I don't have a function generator at my disposal right now. I just want to know what's the typical resistance between + and - terminals of a function generator when it's not putting out a clock?


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    \$\begingroup\$ I don't think you can assume that there is a "typical resistance" when the generator is turned off. This could vary greatly depending on the internal circuitry. \$\endgroup\$ – Elliot Alderson Jul 31 '19 at 18:32
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    \$\begingroup\$ Check the manual. For a HF generator, I would expect it to be 50Ω, just the same if it was on. For an NF generator, it may be an open circuit unless a protection or parasitic diode kicks in. You could measure about 0.7V then, nearly regardless of input current. \$\endgroup\$ – Janka Jul 31 '19 at 18:33
  • \$\begingroup\$ Thanks. That helps. \$\endgroup\$ – Ash Jul 31 '19 at 18:36
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    \$\begingroup\$ There is no typical, it depends how it's been implemented internally, there are many different choices the designers could have made. Measure yours. \$\endgroup\$ – Neil_UK Jul 31 '19 at 19:16

It depends on the details of how it is implemented. If the power is applied then output impedance is probably going to be 50\$\Omega\$ if the amplitude is turned down to zero, but if the output is turned "off" (with power applied) then there may be a series analog switch or relay that actually makes the output high impedance (not to be confused with the "High-Z" setting which really only affects the output amplitude).

In the case of the Rigol DG4062, there is a series mechanical relay contact that disconnects the output when it is turned off or when power is removed from the instrument. Otherwise it is 50 ohms. I believe this is also true of the Agilent 33220A, but I can't verify it at the moment.

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  • \$\begingroup\$ Interestingly, my 33120A shows an impedance of 83Ω while powered off. I haven't fully dug into the details, but I suspect it's related to the default state of the output attenuator. \$\endgroup\$ – duskwuff -inactive- Jul 31 '19 at 20:40
  • \$\begingroup\$ @duskwuff If I recall correctly the 33120A has some high slew rate power bipolar op-amps driving resistors that, in parallel, equal 50.0 ohms. When the power is removed, if there is no series relay contact, the output Z is likely going to be nonlinear (dependent on any protection diodes and op-amp internal parasitics). I don't think there is an attenuator directly on the output. \$\endgroup\$ – Spehro Pefhany Jul 31 '19 at 20:42
  • \$\begingroup\$ Page 138 of the service manual tells me there is a multi-stage output attenuator, and that its relays default to an attenuated setting. The attenuator itself looks like it should come out to under 69Ω, but L801/F801 probably add some resistance. \$\endgroup\$ – duskwuff -inactive- Jul 31 '19 at 21:04
  • \$\begingroup\$ Cool, thanks for the info! \$\endgroup\$ – Spehro Pefhany Jul 31 '19 at 21:05
  • \$\begingroup\$ What is the Rout of push-pull NPN-PNP output? depends on the test voltage. Below 0.6v, probably highZ. But with enough Vtest, the emitters and the bases will really move around, and perhaps turn on other transistors within the output stage. \$\endgroup\$ – analogsystemsrf Aug 1 '19 at 2:37

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