The RTL-SDR dongle is a real workhorse for radio hacking. However, the 28.8 MHz oscillator onboard isn’t as stable as you might wish. It is fine for a lot of applications and, considering the price, you shouldn’t complain. However, there are some cases where you need a more stable reference frequency.
[Craig] wanted a stable solution and immediately thought of a TCXO (Temperature Compensated “Xtal” Oscillator). The problem is, finding these at 28.8 MHz is difficult and, if you can find them, they are relatively expensive. He decided to make an alternate oscillator using an easier-to-find 19.2 MHz crystal.
How do you convert a 19.2 MHz signal to 28.8 MHz? First, you need a flip-flop to divide the output frequency by two. That gives you a 9.6 MHz square wave. That doesn’t seem much better until you think about what makes a square wave. The Fourier series tells you that a square wave is an infinite sum of sine waves. One sine wave is at the fundamental frequency and then other sine waves at each odd harmonic (that is, 3X, 5X, 7X, and so on). And 3 * 9.6 is 28.8, just what you need!
All [Craig] had to do was filter the output of the flip-flop to produce an accurate 28.8 MHz signal. He provides all the necessary data to duplicate his design, and you can see him put it through its paces in the video below.
The next step up from a TXCO is one with an oven (an OCXO). We’ve seen some homebrew OCXOs that ought to work with the same trick.