DDS Output Frequency Calculator

This DDS Output Frequency calculator determines the DDS frequency based on the tuning word, clock frequency, and number of phase accumulator bits.

The DDS frequency calculation formula is also provided.

Introduction

DDS (Direct Digital Synthesis) is a technique that generates a frequency/phase tunable output signal based on a fixed frequency clock reference. Internally, a DDS consists of a phase accumulator and a phase-to-amplitude converter. The binary tuning word (N) is provided as input to the phase accumulator. The phase register then generates binary phase information using a certain number of bits.

Figure: Simple DDS Block Diagram

As illustrated above, the DDS includes a clock reference, address counter, PROM (Programmable Read-Only Memory), and DAC (Digital-to-Analog Converter).

In DDS, digital information representing the amplitudes of an entire 360-degree cycle of a sine wave is stored in the PROM (also known as a Sine lookup table). This amplitude information is fed to the DAC’s input at each clock cycle. This is achieved with the help of an address counter, which provides the address information needed to read the stored data from the PROM. Consequently, the DAC generates an analog sine wave based on the digital input words stored in the PROM.

DDS Output Frequency Calculator

The calculator computes the DDS output frequency.

EXAMPLE:

INPUTS:

• Tuning word = 163200000 (decimal) (i.e., 0009BA3C00 hexadecimal)
• Clock Frequency = 77.76 MHz
• Number of bits in phase accumulator = 32

OUTPUT:

• DDS Output Frequency = 2.95 MHz

DDS Output Frequency Formula or Equation

The following formula is used for DDS output frequency calculation: