I have made a printed circuit board to test different inductor and capacitor values at their resonant frequencies. I am trying to measure voltage drop when two coils are resonantly coupled at different distances. It is a primitive RFID reader with no data transfer.
When the coil isn't driven, ripple on LM317 (powered from a switched-mode power supply) is small, below 100mV(node 1). When I connect coil and drive MOSFET with function generator I get a sine wave but, the amplitude of the sine is not constant. The voltage from LM317 is 3V, with potentiometer set to 10 Ohms the peak voltage is about 10V. Amplitude varies around 500mV, Noise on the Power supply is more visible(node 1).
So I observe a waveform at node 2, which varies between 10V and 9.5V on positive cycle, ten seconds later it changes to 10.2V and 9.7V, later on, its 9.8V and 9.5V, it keeps changing around that 10V.
I think the reason why this happens, is because the capacitors are not able to supply nessecery power to drive a load at that frequency. I was looking around the internet for equations to calculate a proper capacitors values, but with no luck. Or maby LM317 is a poor choice for regulator when driving transient load? Any ideas on how to improve this circuit to have nice stable voltage on envelope detector?
EDIT: R5 potentiometer serves a purpose of increasing or decreasing damping factor and bandwidth of RLC. D1, C2, R2 forms peak detector to measure signal amplitude.