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I am using a function generator with output in units of dBm. It has an output impedance of 50 ohms. I am measuring this output on an oscilloscope, and am failing to understand the waveform's amplitude. With the scope input impedance at 50 ohms, a 0dbm signal has amplitude ~640mV. A 10 dbm signal has amplitude ~1.28mV. With a scope input of 1M ohms, a 0dbm signal has amplitude ~2.04V. A 10 dbm signal has amplitude ~4.08V. I would expect at 1M ohm input impedance and with 0dbm, to get an ~.244 V amplitude, yet this is not the case. Would someone be able to explain this behavior?

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  • \$\begingroup\$ Are your voltage measurements rms or peak-to-peak? Just to be sure, are the units dBmV or dBmW? Changing the input impedance of the scope from 50 ohms to 1 megohm should cause the voltage amplitude to double (within the scope's resolution) so your numbers don't make sense to me. Please provide links to the manufacturer's datasheets for the function generator and the oscilloscope. \$\endgroup\$ – Elliot Alderson Jun 24 '19 at 13:33
  • \$\begingroup\$ a 0dbm signal has amplitude ~640mV I would define 0 dBm in 50 ohms as 224 mVrms or 316 mVp or 632 mVpp. To me the amplitude would be 316 mVp. Always pay care when talking about amplitudes. A mistake / misinterpretation is easily made and can make a difference of a factor 2. \$\endgroup\$ – Bimpelrekkie Jun 24 '19 at 13:37
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I would expect at 1M ohm input impedance and with 0dbm, to get an ~.244 V amplitude, yet this is not the case.

At 0 dBm (1 milliWatt) into 50 ohms I would expect an amplitude of:

224 mVrms = 316 mVp = 632 mVpp.

However, you removed that 50 ohm load! So now you get double that voltage:

448 mVrms = 632 mVp = 1.267 Vpp

This schematic explains what happens:

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Your schematic shows a 1 dBm sine source but your text talks of a 0 dBm source - why the difference? \$\endgroup\$ – srl100 Jun 24 '19 at 14:30
  • \$\begingroup\$ @srl100 1dBm is a typo! I'll change that \$\endgroup\$ – Bimpelrekkie Jun 24 '19 at 17:39

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