I was wondering if it would be possible to increase the frequency of a signal by a fixed amount. Basically, I have a negative wire soldered to a speaker and a positive wire that very quickly breaks/unbreaks the circuit. As expected, this causes the speaker to pulse each time the positive wire breaks and unbreaks. However, the frequency is very low and I was wondering if it would be possible to have a circuit that "adds some frequency" to the current frequency so that I would get a continuous beeping instead of some pulsing with noticeable space in between. The circuit would have to be pretty simple since I'm making it myself. Is this feasible or should I just be happy with what I have?
Answer
A mixer might be what you're looking for.
A mixer multiplies two signals together. \$x_{out}(t) = x_1(t) \times x_2(t)\$ if \$x_1\$ and \$x_2\$ are two input signals. From the multiplicative property of the Fourier transform, this means that in frequency space, the output contains sum and difference frequencies of the input.
Say \$x_1(t)\$ is the signa produced by you moving your wire around. And \$x_2(t)\$ is a sine wave at some frequency \$f_0\$. Then if you put these two signals into a mixer, the output will be your \$x_1\$ signal shifted up in frequency by \$f_0\$. There is also a shifted version of the negative-frequency part of your input signal, but that might not be important or it might be what you actually want, depending what you're trying to do.
You can also see that if \$x_1(t)\$ is a square wave that's sometimes 1 V and sometimes 0 V, and you multiply it by a sine wave, you get out a pulsed sine wave, or a series of beeps, if \$f_0\$ is an audio frequency.
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