Subelement D: Circuits— Topic 31: Phase Locked Loops (PLLs); Voltage Controlled Oscillators (VCOs); Mixers
Question 3-31D4
Element 3 (GROL)What spectral impurity components might be generated by a phase-locked-loop synthesizer?
Explanation
Phase-locked-loop (PLL) synthesizers inherently generate broadband noise. All active electronic components within the PLL, such as the Voltage-Controlled Oscillator (VCO), phase detector, and loop filter amplifiers, contribute thermal noise. This noise, particularly phase noise from the VCO and noise injected from the reference, phase detector, and dividers, is processed and amplified by the loop. It manifests as a general increase in the noise floor across a range of frequencies surrounding the desired carrier, which is characterized as broadband noise.
Spurs (A) are discrete, unwanted frequency components, typically resulting from harmonic leakage, mixer products, or digital clocking signals, rather than the inherent random noise floor. Randomly drifting spurs (B) are not a characteristic impurity. Digital conversion noise (D) is a term more applicable to ADCs/DACs and doesn't accurately describe the spectral impurity of a PLL synthesizer.
Related Questions
3-31D2 A circuit that compares the output of a voltage-controlled oscillator (VCO) to a frequency standard and produces an error voltage that is then used to adjust the capacitance of a varactor diode used to control frequency in that same VCO is called what?3-31D3 RF input to a mixer is 200 MHz and the local oscillator frequency is 150 MHz. What output would you expect to see at the IF output prior to any filtering?3-31D5 In a direct digital synthesizer, what are the unwanted components on its output?3-31D6 What is the definition of a phase-locked loop (PLL) circuit?3-32D1 Given the combined DC input voltages, what would the output voltage be in the circuit shown in Figure 3D7?