DDS
Understanding DDS
DDS provides frequency agility and resolution that are impossible with PLL-based synthesizers. While a PLL takes microseconds to settle on a new frequency, a DDS switches instantaneously. The trade-off is that DDS has higher spurious levels and lower maximum frequency than PLL/VCO sources.
DDS Architecture
- Phase accumulator: N-bit register that increments by the tuning word each clock cycle. Overflow rate = output frequency.
- Phase-to-amplitude converter: ROM lookup table converts phase to sine amplitude values.
- DAC: Converts digital amplitude values to analog output.
- Low-pass filter: Removes DAC images and clock artifacts.
DDS Advantages
- Resolution: 2^-N x f_clk. For 48-bit accumulator at 1 GHz clock: 3.5 microHz resolution!
- Switching speed: One clock cycle (nanoseconds).
- Chirp generation: Linear frequency sweep by ramping the tuning word.
Frequently Asked Questions
What is DDS?
DDS generates analog waveforms digitally using a phase accumulator and DAC. It provides micro-Hz frequency resolution and nanosecond frequency switching. DDS is used for agile LOs, chirp generation, and precision signal synthesis.
What limits DDS frequency?
Maximum output frequency is about 40% of the clock frequency (Nyquist limit). A 10 GSPS DDS can generate signals up to about 4 GHz. For higher frequencies, DDS is followed by a mixer/upconverter.
DDS vs PLL: which is better?
DDS provides faster frequency switching and finer resolution. PLL provides lower spurious levels and higher maximum frequency. Many modern synthesizers combine both: DDS for fine resolution and agility as the PLL reference, PLL for frequency multiplication and spectral purity.