How to amplify a square wave clock signal received from 50 ohm coax?

Question

I want to use the signal from a GPS disciplined oscillator (GPSDO) as a clock signal for a microcontroller.

The GPSDO outputs a 10 MHz square wave between 0 V and 3.3 V designed to drive into a 50 Ω characteristic impedance.

This is delivered to my circuit over a short (500 mm or so) length of coaxial cable. My circuit terminates it with 50 Ω to GND (which also reduces the peak voltage of the square wave to about 1.5 V).

I'm using a Schottky diode to clamp the signal (through a 2200 Ω resistor) to the 3.3 V rail to protect a microcontroller from being powered by the square wave when the 3.3 V rail is turned off.

As a result of this, the final clock signal is reduced to a slightly curved triangular wave between 1.3 V and 1.78 V (in other words, a 480 mV pk-pk with a +1.55 V DC offset). The waveform is very clean and appears free from any distortions that might make it unsuitable as a clock signal. However the amplitude is very small and it is currently unbuffered.

^{simulate this circuit – Schematic created using CircuitLab}

What could I use to amplify this signal into the microcontroller? The microcontroller expects a 0 - 3.3 V signal, but I think it's fairly tolerant of anything better than 2 V pk-pk centred on 1.65 V.

So far I have tried to use an NPN BJT transistor (2N2222) as a simple voltage amplifier but I couldn't get that to work. I wondered about using something like an LM741 op-amp but the 1.5 MHz bandwidth is insufficient.

Answer 1

I think the problem here is that you haven't heard of series terminations applied at the source end of the clock signal. If you put a 50 Ω series resistor in-line with a signal output just before it feeds the coax then, the output from the other end of the coax will be 3.3 volts peak and no buffering is needed to put that right.

However, by the sound of it, the GPS output already has this internal output impedance built in so, no need to add anything; just feed the GPS output onto the coax and you should see a clean 3.3 volt clock signal at the far end. You can easily check this of course.

Then if you want to protect the MCU input from surges, you should be able to look at the data sheet of your device and add a series current limiting resistor to your input.

AN-903 A Comparison of Differential Termination Techniques <-- read the section on series termination on page 4.