-FF.

The phase accumulator can be interpreted as a frequency integrator. Note, that the phase is the integrate of the frequency, and the sum is the discrete equivalent of the integration.

t=φ/2π

Where the φ is the phase the ω is the angular frequency, the t is the time and the f is the frequency.

• What is meant by the "phase of the function"? – cm64 Nov 14 '18 at 12:05
• Phase accumulator is integrating what actually? – cm64 Nov 14 '18 at 12:06
• The phase (phi) is the x axis, measured in radian. If you know the period time (or the frequency) you can convert the phase to time. phi = 2pi/f or t=2pi/phi – betontalpfa Nov 14 '18 at 12:11
• Can you show me what do you mean by phi in this context? – cm64 Nov 14 '18 at 12:54
• The phi is the phase – betontalpfa Nov 14 '18 at 13:47