DAC
Understanding DACs in RF
Modern DACs are enabling a revolution in transmitter architecture by directly synthesizing RF signals without traditional analog upconversion. A high-speed DAC fed with digitally computed samples can generate any modulated waveform at any frequency up to the Nyquist limit.
Key Specifications
- Sampling rate: Maximum output sample clock. Determines maximum output frequency (Nyquist: f_max = fs/2).
- Resolution: Number of bits per sample. More bits = lower quantization noise = better SFDR.
- SFDR: Spurious-free dynamic range at the output.
- Output power: Maximum analog output level.
DAC Architectures
- Direct synthesis: DAC generates the signal directly at RF. Simplest but requires highest speed DAC.
- IF synthesis: DAC generates signal at IF, then analog upconverter shifts to RF.
- I/Q synthesis: Two DACs generate I and Q baseband signals, then an I/Q modulator upconverts to RF.
Frequently Asked Questions
What does a DAC do in RF?
A DAC converts digital samples to an analog signal for transmission. In modern transmitters, high-speed DACs directly synthesize modulated RF signals, eliminating traditional analog mixers and upconverters. This enables software-defined waveform flexibility.
What is direct RF synthesis?
Direct RF synthesis uses a very high-speed DAC to generate the modulated signal directly at the carrier frequency, without an analog upconversion stage. This simplifies the transmitter and provides maximum flexibility. Current DACs support synthesis up to several GHz.
What limits DAC performance?
At higher output frequencies, DAC INL/DNL errors create harmonics and spurious tones. Clock jitter degrades SFDR. The sinc rolloff of the sample-and-hold reduces output power at higher frequencies. Digital pre-emphasis and calibration can compensate for some of these effects.