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https://www.thinksrs.com/products/sim.html

The SIMs used here are designed to function as Phase Locked Loops (PLLs).  A PLL is an electronic circuit with a voltage controlled oscillator (VCO) that constantly adjusts to match the frequency of an input signal.  PLLs are used to generate, stabilize, modulate, demodulate, filter or recover a signal from a noisy communications channel.  The main purpose of a PLL is to synchronize the output signal with a reference signal.  It starts with estimating the phase difference between the input and reference signals.  By constantly adjusting the VCO voltage, a PLL reduces phase errors between output and input frequencies.  When the phase difference between these signals is zero, the system is said to be locked.  

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