I have a new function generator - model AFG3000C.

I am having difficulty in understanding certain terms related to this function generator.

I have tried online searches, but I couldn't get what I was looking for. Being a newbie, I request you all to help me understand my queries in simple terms.

Could you please explain my queries below :

  1. In this function generator, we have the option to set "LOAD IMPEDANCE" as 50 ohms, load and high Z.

    • What does each mean? And where to use each one? I guess, it is related to the transmission line characteristic impedance. But where should we use the load option, 50 ohms and the high Z option?
  2. Can we set the impedance using the function generator according to our needs?

  3. This function generstor only provides output voltage right? Can someone impress voltage on the function generstor channels? If so, how and how to check how much voltage can the FG withstand? And what about the current parameter?

  • \$\begingroup\$ Try rephrasing the question to be more general. The question applies to pretty much all function generators, simply making it not specific to the AFG3000C could help people in the future. \$\endgroup\$
    – Joren Vaes
    Mar 24, 2020 at 7:33
  • \$\begingroup\$ Sure. I will. But just for example, I am using this FG. Can you please help me clear my queries regarding this \$\endgroup\$
    – user220456
    Mar 24, 2020 at 7:36

1 Answer 1


Taking your second point first. Most test sources have a fixed output impedance of 50 Ω, and your AFG3000C is no exception. From the datasheet you linked to (Amplitude table, page 4), it says ...

Output impedance: 50 Ω

There are several reasons for this

1) It's not possible to achieve zero output impedance over any useful bandwidth, and if you did, you couldn't maintain it at the end of a cable
2) With a cable and generator of the same impedance, the output impedance at the end of the cable is the same as that of the generator
3) Many things at high frequency require a finite impedance to be stable, when testing RF transistors for instance
4) As a result, 50 Ω has become the de facto standard impedance for cables, connectors and test gear, which makes it very easy to connect things and have them just work.
5) Providing a good 50 Ω is enough of a struggle that no general purpose test gear provides a freely controllable output impedance
6) If you want a specific output impedance for your particular test setup, it's easy enough to use an external resistive pad to provide it.

As a result, the output voltage is usually calibrated so that it displays the voltage or power into a 50 Ω load.

Now to your first point. Your AWG goes one better. Your datasheet, same table, says ...

Load impedance setting: Selectable: 50 Ω, 1 Ω to 10.0 kΩ, High Z (Adjusts displayed amplitude according to selected load impedance)

If you tell it what the load is, it will work out for you the correct voltage scaling to display. The corollary to that is that if you tell it the wrong load, or if your load is varying in a way you don't anticipate, then it will display incorrect numbers.

A point worth reiterating about the fixed 50 Ω output impedance and cable impedance. Even if you are driving a non-50 Ω load, the AWG+cable combo still looks like a 50 Ω source at the end of the cable, and (neglecting cable attenuation) the voltage display on the generator is still valid at the end of the cable, regardless of your strange load.

Your third point. You'd like there to be an 'absolute maximums' section, which specified what the highest external signal that could be applied was. I used to design signal generators, and we sweated bullets so that it would survive if somebody accidentally keyed-up a 50 W transmitter into it. However, all your datasheet says is ...

External voltage protection: To protect signal outputs against external voltages use fuse adapter 013-0345-xx

So I guess it's fairly fragile. Don't reverse drive it with any signal, and buy the fuse adapter if there's any likelyhood of you doing so. You could ask the manufacturer what the specifications of 013-0345-xx are, and how it improves the survivability of the generator.

Always read the data sheet carefully for details of the reverse driving specification. Some instruments can be particularly fragile, some can be surprisingly tough (like my signal generators). You can't tell by looking at the instrument, or at the price tag. You won't know by experiment, until it's too late (my favourite phrase from a high energy forum I use is 'turn it up until it catches fire, then back off a bit'). Your AWG says nothing about survivability, except to recommend an extra protection circuit if you're likely to reverse drive it. So don't.

  • 1
    \$\begingroup\$ The outputs appear to be only DC coupled. If they could be AC coupled, then you would expect it to say so, and to give a maximum voltage across the capacitor. They don't, so they're not. Do not apply any external signal to this AWG, at best it could upset the display or level setting, at worst it could damage the output. If you want to bias an AC signal, then build or buy an external bias-Tee, which is a coupling capacitor, and a bias resistor or inductor. The effects of keying up a transmitter into it could cover the same range, at best wrong readings, at worst a big repair bill. \$\endgroup\$
    – Neil_UK
    Mar 24, 2020 at 9:57
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    \$\begingroup\$ Normally the AWG sends signals out. 'Reverse drive' means trying to send a signal from an external source to the AWG. Only connect passive loads to the AWG, an amplifier input for instance, or a 'scope input, or an RLC circuit. Do not connect things to the AWG output that you would expect to output power. So, don't connect a battery to the AWG output, or a power supply, or another signal generator, or another channel of the same AWG, or a charged capacitor, these could reverse drive the output. I doubt that numericals would clarify these examples, as I mean any battery, any PSU. \$\endgroup\$
    – Neil_UK
    Mar 24, 2020 at 10:43
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    \$\begingroup\$ Ideally, connect the FG to a 50 ohm load, but high impedance will also work. If you connect 12 V across the FG output, then 12/50 = 240 mA will flow, almost certainly damaging your equipment. If the 50 ohm output impedance comes from a passive attenuator, then it will get about 3 Watts dumped in it and probably overheat. If it's a synthetic 50 ohms from an amplifier, then it might survive, it might not, it depends how it's been designed. As the datasheet doesn't say it will, and advises an external protection box if you're likely to do this, then the 'probably won't survive' is more likely. \$\endgroup\$
    – Neil_UK
    Mar 24, 2020 at 10:57
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    \$\begingroup\$ You do realise you can program the FG to provide an AC signal together with a DC offset (generated internally), don't you? \$\endgroup\$
    – Neil_UK
    Mar 24, 2020 at 10:59
  • 1
    \$\begingroup\$ How do I say 240mA will damage the FG? I don't say that, I say that it will almost certainly damage it. It's well above the DC output that the FG is rated to produce, and it's well above the level that the FG is rated to withstand, which is zero. They don't just avoid giving a figure it will handle, they positively say that if you are going to reverse drive it, you should use an external protection unit and even give you its part number! But, you have the equipment in front of you, do the experiment. Let me know whether smoke comes out, I'd be intrigued. \$\endgroup\$
    – Neil_UK
    Mar 24, 2020 at 12:34

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