Allison,
I'm afraid if you tested the way you described in your original post on this thread, you weren't testing for crosstalk.? I believe what you measured wasn't cross-talk at all, but rather ground bounce due to current starvation of the Si5351 output buffers.
I've attached the Si5351 datasheet.? On page 5, Silicon Labs gives the following specs:
Vddox, output buffer voltage => 3 ranges, 1.8V +/- 0.9V (5%), 2.5V +/- 0.25V (10%), and 3.3V +/- 0.3V (9.09%).? ?The Raduino uses a 3.3V supply for Vddox.
Iddox -? output buffer supply current per output => 2.2mA typical, 5.6mA max
Zo, output impedence @ 3.3V => 50 ohms
Note this is output impedence, which is very misleading.? The spec is not saying to use an output load of 50 ohms (Rload).? It is saying we need to drive thru a 50-ohm impedence to reach the destination Rload.? So we need to calculate the mimimum output load (Rload) that the output driver can drive.
Rload = Vddox/Iddox = 3.3 / 0.0056 = 589.3 ohms.? This is the minimum load the output can drive, as it gives the maximum allowed output supply current.? Exceed this spec and you enter current starvation on the power supply to the output buffer.
You stated that you used both a 50-ohm load (not impedence) and a 25-ohm load (not impedence) for your cross-talk testing.
With a 50-ohm load, Iddox = V/R = 3.3 / 50 = 66mA, which is more than 10x the maximum allowed output buffer supply current.
With a 25-ohm load, you subjected the part to 132mA Iddox!?
In both cases, the output buffers are so power starved that the chip internally can't supply enough Vddox to control the output buffers.? The result is ground bounce on all the outputs, not cross-talk.? This is typical behavior for digital chip output drivers whose output specs are violated in such a manner.? It also explains why you didn't see as much signal on the Si5351 outputs as you expected.?
The Zo spec was meant to say that the chip was designed to drive thru a 50-ohm impedence (either a 50-ohm coax or a 50-ohm micro-strip line on a PCB) to its load (Rload), not that you could use a 50-ohm or 25-ohm termination as the Rload.? An example of this appears on page 22 of the Si5351 datasheet.? Here the datasheet describes using the part to drive 2 clock outputs with 180 degree phase difference, but the example circuit shows driving the Si5341 clock outputs thru a 50-ohm coax to a voltage divider for the receiving circuit that presents an Rload of 511 ohms + 240 ohms = 751 ohms to each? Si5351 output buffer.? This gives an Iddox = 3.3 / 751 = 4.4mA, well within the 5.6mA max Iddox spec.
I suggest you repeat your cross-talk tests using an appropriate Rload value at the end of a 50-ohm line for each clock output and see how much cross-talk you see.
73,
Carl,? K0MWC
