Subelement D: Circuits— Topic 31: Phase Locked Loops (PLLs); Voltage Controlled Oscillators (VCOs); Mixers
Question 3-31D2
Element 3 (GROL)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?
Explanation
The circuit described is a classic example of a Phase-Locked Loop (PLL). A PLL works by comparing the phase (and thus frequency) of a voltage-controlled oscillator (VCO) to a stable reference frequency standard. Any difference detected results in an error voltage. This error voltage is then fed back to the VCO, typically adjusting a varactor diode's capacitance, which in turn precisely corrects the VCO's frequency to match the standard. This continuous feedback loop locks the VCO's output to the reference.
A **Doubly balanced mixer** (A) is primarily used for frequency conversion or modulation, not for frequency stabilization of an oscillator. A **Differential voltage amplifier** (C) amplifies the difference between two voltages but is a generic amplifier and not the complete system described for frequency control. A **Variable frequency oscillator (VFO)** (D) is the component whose frequency is being controlled (the VCO itself is a type of VFO), not the entire control circuit.
Related Questions
3-30D6 In the op-amp oscillator circuit shown in Figure 3D6, what would be the most noticeable effect if the capacitance of C were suddenly doubled?3-31D1 What frequency synthesizer circuit uses a phase comparator, look-up table, digital-to-analog converter, and a low-pass antialias filter?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?3-31D4 What spectral impurity components might be generated by a phase-locked-loop synthesizer?3-31D5 In a direct digital synthesizer, what are the unwanted components on its output?