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":
What does "loaded resistor" exactly mean in this context?
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asked Feb 2, 2013 at 19:15
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1\$\begingroup\$ 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. \$\endgroup\$– user16324Commented Feb 2, 2013 at 21:17
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\$\begingroup\$ 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? \$\endgroup\$– RandomblueCommented Feb 2, 2013 at 23:54
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2\$\begingroup\$ 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? \$\endgroup\$– user16324Commented Feb 3, 2013 at 0:02
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\$\begingroup\$ 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" \$\endgroup\$– DrFriedPartsCommented Apr 4, 2013 at 19:00
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1 Answer
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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.