Subelement D: Circuits— Topic 31: Phase Locked Loops (PLLs); Voltage Controlled Oscillators (VCOs); Mixers
Question 3-31D1
Element 3 (GROL)What frequency synthesizer circuit uses a phase comparator, look-up table, digital-to-analog converter, and a low-pass antialias filter?
Explanation
A Direct Digital Synthesizer (DDS) precisely matches the description. Here's why:
* **Phase Accumulator (implied by "phase comparator" in a digital context):** A DDS uses a phase accumulator to generate a continuously incrementing digital phase value. This phase value determines the point on the waveform at any given instant.
* **Look-up Table (LUT):** The digital phase value addresses a look-up table, which stores pre-calculated amplitude samples of a sine wave (or other desired waveform).
* **Digital-to-Analog Converter (DAC):** The digital amplitude values from the LUT are fed to a DAC, which converts them into an analog voltage signal.
* **Low-Pass Antialias Filter:** Because the DAC's output is a "stepped" approximation of the desired waveform, it contains high-frequency components (sampling images or aliases). A low-pass antialias filter is crucial to smooth the waveform, remove these unwanted frequencies, and leave only the desired fundamental output frequency.
A **Phase-Locked Loop (PLL) synthesizer** (B) operates on a completely different principle, using a phase detector, loop filter, and voltage-controlled oscillator (VCO) in a feedback loop. **Diode-switching matrix synthesizers** (C) are an older, less common method. A **hybrid synthesizer** (D) might incorporate a DDS, but the described components specifically define the DDS section itself.
Related Questions
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