# How can I multiply square wave circuits?

I would like to generate a signal that consists of a 1 kHz signal, which switches to a 100 kHz signal, flipping from one to the other at a rate of 10-100 Hz. I have access to signal generators to produce each of the three input signals.

How can I combine (mulitply?) the three signals to produce the desired product signal?

• What, like logic gates?
– user76844
Aug 20, 2017 at 23:01
• Generate the two signals, feed them into two 2 logic gates like AND or OR and drive those gates with a signal toggling the outputs in inverse of each other. Aug 20, 2017 at 23:15
• You are asking for a frequency-shift-keying (FSK) signal. Many signal generators are designed to directly produce such a signal. Often a generator with a frequency-modulation (FM) input can also do this job. Your 10-100 Hz signal would be used as the FM input and the generator would be adjusted to generate the 2 frequencies in response to that signal. By the way your output signal is not a product since that would produce a much different signal; instead it is, as I pointed out, an FSK signal which is switching between 2 frequencies based on the polarity of a third signal. Aug 20, 2017 at 23:36

Your picture makes it look like that you do not care for any signal edge synchronization at all. If that us indeed the case then the following circuit consisting of a single quad NAND gate shall do the trick for you.

If you do care about edge synchronization and generation of no runt pulses at the time of the frequency shifts than much more work must be done. That could take on the form of a circuit that contains a batch of additional flip-flops or a generator circuit that uses a common source clock and dividers to generate desired frequencies shown.

Technically, you are not multiplying. You are multiplexing (switching) between two signals.

Logical multiply would be an XOR (Exclusive-OR) function. Multiplexing (or MUX for short) is basically the schematic below. A low on the control pin (V1 signal) picks input 0 and a high on the control pin picks input 1. The output follows which ever input is selected.

You can use @Michael Karas MUX circuit or you can use an IC specifically designed as a MUX such as 74HC153, 74HC4052 and many more.

Same disclaimer as Michael's circuit. There is no synchronization so you can end up with narrow little pulses when you switch frequencies.

simulate this circuit – Schematic created using CircuitLab