I am using some Si5351 PLL clock chips in my homebrew HF transceivers for the VFO. I have noticed that there is a small drift in frequency after switch on of around 50Hz. I have not done anything inside the VFO to stabilise the temperature or lock the Si5351 to a more accurate crystal or source. I am just using stock standard clock modules from EBay with the small SMD 25MHz crystals. In this video I am demonstrating the drift in frequency of the Si5351 used in my homebrew signal generator by comparing the output on a frequency counter which is calibrated using a low cost GPS module locked to a satellite and producing a 10MHz output. You will see that after a few minutes the drift has been reduced quite considerably and after 40 minutes is very close to zero. Of course, if one moves the test Si5351 or the room conditions change, then one would expect a small change in frequency. During this test, room conditions were held constant. So the drift was caused by the environmental changes inside the signal generator.