# How can I create signal interference in a circuit? [closed]

I understand that this is counter intuitive but, as an experiment I want to create interference in a circuit.

I want to expose an AC sinusoidal signal to a second sinusoidal signal. This will cause some type of interference and produce a resultant (third), output voltage signal across a passive element (such as a resistor) as shown at the bottom of the image below. If I wanted to do something like this, what sort of circuit would I need to construct? I assume I would need to use two functions generators, how would they need to be set up?

• The word "expose" isn't specific. You could "sum" the amplitude voltage of two signals. Or you could "mix" them (amplitude modulation of one by the other) using a multiplier. And that's just two possible examples. Also, keep in mind that most function generators have their 50 Ohm shield connected to their safety Earth ground mains plug pin and that two such generators, both plugged into the mains, will have their 50 Ohm shields tied together. Just FYI. – jonk Aug 3 '20 at 19:04
• @jonk What do you mean by "will have their 50 Ohm shields tied together"? What does this mean practically? – user10764803 Aug 3 '20 at 19:06
• It means they aren't isolated differential outputs that can be treated any-old-way when connecting things up. And if you imagine they are freely connectable into a test arrangement, that could become a problem depending upon how you decide to combine them into a testing approach. I just wanted to call your attention to it so that you didn't make some gross mistake. – jonk Aug 3 '20 at 19:07
• For example, this would be a "bad way" to sum two outputs from two signal generators. – jonk Aug 3 '20 at 19:17
• What is your end goal here? Are you trying to learn something about how signal interference would behave in a real-world circuit that is subject to external noise? – Martin Stiko Aug 3 '20 at 23:08

You need to add these two voltage signals.

There's many ways of doing that, for example using an opamp in summing amplifier configuration.

I assume I would need to use two functions generators, how would they need to be set up?

Well, you don't need a function generator to generate an oscillation. Any oscillator does that, and there's hundreds of different oscillator circuits.

Realistically, you seem to be looking for a demonstration: Your sound card has two channels, can be instructed to produce arbitrary signals (aside from constant values, typically) up to some 20 kHz or so.

• and if you want to subtract .. Use a differential amplifier. – varun Aug 3 '20 at 16:18

assume I would need to use two functions generators

You would not need to go through the trouble of buying signal generators, rather construct your own oscillators as they are easy to make even on a breadboard, depending on what is your frequency of interest.

If you are looking for fixed frequencies for your signals, a phase shift oscillator would be good since it does not need gain control to generate a sine wave. If you have different frequencies, you can make a Colpitt oscillator and tune it with varactor didoes. You can find all the schematics you need with a quick google.

what sort of circuit would I need to construct?

What you described in your question is a mathematical operation of adding 2 voltages at every given time, pretty much how super-position works with waves. So for that you need a circuit to add your signals together. The easiest way, as Marcus mentioned is an analog adder with an op amp. One thing to make sure, however, is that the op amp or what ever component you will use has enough driving capacity to source/sync the amount of current you need without exceeding limitations (e.g. max output current, max power and ...).