# Can I double (or increase) the frequency of crystal oscillators by combining them in a circuit?

I'm thinking of a circuit which combines two waves of equal frequency (from two identical crystals), and outputs a wave with a higher (preferably doubled) frequency.

Can this be done? (Making 100MHz out of two 50MHz crytals.)

If so, is it possible to repeat this process (at least theoretically) to reach much higher frequencies?

• For exactly double you could set up a rising and falling edge detector. – Parker Feb 10 at 15:13
• A more straightforward approach is a single crystal and a PLL. – nanofarad Feb 10 at 15:18
• A mixer, comprised of a few diodes, will accomplish your goal. – Phil Freedenberg Feb 10 at 15:24
• You can double the output frequency of one crystal by feeding it into a nonlinear circuit and filtering it, no need for the second one. Or force it to oscillate at an odd integer overtone such as 5:1 from a 20MHz crystal by tuning the oscillator circuit. At least in theory. – Spehro Pefhany Feb 10 at 15:28
• @SpehroPefhany , Phil, nanofarad have all made legitimate suggestions. One more approach is an overtone oscillator where the crystal oscillator adds a resonator (usually LC) that preferentially boosts gain - often at 3X crystal's printed frequency. 5X, 7X is also possible, but trickier, since the crystal is usually less active at higher harmonics. Some crystals are manufactured to be most active at 3X their fundamental resonance. – glen_geek Feb 10 at 15:44

Yes. This has been done in the lab to improve the phase noise of reference oscillators, though it gets expensive in parallel hardware if iterated.

Start off with two independent VCXOs of the same frequency. Use a mixer with a DC-coupled output to combine their outputs. The sum frequency is the doubled output. The difference frequency, their phase difference, is used to phase lock them together. The output of the loop goes to increase the frequency of one, and decrease the frequency of the other. The combination can still be voltage controlled as a single entity, by using a tuning signal which increases the frequency of both.

When frequency doubling a 50 MHz oscillator to 100 MHz in the conventional way with a doubler or PLL+divider, the phase noise increases by 6 dB. When adding two 50 MHz oscillators together like this, the phase noise adds as power, so only increases by 3dB. That's only a 3dB improvement, for the expense of two oscillators and their control circuitry, but if you are grubbing around at the margins of what is possible, it's 3dB you won't get any other way.

You could combine four oscillators for 6dB improvement at 200 MHz, and 8 for 9dB at 400 MHz, though you now need to duplicate rafts of hardware for each further 3dB improvement.

It is possible to get the same effect without phase locking the two VCXOs together, if they are both phase locked to a common reference. Adjust the relative phases of the two sources at the mixer, perhaps by changing cable lengths, to maximise the sum output. This also brings the DC output to zero, which is perhaps a more convenient way to monitor their relative phase.

Of course if you are in a lab and can borrow a couple of signal generators, you can play with this effect. Make sure both are referenced to the same 10 MHz external reference, set them to the same frequency, mix them, and adjust their relative phase. This is rarely the best way to get a 2x frequency signal, or a 3dB phase noise better signal, but it's instructive to do it once.

• Is there a search term or specific name for this technique? I'm very curious about it and would love to read more/potentially apply it to some research work I am doing. – nanofarad Feb 10 at 17:59
• @nanofarad I don't know what it might be called, I've not read anything about it. When we did it in the lab it was just an obvious technique. What I've written there is all you need to know, all the rest is implementation details. Like I said, it's very rarely done. There's no point doing it until you're at the very margins of metrology, in which case, you already know what you're doing and then some. – Neil_UK Feb 11 at 6:57

Yes.

If the circuit is an RF mixer followed by the right bandpass filter, you can add or subtract the two frequencies.

For multiplying a single oscillator's frequency, there are other approaches such as PLLs.

What you are looking for is called "frequency doubler" schematic. Schematics can be found online or in the following books: "QRP Classics", "Solid State Design for the Radio Amateur". Both are quite old and are available on web.archive.org. To double the frequency only one oscillator is required. Yes, you can repeat the process several times to get x4 or x8 frequency.

If it's necessary to combine signals from two oscillators, use a mixer. A diode ring mixer is easy to build. The schematic can be found online or in the books named above.