# Do function generators produce AC signals in the sense we know?

When I examine function generators, there are a wide range of products. I see products between 7 USD and 600 USD.

If they can do the same job, why is there so much difference in price? I can't make sense of it. I don't think these cheap products can actually produce a sine wave with a negative voltage portion. Maybe they are producing PWM like Arduino (no negative side). For example, in the video below, do it yourself function generator.

I take the function generator to use in diode tests. When I give an AC voltage without disconnecting the diode on the circuit, I will decide whether the diode is solid or not with the oscilloscope.

This is the function generator that I think can generate the AC signal that I intend to buy. I think there are a few good brands such as RIGOL, UNI-T

• I don't think these cheap products can actually produce a sine wave with a negative voltage portion. Why not? A negative voltage is just a voltage that's lower than another voltage. If I choose the correct reference (different ground) I can get a negative voltage easily. I can make a such a reference (virtual ground) using a couple of resistors and an opamp. Only caveat is that the output ground will no longer be same net as power supply ground. Mar 11 '20 at 8:33
• I'm too lazy to go through the video and find the schematic of the DIY function gen. Just include the schematic here. With AC coupling at the output it is also easy to make negative voltages. Audio power amplifiers (to drive a speaker) have been doing so for decades. Mar 11 '20 at 8:36
• Your question is more suitable for EEVblog site. I have Siglent SDG1032x (almost same price as linked, except it is 150 MS/s)and SDS DSO, when connected together they can do Bode plot. You have to ask yourself for which purpose you will use it. There are also chinaware FY-6600, but nobody will buy it second hand, if one day you want to sell it. Mar 11 '20 at 8:52

I bought the ultra-cheap function generator last year. It is based on an XR2206 IC clone. It is barely usable. The sine and triangle waves have DC superimposed, you need to add your own cap to remove it (or a bias circuit). the square wave amplitude and duty cycle is fixed. One of the knobs is backwards. The harmonic distortion on the sine is horrible, about 5%. To keep it cheap, they took a lot of shortcuts, they didn't even include the symmetry trim (most users won't be able to set it properly anyway).

I thought that I could get by with it, but I couldn't, I just bought a used HP33120A on ebay.

What do you get for more money:

• Better sine waves.
• Adjustable DC offset.
• Adjustable square wave amplitude.
• Lower and higher frequency capability (cheap one is 1 Hz to 1 Mhz).
• Digital display.
• Digital parameter control.
• Proper 50 ohm source impedance (cheap one has about 600 ohms).
• Sawtooth wave.
• AM modulation.
• FM modulation.
• Frequency sweep capability.
• Burst capability.
• Arbitrary waveform capability.
• Remote programming.

I am usually pretty good at work-arounds and making due with what I have, but the cheap version is below my threshold. Depending on your needs, it might be usable for you.

• What you get for $600 as an employee: not wasting$1000 of time trying to get it to work right. What you get for $600 as a hobbyist: not wasting$0 of time trying to get it to work right. Mar 11 '20 at 16:55
• @Mattman944 - sorry but make do, not make due. Too short for an edit. :-) Mar 11 '20 at 19:40
• By "better sine waves", can we assume that you mean harmonic-free? Mar 11 '20 at 19:49
• @MikeWaters - yes, better sine waves = less harmonic distortion. The HP spec is 0.04%, 100 times better. Mar 11 '20 at 21:29

An AC signal is a signal that Alternates, which means it changes direction over time.

Even the cheap DIY "function generators" can generate such a signal (if I apply that signal to a capacitor, the capacitor will charge and discharge so the current changes direction) so yes, that's AC.

Indeed some might not be able to produce a negative voltage and can only output the AC signal superimposed on a DC signal. You could use a capacitor to block that DC voltage. You could use a DC voltage source as a reference such that it compensates for the DC voltage.

Making a "proper" balanced output signal with no DC present requires a slightly more complex circuit. For a DIY function generator having that DC voltage present might actually not be an issue.

• I would have thought that an AC signal is one whose polarity or direction alternates over time. i.e. It switches from positive to negative. Ripple, for example, is considered AC with a DC offset. Mar 11 '20 at 8:56
• @Transistor I think you're correct as the signal would need to be Alternating and that means change direction. I'll improve my answer. Mar 11 '20 at 9:03

Function generators differ in price due to the following factors:
- Frequency range
- Voltage range
- Accuracy
- The amount of power it can deliver (this might be the most important for you). Normally a function generator is used through an amplifier which then powers the load.
- Whether they can create arbitrary wave forms etc.

I can't make sense. I don't think these cheap products can actually produce a sine wave with a negative voltage portion.

Why not? Split voltage supplies and amplifiers are trivial.

• I bought one of these, with no modifications, the outputs have DC superimposed. Mar 11 '20 at 13:30
• Another feature: resistance to damage from accidental incorrect usage. Mar 11 '20 at 19:35