Kaiwalya:
A PLL is the usual way to up convert frequency. If you use it you should expect to find some error in the translated frequency. After all the PLL is an analog circuit.

It is highly unlikely that the output variation you are seeing is due to a harmonic frequency. Far more likely the output variation is caused by frequency component(s) that are allowed to pass through the filter on the output of the phase comparator. I suggest to you that the PLL can be made more suitable to your measurement application by carefully adjusting and tweaking your PLL and its filters. Decreasing the PLL filter bandwidth will make the output frequency of the PLL more stable. This is of course at the expense of capture and lockin time such that a stable output takes longer to attain when there has been a change of input frequency.

It is a common plight for engineers in this digital world of today to have difficulties with certain types of analog circuits. Combine that with the fact that often the PLL is one of the least understood type of circuits and it is easy to see why PLL applications many times do not live up to expectations.

Do note that PC board layout for circuits using PLL components require a rigorous consideration of details including analysis, isolation, grounding, shielding, and bypassing. This should not however discourage the use of PLLs in a design. Through the use of careful design and layout it is possible to build a successful implementation !!!


Michael Karas