I'm working on a PCB for a reference clock that will connect to a UFL connector, but I'm facing some challenges with impedance matching. Most TCXOs I've found, like the X1G0054210216, have an output impedance around 10kΩ || 10pF. I know I need a buffer, but I haven't been able to find one that can handle such a high impedance. Any tips or suggestions would be super helpful!
3 Answers
For driving transmission lines, you should look into line drivers—they're designed for this purpose! Specifically, video line drivers could be a solid option for your needs. You generally want the input impedance of the driver to be high so it doesn't load down your TCXO. If you're feeling adventurous, you can also set up an operational amplifier with an emitter follower to get your output impedance down to 50 ohms. Just be sure your components can handle the rise and fall times you’re aiming for!
That output impedance you mentioned is likely a recommended load rather than the real output impedance. It should be fine for routing short distances on a PCB. If you're considering a buffer, keep in mind it might introduce some phase noise. In the past, I've had good success using LVDS receivers to buffer TCXOs, especially those with clipped sine outputs. Just make sure to pick one without built-in termination if you're going for a 50-ohm match.
I previously used an OnSemi FIN1101 with a 4.7k resistor and a 1nF capacitor for this type of setup. The LVDS outputs worked great, but you might want to find a buffer with a single-ended output for driving your 50-ohm line.
If you want a basic solution, why not try a vintage crystal oscillator? I found an old 10 MHz ovenized oscillator on eBay and it runs perfectly for test setups. Just throw it in a metal box with a 15V supply and you're good to go! Plus, it gives off a solid +16 dBm of RF power, which is a nice bonus!
Do line drivers exist for frequencies above 1GHz, though?