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DDS (Direct Digital Synthesizer) basics

This page covers DDS (Direct Digital Synthesizer) basics.It provides link to RF synthesizer design example using DDS Chip.

RF Synthesizer is in use since years in the wireless communication systems as tunable source. This helps in RF channel selection in variety of applications such as TV, radio, satellite and so on. It is also widely used in test and measurement equipments such as synthesizable sweepers, RF vector signal generators(VSGs) and RF vector signal analyzers(VSAs) etc.

DDS is a technique which generates frequency/phase tunable output signal based on fixed frequency clock reference. The full form of DDS is Direct Digital Synthesizer. It is also referred as Direct Digital Synthesis.

DDS, Direct Digital Synthesizer Block Diagram

There are many benefits(advantages) of DDS technique as mentioned below:
• Step of size in micro-Hz can be achieved.
• Fast hopping speed
• Avoids tuning needed in analog counterpart due to component aging as well as temperature drift.
• As the DDS chip provides digital control and hence it can be remotely controlled using microcontroller/microprocessor/DSP/FPGA.

Figure-1 depicts simple DDS Block Diagram. As shown it consists of clock reference, address counter, PROM and DAC. In DDS, digital information of amplitudes of entire 360 degree cycle information of sinewave is stored in the PROM as mentioned(Sine lookup table). This amplitude information is provided at the input of DAC(Digital to Analog converter) at each clock. This is achieved with the help of address counter which provides address information to read the stored data from register. Hence DAC generates analog sinewave based on digital input words stored in the PROM.

DDS internal functional modules

The figure-2 depicts internal functional modules of DDS chip. As shown it consists of phase accumulator and phase to amplitude converter. The binary tuning word(N) is provided as input to the phase accumulator. Phase register generates binary phase information in few number of bits.

The tuning word specifies output frequency as a function of reference clock frequency.

Following formula/equation is used to determine output frequency of a DDS chip. Using this equation M is determined based on FOUT, Fref and N.
FOUT = M * Fref / 2N
FOUT = The output frequency of DDS
M = Binary tuning word (32 bits binary information)
Fref = Reference clock frequency
N = Length in bits of the phase accumulator(example:32) (Determines frequency resolution)

RF Synthesizer Design using DDS Chip

Refer L Band RF Synthesizer design using DDS Chip and other discrete components. L Band is from 950 MHz to 1450 MHz. It is widely used in satellite communication.

Also refer RF Synthesizer manufacturers or vendors.

RF and Wireless Terminologies

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