15625hz filter
If it is the sweep frequency of your TV set you want to lock to, you can make a pick up coil to place near your TV set. Then follow the coil with an amplifier and a narrow band active filter. This should be good enough. You can further clean the signal by locking a PLL chip running at this frequency. The NE567 types (also called tone decoders) work very well. Select in the design a narrow loop bandwidth. If you want to adjust the phase relationship, you can preceed the PLL with a RC low pass filter and adjust the time constant. You can also use several RC sections if you want a wide phase adjustment without very much signal amplitude reduction. You should orient the pick up coil with its axis on line with your CRT ppicture tube deflection coil axis. Your coil should be only a few times larger diameter than the tube coil. At these low frequencies there is no resonance limit on the number of turns resonating with the inter wire capacitance. You will probably run out of stamina in winding the coil before you reach the self resonance frequency being above the swee frequency.