Subelement D: Circuits— Topic 31: Phase Locked Loops (PLLs); Voltage Controlled Oscillators (VCOs); Mixers
Question 3-31D3
Element 3 (GROL)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?
Explanation
A mixer is a non-linear device designed to combine two input signals, in this case, the RF input (200 MHz) and the local oscillator (LO) signal (150 MHz). Due to its non-linear operation, the output will contain several frequency components.
Specifically, you would expect to see:
1. The original RF input frequency: 200 MHz.
2. The original local oscillator frequency: 150 MHz.
3. The difference frequency (IF): 200 MHz - 150 MHz = 50 MHz.
4. The sum frequency: 200 MHz + 150 MHz = 350 MHz.
Before any filtering is applied, all these components (and potentially harmonics) are present at the mixer's output. Subsequent filtering stages are then used to select the desired IF product (50 MHz in this common scenario) and reject the others. Therefore, 50, 150, 200, and 350 MHz would all be present.
Related Questions
3-31D1 What frequency synthesizer circuit uses a phase comparator, look-up table, digital-to-analog converter, and a low-pass antialias filter?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?3-31D4 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?3-31D6 What is the definition of a phase-locked loop (PLL) circuit?