10 MHz is the quasi-standard for reference clock in measurement equipment. Most boxes have "REF in" and "REF out" or "10 MHz in"/"10 MHz out".

In my case, I have a measurement setup consisting of an FSW, SMW200A and SMF100A (all top notch boxes from Rohde & Schwarz) as well as a Tektronix signal generator.

What is the best way to distribute the 10 MHz reference clock along >2 devices?

  1. Daisy-chaining
  2. Use BNC Tees (for 3 devices one tee, for 4 devices 2)
  3. A combination: E.g. take the FSW as master, use a BNC tee to connect to the SMW and SMW via RefIn. Then take RefOut from the SMW and use it for the Tektronix signal generator
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    \$\begingroup\$ Unless you have LONG CABLES between the boxes, this likely does not matter. You should have these boxes on one power system, to remove 60Hz GND trash from modulating the 10MHz amplitude and thus modulating the zero-crossings. \$\endgroup\$ – analogsystemsrf Sep 6 '18 at 3:50

The 'Best Way (TM)' can vary with what you are trying to achieve. Here are some considerations. I'm not going to use the term 'daisychain', as it can mean different things to different people.

Most 10MHz I/O are designed with roughly 50 ohm output impedance and high input impedance. Most have enough sensitivity to work well with a terminated link driven by 0dBm. Most drive at least 0dBm.

This means point to point links between 2 instruments can be connected with impunity. The far end of the cable will receive clean transitions, whether terminated or not, whether driven with square or sine.

A single Tee'd connection is different however. If terminated, all points on the cable get clean transitions. If left unterminated and driven with squarewave, then only the far end sees nice switching. All other points along the cable see the voltage rise to 50% as the outward wave passes, and it dwells there waiting for the reflection from the high impedance far end to continue up to 100%. This midpoint voltage is the worst possible place to wait, in terms of noise and even possible mis-clocking. A Tee'd connection must be terminated at the far end, with all intermediate nodes high impedance. This is less important if you know you have sinewave drive, and will always use sinewave.

Other considerations. You might want to look at the specifications for the internal standards, and choose the best to be the master. Make sure it's got a separate IN and OUT connection, some devices have a single I/O port. The reason? If you decide later to use a higher quality external reference, then plug it in here, and you don't have to re-cable your system.

Having a Tee on the back of the instruments may make cable identification easier when you are crawling around trying to change connections.

You might find that some inputs are just not sensitive enough for good clocking from some outputs, especially if terminated. If you get problems with one configuration, then try another. Better still, measure the sensitivities and output levels, and actually avoid any dodgy links.

Whatever you do, write down how you've connected them, and note which are high Z and which are terminated inputs. It will save grief when you come to add a new box, or exchange it for one with different ref I/O provisions.

If you run point to point links, then it saves Tees, and it may save thinking about terminations (but see below). If any box is switched off, then everything downstream will (may) not work, which is a good failure! If you run a Tee'd connection, then the system may still work with an intermediate box being off or failed. This box may be degrading the standard without you noticing.

If you run point to point links, each box has the option of passing the input straight to the output, or buffering the reference signal. If it passes it straight on, then electrically it's a Tee, and you will need to terminate the far end for it to work properly. If it buffers the signal, it will add noise, which will usually be at a level irrelevant to your measurements. Should you come to investigate an anomalous system close to carrier noise floor, revisit your reference distribution arrangements to make sure that's not it.


For a 10 MHz signal the wavelength is 30 meter so when the cable length approaches that 30 meter you might run into issues if you just split the cable using a BNC-T.

If you stay far below that 30 meters then this is generally not an issue.

If there is the possibility of daisy-chaining then I would use that as the first choice. Do note that if one of the devices in the chain is off or not receiving mains power, the devices further up the chain might not receive the clock.

I would limit the amount of splitting the signal using tees to a minimum.

Sure you can use a combination of daisy-chaining and splitting.

From practical experience, if the number of devices is limited then it does not really matter much what you do. They are still able to use the clock signal whatever the way you choose to distribute the clock.


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