Etherkit, Homebrewing, Test and Measurement

Si5351A Investigations Part 8

In looking through the analytics here on the blog, I noticed a search term that has been regularly coming up near the top: Si5351 crosstalk. Realizing that I haven’t yet presented data on this, it seemed like a good time to knock this one out, since it isn’t that difficult of a measurement to make.

It appeared to be a wise idea to choose output frequencies that were non-harmonically related, so I decided on the following outputs:

  • CLK0: 22.444555 MHz
  • CLK1: 10.140123 MHz
  • CLK2: 57.456789 MHz

Each output was set to the maximum 8 mA current and each one was locked to PLLA, which was set at 900 MHz.

The measurement procedure was simple. I connected the spectrum analyzer to each output sequentially. The unused outputs were terminated in 50 Ω. For each measurement, I used a delta marker to measure the difference in amplitude between the desired signal from that output and the frequencies of the other two outputs.

Without further ado, allow me to present the spectrum analyzer plots.

Output port: CLK0 Crosstalk signal: CLK1
Output port: CLK0
Crosstalk signal: CLK1
Output port: CLK0 Crosstalk signal: CLK2
Output port: CLK0
Crosstalk signal: CLK2
Output port: CLK1 Crosstalk signal: CLK0
Output port: CLK1
Crosstalk signal: CLK0
Output port: CLK1 Crosstalk signal: CLK2
Output port: CLK1
Crosstalk signal: CLK2
Output port: CLK2 Crosstalk signal: CLK0
Output port: CLK2
Crosstalk signal: CLK0
Output port: CLK2 Crosstalk signal: CLK1
Output port: CLK2
Crosstalk signal: CLK1

I thought that perhaps these measurements would be a best-case scenario, and that leaving the unused output ports unterminated might produce even worse performance, but it turns out I was wrong. Below are the same measurements, but with an open circuit on the unused ports.

Output port: CLK0 Crosstalk signal: CLK1
Output port: CLK0
Crosstalk signal: CLK1
Output port: CLK0 Crosstalk signal: CLK2
Output port: CLK0
Crosstalk signal: CLK2
Output port: CLK2 Crosstalk signal: CLK0
Output port: CLK2
Crosstalk signal: CLK0
Output port: CLK1 Crosstalk signal: CLK2
Output port: CLK1
Crosstalk signal: CLK2
Output port: CLK2 Crosstalk signal: CLK0
Output port: CLK2
Crosstalk signal: CLK0
Output port: CLK2 Crosstalk signal: CLK1
Output port: CLK2
Crosstalk signal: CLK1

I’m not quite sure what to make of that. In practice, I haven’t seen any problems in my receivers so far that I can trace back to crosstalk from adjacent channels. Of course, this probably won’t do in a higher-performing receiver, but if you wanted to use the Si5351 in such a receiver perhaps you could find a way to put two or more on an I2C bus at the same time, then use only one output from each. My advice would be to turn off any channels you are not currently using, just to keep the other outputs clean.

I have no doubt that this data will be more ammunition for those who are convinced that the Si5351 is a terrible LO. I stand where I always have: this is an excellent IC for the price and you are hard pressed to find such capability and stability for such a low price anywhere else. If, knowing its limitations, the Si5351 meets your needs, then excellent! If not, that’s fine too. Neither I, nor anyone else I have heard, has suggested that the Si5351 is a panacea or a substitute for a better oscillator such as the Si570. It’s another tool to be put into our toolbox in the quest to stay relevant with the march of technology.

Quite a bit of work has been done in quantifying the performance of the Si5351 for amateur radio use, within the limitations of our modest home labs. Something that you don’t see done with a lot of other new components these days. Have I made mistakes or overlooked some things? Almost certainly. I’m still learning how to apply a strict measurement discipline over all of my serious building activities, so this is a learning process for me as well. If you have some constructive criticism of any of my measurements or feel that I have neglected things, I absolutely welcome an email or comment on the blog. Let’s try to hold ourselves to high standards as home experimenters without being unduly negative, as many of us continue in the journey of RF experimentation.