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I have a very simple setup that is giving me problems I cannot understand: Electrical Scheme

Apparently, the reflected power is the same as the power generated by the amplifier (both measured by the amplifier, although I double checked with a power meter). The load, a 50 ohms ultrasonic transducer in this case, seems to be completely mismatched. HOWEVER, this problem sometimes disappear after some minutes (5-10 min) without changing anything.

I have switched all the equipment, transducers and cables (+connections). Can someone rationalize this behavior? Could it be due to the driving frequency?

Some details if necessary:

  • The function generator is one from Agilent which I double checked by means of an oscilloscope.

  • The RF amplifier is from Electronics & Innovation

  • The ultrasonic power transducers are from American Piezo. The behavior described is the same for everyone of them.

Note: Obviously, my background in electronics is very limited.

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  • \$\begingroup\$ how do you measure the reflection? do you disconnect the amplifier while measuring, right? \$\endgroup\$ – berto Mar 7 '16 at 3:14
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    \$\begingroup\$ The transducers have a high Q factor (typically > 400), meaning that they only operate in a narrow frequency range, and that range is somewhat affected by the temperature. As you are driving the transducer with a fixed frequency from your signal generator, the transducer is initially out of tune and exhibits poor coupling. Only after warming up for some time its resonant frequency gets closer to the drive frequency, enabling significant power transfer. \$\endgroup\$ – jms Mar 7 '16 at 3:24
  • \$\begingroup\$ @berto I tried two amplifiers: one measures the forward and load power; the other gives me the reflected power. I have a power meter that confirms my readings. \$\endgroup\$ – FZNB Mar 7 '16 at 14:45
  • \$\begingroup\$ @jms Your explanation makes sense, I will try to manually tune the frequency (although did that already with no success) \$\endgroup\$ – FZNB Mar 7 '16 at 14:54
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As @jms mentioned, the problem was the (very) narrow frequency range of the transducer.

I could find the freq. sweeping and monitoring the power consumed.

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