# Is it possible to convert a low frequency signal into a higher frequency signal?

I have a signal generator that has an adjustable frequency range that is below the required frequency that I need.

Is it possible to input a signal from my generator into a passive series RLC filter that will output the frequency that I want? I would like to do this using a low-pass filter.

• 40 KHz is not remotely challenging, you are better off creating it from scratch than trying to mix or multiply your existing generator to it. Commented Feb 28, 2016 at 5:55
• Wanting to use a LPF filter is like constraining a walk in the park by suggesting you are only allowed to do it blindfold and on your hands. Watch that busy road you have to cross. Commented Feb 28, 2016 at 10:12

If you have a square wave generator, then the output will be relatively rich in odd harmonics. The third harmonic for instance is only about 10dB down on the fundamental.

If you set the generator for 1/3rd of the output frequency you need, and connect it to a passive high pass filter to reject the fundamental and pass the 3rd harmonic, or better still a band pass filter tuned to the 3rd harmonic, you will get some output. A low pass filter will not do.

With good filters with low losses, you may approach an output voltage of 30% of the fundamental input voltage, which is what -10dB means.

Whether you can build good passive filters, which means appreciating the effect of source impedance and loading on its response, and designing for it properly, and whether that is enough output for you is another matter.

If this is an interesting exercise in 'can it be done', I'd say go for it for the lols. If this is a real attempt to build bench test gear that you don't have to worry about and nurse, then I'd say build the proper test gear from scratch, or buy it.

There is one consideration I have. If that '40kHz' means you want to drive an ultrasonic transducer, then you may have it fairly easy. The transducer itself is fairly 'bandpassy', and a small series capacitor may be all the high pass filter you need. Whack a 13.3kHz square wave into it through a small capacitor and see whether you get what you need.

Yes but with reduced effect; and no you aren't going to use a low pass filter to do this. (Refer to the Fourier expansion of a square wave.) A square wave can be decomposed into a sum of harmonic sinusoidal frequencies however The farther away from your starting frequency, the more attenuated they become. There is a formula for this and you can predict approximately how much attenuation to expect. Design the filter so that it doesn't attenuate the signal at your desired frequency, and is a harmonic frequency of the square wave and apply amplification as necessary. Don't expect to get anything if the harmonic frequency you desire is below the signal to noise ratio.

If you have a multiplier in the lab it would be a slightly better solution, what frequency are you looking for?

• Thanks for the reply! Is it possible to do this with just a resistor, a capacitor, and an inductor in some series? I would like a 40kHz frequency. I can input from 1Hz to 30kHz. Commented Feb 28, 2016 at 5:33