I connected a 50 ohm function generator to an oscilloscope set to have an input impedance of 1 MOhm. I set the generator to 0.5Vpp and saw a 1Vpp signal on the oscilloscope. The generator display is set for a 50 ohm load, so this all makes sense so far.

Then, I switched the oscilloscope to 50 ohm impedance, and I was expecting to see a 0.5Vpp signal, but I got no output at all. The scope has 10:1 probes and the input is rated for < 5 Vrms at 50 ohms. The current through the generator would be 10 mApp for a 50 ohm load, which it should be able to handle. There is no external circuitry. Function generator is HP33120A and scope is RTM2034.

So the question is, why does setting the oscilloscope to 50 ohms completely kill my signal?

  • 1
    \$\begingroup\$ 10:1 probes have an input impedance of 10M using a series R of 9Meg. with the scopes 1M shunt R. \$\endgroup\$ Mar 1 '17 at 18:04
  • \$\begingroup\$ What is the frequency your measuring? What is the impedance of the source? \$\endgroup\$
    – Voltage Spike
    Mar 1 '17 at 18:56

You're not using the 10X probes are you? When the scope is set to 50 ohm input impedance you should connect the signal generator directly to the input BNC on the scope, you should not use the probes which are designed for a 1M input impedance on the scope.

  • \$\begingroup\$ Ok, I think I get it. With 10:1 probes, the 1M oscilloscope impedance sees 10% of the input voltage because of the 9M series resistance from the probe. By switching to 50 ohm input impedance, the scope now sees only 0.00056% of the input voltage because of the voltage divider. That's why the signal doesn't appear on the display, correct? \$\endgroup\$
    – pr871
    Mar 1 '17 at 18:23
  • \$\begingroup\$ That's exactly correct. \$\endgroup\$
    – John D
    Mar 1 '17 at 18:38
  • \$\begingroup\$ Or he can use a tee with a 50 ohm termination: electronics.stackexchange.com/questions/101634/… (works with my bottom of the barrel equipment) \$\endgroup\$ Mar 1 '17 at 18:51

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