Si5351A Update

I don't have a ton of new updates for you from the NT7S bench, but I wanted to give you a heads up on an error on the Si5351A Breakout Board Rev B and let you know about some exciting stuff brewing on the Si5351 scene.

When I created Rev B of the Si5351A Breakout Board, I attempted to create a PCB footprint that would allow dual-use of both the standard 25 MHz ECS reference oscillator crystal, but also a 25 MHz Fox TCXO in the same footprint. It was almost a good plan, but somehow I overlooked one detail. On the TCXO, VCC is applied to pin 4 and ground is pin 2. Unfortunately, on the ECS crystal, those two pins seem to be internally connected, but I missed that. So if you want to use the ECS crystal with the board, you can simply cut the trace providing +3.3V to pin 4 of the footprint, and your Si5351 will work fine. Otherwise, the power rail will be shorted to ground and you will have an unhappy board. I apologize for that oversight and I will work on getting a corrected PCB up on OSH Park in the near future.

In happier news, it seems to me that interest in the Si5351 is finally starting to really gain momentum. I've had a fair number of ham homebrewers contact with inquiries about the Si5351 and sharing their experiences with the IC so far. Although I don't subscribe, I've heard that there is quite a bit of chatter on Farhan's Minima mailing list about the possibility of substituting the current Si570 for a Si5351. Given that the Minima is meant to be a minimalist rig, I think this makes a lot of sense. There even some blog posts from others coming out now. For example, the always entertaining Shack Nasties blog by M0XPD now has some blog posts about initial experimentation with the Si5351 in a transceiver. I would put Paul's blog in your feed reader if I were you, even if you are only interested in RF experimentation in general.

This should be a good upcoming time for the cheap and cheerful little clock IC. I'm going to be experimenting with it in the next CC1. It should work great, with one output for the VFO, one for the BFO, and the last for the TX drive. As always, keep an eye on the blog and my Twitter account for further updates.

9 thoughts on “Si5351A Update”

  1. Jason
    I just fired up an AdaFruit Si5351 board with your (?) library code. 1st reaction is that there's a lot of low-level spurs about 60-65 db down and crosstalk from the other channels about 50 db down. Do you see this on your board?


  2. Hi Joe,

    I assume you are talking about the Etherkit code from GitHub, which is mine.

    In regard to the spurs, I didn't notice many close-in spurs on my spectrum analyzer (where I had at least 80 dB of dynamic range). In wide bandwidth mode I didn't see much either, but my dynamic range is probably only around 60 dB there, so it is possible that I didn't see what you saw. Can you tell me some fundamental and spur frequencies to look for? It's my opinion that this PLL IC has much better spurious performance than the common DDS ICs you see used in ham radios like the AD9834. I have encountered very few spurious responses in the receivers I have built using the Si5351.

    As for the crosstalk, that's in line with what I measured. On my Rev A board (which does not have shielded output traces), the crosstalk is about 40 dB. I feel that I did a fairly good job of using good RF layout practices in the Rev B board, so I suspect that the 50 dB crosstalk may just be the best you can get from the IC. The Adafruit board doesn't seem to use any shielding, so it's not bad that you are getting 50 dB. In practice, I haven't found it to be a problem so far. but perhaps you have a more stringent application for it. I feel like for about $3 total, the Si5351 and crystal make a heck of a oscillator system for a radio, even if it has some flaws.

    Thanks for the feedback!

  3. Jason
    Thanks for the reply. If you can contact me off-line, I'll send a spectrum plot and a piece of the dialog I had with Thomas, LA3PNA


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