Si5351-based Clock Synthesizer board

By: Bertrand Zauhar, VE2ZAZ
Last updated: 20/11/2019

On this page, I describe my design of a multiple output RF clock synthesizer board based on the Silicon Labs Si5351 chip. This integrated circuit provides a cheap and easy way of producing up to six independent clock outputs with frequencies ranging from a few KHz to more than 200 MHz. This board supports both the A and C version Si5351 chips, the latter being more valuable as it allows to synchronize to an external reference. If soldering a fine-pitch leadless QFN package C version chip is not possible, the builder can instead equip the board with an off-the-shelf Chinese-made Si5351A mini-module, and use the board in free-running mode.

his board includes an external reference scaling/shaping circuit, which provides the proper amplitude to the Si5351C, regardless of the shape or amplitude of the input signal. Since the Si5351 chip configuration is volatile (configuration lost when power is removed), an accompanying micro-controller is required to re-program the chip at power up. I chose an Arduino Nano to perform this task. In order to create the desired Si5351 configuration for the Arduino, I also developed Python language computer scripts (programs) (simple-mode and manual-mode versions) that provide the ability to define the parameters and perform the transfer. These can run on Linux, Windows and Mac OS.

To understand this board's capabilities, the reader should become knowledgeable of Si5351 chip features, possibilities and limitations. This web page assumes so, and does not expand on those.


The best way to understand what this project offers is by first consulting the Circuit Schematic of the board below (click on image to enlarge):

The Chinese Off-the-Shelf Si5351A mini-module

The Arduino Nano v3 board
The circuit is rather straightforward. The main features of this printed circuit board are:
  • Supports:
    either a 20-pin Si5351C chip (QFN-20 leadless package), with the benefit of  external reference (for example, 10MHz) support through a back plate SMA connenctor, 
     - or An off-the-shelf Chinese-made Si5351A mini-module, which will rely on its own 25 MHz crystal for output accuracy and stability.
  • Provides 3 square wave clock outputs via face plate SMA connectors, and three additional on-PCB clock outputs,
  • Provides an external reference input scaling/shaping circuit, ensuring proper reference amplitude to the Si5351C, regardless of the shape or amplitude of the input signal.
  • Uses an Arduino Nano board version 3 (original or clone).
  • Has two face plate LEDs, one showing power, the other showing the internal PLL status.
  • Includes 0805-size component footprints to build attenuators or filters on both the reference input and the three RF outputs,
  • Runs off the USB +5V Supply,
  • Has built-in +3.3V regulation for the Si5351 and the 65LVDS34 supplies.
  • Provides +5V <-> +3.3V bidirectional I2C signal level conversion between Arduino and Si5351.
  • Designed to fit into a 80x50x20 mm extruded aluminum enclosure, readily available from eBay, AliExpress or Banggood for a few dollars. Both the face plate and the back plate need a few holes to be drilled/filed. A template drawing is provided.


The Printed Circuit Board is a double-sided design with plated-through holes, solder resist and silkscreen (marking) on both sides. I provide the design files, and the gerber and drill files for anyone who would like to replicate it. Such PCB can be ordered for around 20$ per lot of 10 PCBs. Simply provide the gerber files and the drill file to the manufacturer. I ordered from JLCPCB, but other manufacturers should produce the same quality. In your order, select 0.062" (1,6 mm) thickness FR-4 glass-epoxy material, the standard stuff.


The following is a list of all required components and sub-modules to put together this project. Note that a number of components are required
only when the Si5351C option is used. Components that are required in both Si5351C and Si5351A options are shown in bold font.

Some procurement recommendations are provided in the right hand side column. Other than the details provided, part selection is not critical.

Quantity Designator Designation Package Procurement Recommendation
2 C3,C5 Aluminium Electrolytic Capacitor, 100 uF, 10V, 20% tolerance
SMD 5.3x5.3mm Digikey PCE3867CT-ND
10 C1,C2,C4,C6,C7,C8,
C9,C10,C11, C12
Capacitor 100 nF, 25V, Ceramic X7R type SMD 0805 eBay
2 D1,D2 LED, 3mm, green Through-hole 3.0mm diam. Digikey 516-1293-ND or eBay
4 J3,J10,J11,J12 SMA Connector, right angle Through hole jack, right angle eBay
1 J4 Arduino Nano Version 3 Through-hole DIP-30 (Unused pins cut off) eBay
1 J9 Si5351A mini-module (use only when not populating the Si5351C chip) Through-hole 7 pin header, 0.100 pitch eBay
2 Q1,Q2 N-Channel MOSFET (BS170 or 2N7000 typical) SMD SOT-23 Digikey BS170-ND
1 R2 51 Ohms Resistor SMD 0805 eBay
2 R1,R3 470 Ohms Resistor SMD 0805 eBay
4 R4,R5,R6,R7 1K Resistor SMD 0805 eBay
1 R8 4.7K Resistor SMD 0805 eBay
1 R9 220 Ohms Resistor SMD 0805 eBay
1 U1 LM1117-3.3 +3.3V Regulator, SMD SOT-223-3 Digikey
1 X1 SN65LVDS34 SMD SOIC-8 Digikey 296-39187-1-ND
1 X2 SI5351C SMD QFN-20 Digikey 336-5160-1-ND
1 Y1 Crystal, 25.000 MHz, 10ppm tolerance, 15ppm stability, 10pF load capacitance, 50 Ohm ESR SMD Crystal 3.2x2.5mm size Digikey 535-13468-1-ND
24 A1-A24 Optional for Filter or Attenuator design SMD 0805


Early on, the builder must decide whether an Si5351C chip or the Si5351A mini-module will be used. If the mini-module is selected, a number of components can be omitted. The components that are required in all situations are shown in bold font in the component table above.
One major challenge in assembling this board is
the Si5351C soldering, which requires a hot-air rework station, some magnifying apparatus (camera or microscope), some soldering flux and a lot of skill. This is one VERY SMALL package! If you have never soldered this kind of device (QFN leadless), I recommend that you document yourself on the Internet prior to performing the work, or find someone who is willing to help with that task. This is an expensive chip, and you do not want to spoil it!

The remaining of the board is not particularly difficult to assemble by hand if you can handle common surface-mounted components. A magnifying lamp, a fine tip soldering iron and a pair of tweezers are required.

 The lowest profile components should be installed first, starting with the Si-5351C chip, followed by the

The Arduino Nano pins to keep. Other pins should be removed.
remaining surface-mounted components. The electrolytic capacitors, the Arduino, the Si5351A mini-module (if required), the LEDs and the SMA connectors should be installed last. All through-hole pins should be trimmed down to the solder blob with the bottom PCB surface, otherwise there will be insertion or short circuit issues with the enclosure.

Prior to installing the Arduino Nano, its unused pins must be removed (or cut off flush) from the two rows of pins. Only the pins shown on the circuit schematic should be kept (see figure to the right).

When preparing the
Si5351A mini-module for installation on the board, do not solder the three SMA connectors on the mini-module. Output signals get extracted through the 7-pin header and are sent to the board's three SMA connector instead.

The suggested enclosure is an 80 x 50 x20 mm extruded aluminum enclosure. The PCB dimensions were set for an exact fit into this enclosure. This two-sided clam shell enclosure can be easily found on most electronic component sites (
eBay, AliExpress, Banggood...) for a few dollars. I provide a template drawing to help drill and cut the various holes on the front and rear faceplates.

The recommended enclosure



The Arduino firmware (a.k.a. the sketch) is written to be compiled in the Arduino IDE environment. The Arduino sketch uses Jason Mildrum's Etherkit Si5351Arduino library ( That library must be installed in the Arduino IDE via the Library Manager (Menu: Sketch -> Include Library -> Manage Libraries...) prior to compiling the sketch.

The Arduino must be sent an Si5351 configuration from the software described below, otherwise the synthesizer board will not operate.


This software includes comprehensive help files (About.html and About_Raw.html) that describe the prerequisites, the installation process and the operation in detail. Please refer to the software package content to get these files.

The software allows to configure the Si5351A/B/C Synthesizer chip when supervised by the Arduino Nano. Once the Arduino has received a configuration from this software, it will re-load the Si5351 chip with that same configuration at every power up or reset. The Arduino (properly configured by this software) is required as the Si5351 chip does not retain its configuration when power is removed; it must be re-configured at power up.

The software is written in Python 2.7 programming language. Thus it is fully compatible with the Linux and Windows (7, 8, 10) operating systems. Although it has not been tested, the software should also run properly on the Mac OS.

The software covers a superset of features offered by the Si5351 family of chips. The user must understand the limits imposed by the Si5351 model (A or C version), such as the number of channels available or the support for an external reference input. A good comprehension of the Silicon Labs' Si5351 Datasheet document is recommended.

The software is actually made of two python scrips. The "Si5351 Synthesizer Configuration" script was designed with the objective of exploiting the most common Si5351 features. Thus, it allows to configure the chip for most applications, but not all of them. Examples of features that are not implemented are VCXO support (the Si5351B chip), phase adjustment (other than simple inversion) and support for output channels 6 and 7. In an application where one or more of the missing features is required, the user can still use this hardware by programming the Arduino Nano using another python script, the "Si5351 Raw data Transfer" program. That Python script is also included in the package. In such case, various Si5351 registers will need to be programmed separately, which is more tedious.

Obviously, the accompanying Arduino
firmware (sketch) must be used in conjunction with this software, otherwise the latter will not function. The sketch is designed to interact with both versions of computer python scrips described here.

Access must be granted to the computer's virtual serial port associated with the Arduino's serial-USB adapter, otherwise an error message will be displayed.

The software for the synthesizer board can be obtained on GitHub by clicking on the link below.

Software on GitHub


One cool application of this Si5351 Clock Synthesizer is a QRSS (very slow transmission rate) low power Beacon transmitter. I have put together the proper software and Arduino firmware to accomplish this task. Please refer to this GitHub link for more information on implementing the beacon.

The output clock shape generated by the Si5351 chip is a square wave. One must include proper lowpass or bandpass filtering if the output is to be used in radio applications, especially if it is to be used as a low power transmitter connected directly to an antenna.