The most common type of PLL is a single loop with the voltage controlled oscillator running at the output frequency, but this configuration is not very practical at a VCO frequency of 50Hz for a variety of reasons. If you must use a PLL to generate sinusoidal 50Hz, (which is an odd requirement, so seems like a classroom exercise), one way to do it easily is to build two PLLs. They would both use the same crystal oscillator as a reference. Design them both to run at a fairly high frequency so that you can use a simple LC oscillator with varactor frequency control for each one. Perhaps somewhere around 2 MHz? Design one to run at exactly 50Hz higher in frequency than the other one. Then, when you have both of them working, you take the two outputs and feed them into a double balanced mixer rated for an IF output down to DC. Follow the mixer with a simple 50Hz low pass filter, and perhaps a buffer amplifier to boost the level a bit. Your output should be a very clean 50Hz sinewave with the same frequency accuracy as your crystal reference oscillator.