External clock input - what circuit for interface?

[max_torque]

My current precision counter project will have an internal 20Mhz oscillator, however i'd like to provide and "external clock input" so that the device could use 'any' external frequency source  (making it more useful for other stuff later)

So the question is, what interface circuitry do i need to provide for a universal external clock input?

What sort of voltages and signal levels are typical external clock sources? Sinusoidal, Square wave? AC or DC coupled? 50 ohm or high impedance?  What sort of connector (BMC socket?)

Anyone done this before, or can provide an example?

(the internal source is a +-1vrms sinewave, that i square up with a comparitor and use the rising edge as my reference trigger edge)

[t1d]

More information, please. A schematic, of what you have, so far, for sure. Are you using a controller chip? If so, the chip probably accommodates this situation.

[PTR_1275]

The references I have in the workshop are sinewave output, some are 2vpp others are 10vpp

I would suggest looking at the input specs for something like a Agilent 53132a counter to see what specs they say the clock needs to be. You might also get the clock input schematics in the service manual.

[DaJMasta]

The safe way is to assume nothing aside from not being continuous overload conditions on the input.

High impedance input (not every 10MHz reference drives 50 ohm well and this is standard)
Add a DC blocking cap where you need to in your design, probably better not to assume it
Diode clamping protection is nice, but not always present in gear I've seen and adds some capacitance
Put in a narrow bandpass filter around 10MHz (20, if that's what you're looking as an input, but 10MHz is the standard) and you'll always be getting something close to the right frequency and will always be getting a reasonable sine wave
BNC female (like the front of a scope) is standard


That all said, you can sort of get away with less.  This is the 10MHz reference input SMA connector (the cable goes to a BNC input on the back panel) inside a LeCroy WavePro 7200's timing board, and it's got a resistor divider, a DC blocking cap, and some diode clamping, then it's off to what I think is a PIN diode switch to go between the internal and external source.

[max_torque]

I'll use a BNC socket for the connection as that suits most things and space is not an issue

if i also wanted to avoid sharing ground level voltages, should i also AC couple the ground input?

I could use a suitable series resistance and clamp my input to say +- 2v pk-pk, ac couple it to a 2.5v mean level, and set my comparitor to trigger at say 1.75 (low) and 3.25 (high).

I guess i need to keep to a reasonably small amount of parasitic capacitance to avoid excessive distortion of the signal form, but perhaps i do need some form of pass filter to reject any significant out of frequency noise??

[David Hess]

If you can assume a roughly 50% duty cycle, then AC coupling before driving a ground referenced comparator in some form or another can accept practically any input level.  High overload protection can be provided by using a high input impedance buffer like oscilloscope vertical and trigger inputs do.

Low phase noise applications will require care but in many cases, the external clock input only serves as a low frequency reference for an internal phase locked oscillator.

Most interesting designs accept multiple input frequencies like the sampling based design Racal-Dana used in their frequency counters.

[awallin]

FWIW was just looking at an Ettus SDR schematic which has a software-settable reference: internal, external, gpsdo
first page here: https://files.ettus.com/schematics/b200/b210.pdf
if you need inspiration