# Can you generate a pure sine wave by mixing a fixed frequency sine wave with a variable frequency square wave?

Let us suppose I don't have a DDS available, and instead I have two square wave clocks with frequencies of my choosing. I have the impression that creating a wide bandwidth (multi-decade) sine wave could from this signal could be done two ways. In the first way, you use only one square wave clock, and you low pass filter through a switched filter bank, with a cutoffs positioned to deal with the odd harmonics (and I suppose whatever even harmonics are generated by parastics). Alternatively, it seems like you could fix the frequency of one square wave, use a crystal filter to get a pure tone, and then mix it down by driving a mixer using the other square wave as a swept frequency LO. As long as the square wave is of high enough frequency, you only deal with harmonics of the LO, which can be trivially filtered. I also understand that a properly designed mixer will suppress harmonics of the output signal... Marki's mixer have excent nlo+nrf figure. This second strategy would seem much easier as it doesn't require a switched filter bank. Have I understood this all correctly?

• The third harmonic of one frequency (fixed) will mix down with the third harmonic of the variable frequency and this is the problem you face. – Andy aka Jan 3 at 8:13
• I've specified that the fixed frequency has been put through a crystal filter to try and reject the third harmonic. We're supposing the fixed frequency source is monochromatic. – Evan Jan 3 at 8:15
• Well, that's not as easy as you think and anyway, harmonics of the square wave can still mix down to the wanted base band area. – Andy aka Jan 3 at 8:17
• You can output “any” waveshape of the fixed f signal using S/H mixer with this method spanning many decades from DC using clock edge . What range? – Sunnyskyguy EE75 Jan 4 at 12:58