Versatile input termination / translation for reference clock?

Question

I've got a microcontroller I want to clock from an external 10 MHz reference. I'd like the reference input to be as versatile as possible. At the moment, my reference is an LVCMOS square wave, but other folks have various levels of sine wave references.

I'm going to use a 74LVC1G17 to condition the input, fed with 3.3 volts (the microcontroller runs at 5v, but I've already tested that it will accept a 3.3v clock).

The remaining question is how best to terminate and/or condition the input, and what range of input signals it will accept (assuming regular, stable, 10 MHz, 50% duty cycle input).

I've googled around and learned about AC parallel termination and Thevenin termination, but in simulating them in circuitlab, I don't see the transitions lining up the way I'd expect.

In particular, a series cap feeding a voltage divider on the input doesn't give me a square wave centered on Vcc/2 like I'd expect.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

Transient simulators typically start all capacitors with 0 V across them. That will put the buffer input initially at 0 V. You will need to run the simulation for a few times the time constant (100 nF * 50 ohms, or about 5 us) to get C1 charged up and put the buffer input voltage due to V2 near the steady state value.