If you mean the input frequency of the sensor stage you can only calculate it theoretically with the values of L1 / C10 + about 10pF of the parasitic capacitance of the TR2 base / emitter, or you can use an external oscillator and check for what frequency there is a variation to the out of the sensor stage. However the most important thing is to establish which is the best frequency on the test field, C10 can be 10 or 15 or 20pF, or nothing, only the base / emitter capacity.
|