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I have an ultrasonic receiver (datasheet available here). On page 2, there are two graphs plotting sensitivity variation and center frequency shift versus "loaded resistor":

enter image description here

What does "loaded resistor" exactly mean in this context?

m.Alin
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Randomblue
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    They must mean "load resistor". Operating the transducer as a microphone, it means the input impedance of the mic amplifier. So you need 39k or more input impedance for highest sensitivity. –  Feb 02 '13 at 21:17
  • So if I don't connect an amplifier (say I just look at the signal on a scope), I shouldn't expect it to work? – Randomblue Feb 02 '13 at 23:54
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    Should work just fine. The scope input impedance is probably 1 Megohm (use a x1 probe) - as I said, more than 39k. Just bear in mind that -60db ref 1V is 1 mv so there might not be much to see. What's the highest sensitivity range on your scope? –  Feb 03 '13 at 00:02
  • An oscilloscope is designed for speed not sensitivity, so as @Brian points out, you may need/want a pre-amp or amplified (Active) probe for your scope in this situation -- but it isn't because the input impedance is "wrong" – DrFriedParts Apr 04 '13 at 19:00

1 Answers1

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I guess they mean load resistor, not loaded resistor.

A load resistor is like a dummy. You can use it to test a power supply, antenna tuner, .... Instead of the device you'd connect normally to the device you want to test, you connect a dummy load or load resistor to it, so that if something goes wrong, you mess up the dummy and not the real device.

About your datasheet: the manufacturer did some tests with his device and a dummy load or load resistor. The diagrams give explanation on that.