si5351 and pulsing noise

Discussion in 'Homebrew and Kit Projects' started by AJ4SN, Sep 12, 2021.

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  1. AJ4SN

    AJ4SN Ham Member QRZ Page

    I am building a receiver around an si5351 VFO controlled by an Arduino nano. I am using the code written by Julio Cesar that I have modified to meet my needs. see link:

    I am testing the VFO in breadboard fashion. I have it connected to my current receiver (also homebrew) which uses a level 3, TAK3H mixer. I placed an amplifier between the si5351 and the receiver, but I did not include a filter. It drives the mixer well and the receiver works fine except for an annoying pulsing noise which repeats at about once per second. When I check the interconnections among the devices on the breadboard with a scope, the noise seems to match the frequency of the data passing from the controller to the si5351 and the display along the SDA and SCL lines. That is to say that the noise seems to occur at the same time the data updates.

    I wonder if anyone has had experience with this problem using the si5351 and an Arduino controller.

    Thanks for reading my post.

  2. KB1CKT

    KB1CKT Ham Member QRZ Page

    Interesting. I only just started in on something like this--same code, but you are ahead of me, so... sorry, no help here. But wildly curious about the fix, as I plan to do similar in the future.

    Can you post a pic of what you have? I'm a bit concerned that maybe there is some sort of ground loop issue here. When you say it's from the SI5351 being updated, I wonder if it's either a ground loop, or perhaps a noise pulse inside of the '5351 when it is updated... not sure. Maybe it's noise on the +5V line, not enough bypass there?
  3. AD5GH

    AD5GH XML Subscriber QRZ Page


    I haven't looked at the code, but I assume the Si5351 would only be updated when there was a frequency change. I would start troubleshooting by making changes to the SW to initialize the VFO to a fixed frequency and then disable the display updates, and see if the pulsing noise persists. This way you can rule the I2C bus in or out.

    If the I2C bus is the source, then would need to look at the VFO, VFO amplifier, and receiver to determine where the pickup is occurring. Per the previous post, a pic of the breadboard and schematic would be helpful.

    I have not experienced this problem with I2C bus in my homebrew equipment.


  4. AJ4SN

    AJ4SN Ham Member QRZ Page

    Shawn, Rod,

    Thanks so much for your replies. I did try operating the amplifier from a separate power supply, but the noise persists. Rod, glad to know that you did not have this trouble with the I2C bus in your homebrew equipment. That is encouraging. I've attached a picture of the bread boarded set up. Clearly, there is room for improvement in layout. At this point, I just wanted to see if it all worked ok. I added code to control a sotabeams DSP board, the AGC, and the receiver attenuator. I will try to eliminate all the extra code that I added and see if that helps.


    Attached Files:

  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    Some things do not work well on a Breadboard.

    Good Luck on your project.
    KV4PD likes this.
  6. KB1CKT

    KB1CKT Ham Member QRZ Page

    It looks like the RF from the SI5351 is run using a wire to the amplifier(?). I would use a bit of coax; lacking that, twist a ground wire around it. I'm a bit concerned about pickup here and/or distortion. It's working, clearly, but this seems less than optimal. I'm not sure where this amp is getting GND from? through the standoffs?

    But I do think it may just be noise on the supply line. Or on GND? I'm not sure how the copper board is being tied to the various modules, or if it is even being used for carrying GND currents for the various signals.

    If it's noise during display updates, then I'm inclined to think it's noise from that update--which doesn't make sense to me, the display shouldn't pulse lots of current on update I'd think. Is this using one of those I2C to parallel adapters? I'm wondering if it's blinking the backlight during updates. Backlight on/off is one of the bits on the PCA8574 (PCF8574?) and maybe the code is setting that bit only on the last byte going out? or just flipping it at some point.
  7. KK4DAS

    KK4DAS XML Subscriber QRZ Page

    Hi ,

    Good work on attempting the receiver build!

    You are almost certainly picking up noise because of all the unshielded untwisted wires running around. Every one of those wires is an antenna that can pick up noise. All of your RF lines should be coax as someone else suggested or a minimum twisted pair with ground connected at both ends. Also, you don't have a very clear common ground. Make sure all of the boards are exactly the same ground potential. What I do with the signal lines from the front panel to the Arduino and from the Arduino to the display and anything else is make sure that all lines are twisted pairs. As someone else mentioned, you will also pick up stray noise when using breadboards the way you are. And what has become my standard controller board, I solder the SI 5351 module to the same board that has the Arduino, and make the connections directly on the board.

    The basic configuration that you are trying to achieve will work. There are a great many successful receiver and transceiver projects using an Arduino, SI5351, and LCD display. For one such project, check out my blog,

    We have about 16 of these on the air as of this week.

    Good luck and keep us posted on your progress!

  8. AJ4SN

    AJ4SN Ham Member QRZ Page


    Thanks so much. I'll get it cleaned up and report back! I enjoyed your blogspot. Despite what some will say, I think that this is the golden era for hombrew!

    KA9JLM likes this.
  9. KB1CKT

    KB1CKT Ham Member QRZ Page

    No kidding. So many have gone before us, and are now documenting it online. Many of the QRP guys are trying to keep costs in check (go figure) and now we're seeing low cost transistor finals (BS170's etc) and other options now too.
  10. N0ZGO

    N0ZGO Ham Member QRZ Page

    For a few dollars, you can have a real RF power transistor. See here:

    (assuming anyone has stock, which is the current problem..)

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