Si5351A Breakout Board Update

I’ve had a good response to the Si5351A Breakout Board when it was posted on Hackaday last month. There have even been a few folks who went through the trouble of ordering PCBs from OSHPark so that they could build their own copies of the board for experimentation. One of them, Tom AK2B, even constructed a complete receiver using the Si5351A Breakout Board and the RF Toolkit modules from Check out the link to the nice-sounding audio in the embedded tweet below.

When the link to the Breakout Board was posted on Hackaday, I wasn’t even sure that anyone would be interested, so the design was not as robust as it should have been for public use. But thanks to some suggestions from Tomas OK4BX and some of my own ideas, I’ve created a Rev B Breakout Board that has a number of improvements.

Si5351A Breakout Board Rev B
Si5351A Breakout Board Rev B

I increased the size of the board by 10 mm on the short side in order to accommodate some new circuitry. I could have kept the board the same size and put the new components on the back side of the board, but I thought it would be better to keep everything on the front. Thanks to Tomas’ suggestion, I added simple MOSFET I2C level conversion so that the Si5351A can be properly interfaced with a 5 V microcontroller. I also added a 3.3 V LDO regulator and jumper blocks so that the I2C interface voltage and the 3.3 V source can be selected. The traces from the Si5351A to the output transformers were also screened with vias, which improved crosstalk between outputs by about -6 dB. I also increased the pad size for the SMT crystal in order to make it easier to hand solder. In addition, I added a provision for the crystal footprint to double as a footprint for a TCXO. So far, the crystal works fine, but I haven’t ordered the TCXO yet in order to verify that it works as well, but I don’t think there will be any problems as long as the crystal is working.

As I anticipated from a previous post, Adafruit has released their own version of a Si5351A breakout board. It looks like they use the same I2C level conversion scheme as my board, but that is where the similarity ends. The Adafruit board seems to be geared to using it strictly as a clock generator, where the Etherkit board is designed to be used in RF applications by providing output isolation via broadband transformers and screening of the output traces. The Etherkit board also has more flexible options for using the board in 5 V or 3.3 V environments.

You can order the new board from OSHPark here, and find the documentation for it on GitHub.

I need to do a bit more testing to ensure that everything is working as it needs to, but so far the preliminary tests look great. Assuming that everything with the new board checks out, there’s a decent possibility that I will kitting at least one batch of these boards for sale. Stay Tuned.

14 thoughts on “Si5351A Breakout Board Update

  1. Hi Jason. I’m thinking of using your breakout board with an RTL-SDR dongle. I’m just wondering if there’s an easy way to convert the square wave output to a sawtooth one, since that seems to be the wave form of the clock used by the R820T based dongles. I remember to have seen such a conversion circuit in the past but I’m not sure if it is something doable at 28.8 MHz … I admit to be almost a newbie…. 🙂 Do you have some suggestion or could you point me to a schematic or to some knowledge resource?

  2. Mario,

    I think it’s tricky to turn a square wave into a sawtooth wave, and I can’t think of a method to do it off of the top of my head. It’s easy enough to get a triangle wave from a square wave using a high-speed op amp integrator, but the sawtooth is another beast all together.

    What I don’t understand is why you need a sawtooth wave when substituting an oscillator for the stock crystal on the dongle. The crystal is going to make a waveform more akin to a square wave or sine wave than a sawtooth. If a sawtooth is needed, I would think it would have to be generated after the 28.8 MHz oscillator. I wonder if the people making measurements aren’t introducing some kind of loading which is giving them a strange looking measurement.

  3. i agree. I was just unsure about that topic (I’m not an expert) : I will try with square wave and see the tuner behavior…Thanks !

  4. Hi Jason,
    I tried to use a ECS-250 crystal with your new board and it caused the 3.3v line to short. It took me a while to figure out that the new board was designed for the FoX924B. I ended up cutting the traces around the pad that connected to 3.3V and jumpering the 3.3V to pin one of the Si5351A. The opposite sides of the ECS-250 that are not the crystal connectors are shorted to each other.
    Tom, ak2b

  5. Thanks for the report Tom!

    I used the ECS-205 on the Rev B board that I built and it worked fine, but I did not solder the unused pins to the board, so I wonder if that makes the difference. I’ll look into this further when I get some more parts on hand!


  6. Hi Jason. I was looking at your design as I need a clock source for a project I’m working on. I had a couple of questions about the board…
    1. How did you determine the dimensions for the 50 ohm traces on the board? When I use the coplanar waveguide with ground plane calculator, your dimensions don’t give me 50 ohms. The results I get with your dimensions is about 88 ohms.
    2. Your stitching on the ground plane next to the traces uses 6 mil drill size. OSHPark specifies a minimum drill size of 13 mils. Are they actually ok with 6 mils?

  7. Hi Will,

    1. In the KiCad transmission line tool, if you select Coplanar Waveguide with Ground Plane, select an H of 1.6 mm and a trace width of 1 mm, it should come out around 51 ohms. I had some experienced microwave folks double-check me on this, so I’m fairly certain it is correct, but let me know if you still get a conflicting figure.
    2. That’s probably an oops on my part. I’m guessing they just used the smallest drill size they had, as I received no report of a design violation when they accepted my Gerbers.

  8. Thanks for the response, Jason. I was looking at the pcb layout file for your rev B board. It has a trace width of 0.20 mm. But I see you now have a rev C with a wider trace. 1 mm does give me 51 ohms.

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