135khz ultrasonic transdcuer

I have a 135khz tranasducer attached to a generator. The generator has a choke on the output and when attached to a transducer forms a resonant circuit f= 1/2pi √LC.
When I run the generator the resonant point ie were the voltage amplitude across the transdcuer is greatest appears to be at 130.5khz.
Any ideas why?
Also C the static capacitance of the transducer =3.669nf I have a choke value of 278uH. The choke value can be measured straight across the transducer when attached to the generator. If I then measure capacitance the result is about 90uF which is wrong. So I have measured the static cap of the transducer whe nnot attached to the gen.
If I do the calculation to find out what value C should be it comes out at 5.3nF when runnnig at 130.5khz.
If this is the case I should be able to find the inductance value at 135khz. I have done this but find the activity is much better at 130.5khz

If you can prove that your transducer delivers more output at 130.5 kHz, stay there. Transducer resonance is defined by the ceramic material and thickness. Also a load pulls the frequency.

The best way to tune up your ultrasound generator is to use a test microphone (new MEMS microphones may have some response at 130 kHz), connect it to an oscilloscope, then tune the "transmitter" for a maximum response.

The ceramic transducers have internal capacitance which can be tuned by the parallel inductor(or transformer) to resonance. At 130 kHz, I think you can connect an air variable capacitor (from an old AM radio), 30-300 pF in parallel and try a fine tuning. Tuning inductors is possible, too. Yours at 130 kHz may have a ferrite core on a screw, or, use a gentle pressure on the core to tune it.

Your generator's output impedance may be affecting your results. try again with a low value resistor strapped across its output (10 ohms?). This will reduce the voltage available, but will increase the Q of the circuit. The tuned circuit you have is generator output Z, then series L, the inside the transducer, series cap the power loss resistor, then earth. What you are trying to do is to remove the effect of the series transducer cap, so you can deliver power into the power loss resistor. This will then cause the transducer to emit ultra sound, but the actual efficiency of this conversion process may lead to more power being radiated at one frequency rather then another (resonant mechanical fixturing?).
Frank

Hi Frank ,
The problem with that is this; I'm getting approx 90V out of my generator and when connected to a transducer I'm getting approximately 348V. I'm drawing approx 0.5A. So you can see the problem I would get with chooseing a resistor i.e The wattage would have to be very high.