![]() The LTC6900 supports frequencies from 1 kHz to 20 MHz. Instead of a custom-designed circuit such as a variable-frequency Colpitts oscillator, I decided to save myself some serious time and energy by using a macromodel for a Linear Tech resistor-set oscillator IC. The less-straightforward portion of the system is the VCO block. It seems to me that the additional ripple suppression provided by a second-order filter is simply not necessary. I tried a second-order filter in my simulation, and the RC circuit worked better. From what I understand, even professional-quality PLLs can often achieve adequate performance with nothing more than an RC low-pass. The phase detector is an XOR gate (I’m using the library discussed here), and the low-pass filter is a single-pole RC circuit. Two of the three functional blocks are very straightforward. ![]() There is nothing particularly special about the circuit that I created, but I think it’s a good place to start, for two reasons: it’s conceptually simple and it doesn’t overburden the SPICE engine (in other words, there’s no need to take a coffee break while you’re waiting for the simulation to finish). There are many ways in which you could simulate a PLL. In fact, the abstract of this academic paper indicates that even “accurate numerical simulations” can be problematic because they require so much processing time. The transient response of a PLL is not only elusive when we try to achieve an intuitive understanding-it is also nonlinear and not easily translated into equations. This is especially true with the initial transient response, i.e., the behavior exhibited by the PLL when it is attempting to lock onto an input signal. This applies to just about any circuit, but it is especially important with PLLs because analytical exploration of PLL functionality is, in my opinion, confusing and inadequate. I prefer to perform simulations myself rather than rely completely on plots and equations and explanations created by someone else. Under the right circumstances, a PLL will ensure that the frequency of the output signal is exactly equal to the frequency of the input signal. The periodic waveform produced by the VCO is the primary output signal. As you may recall, the most basic PLL consists of a phase detector (actually a phase difference detector), a low-pass filter, and a voltage-controlled oscillator: In a previous article I introduced the fundamental concepts and the core functionality of a negative-feedback system known as a phase-locked loop (PLL). This article presents an LTspice circuit that can be used to explore the behavior of a phase-locked loop.
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