Introduction: Si5351C Auxiliary Board for GPSDO 10 Mhz

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About: An old time engineer (who learned about tubes at school) and who after a long time come back to be an hobby constructor :-)

I made GPSDO, a precise 10MHz oscillator designed by Yanick. (https://www.instructables.com/GPSDO-YT-10-Mhz-Lcd-2x16-With-LED/) I made some changes in the control loop and the output. I did some circuit test, I tested different crystals and I'm still working on the perfect final version.

This board I actually made to tests IC Si5351C for potential future using. Using this simple board plus Arduino Uno allows you to set output frequency to any frequency in range 4kHz to 225MHz.

And now the best: Output stability is given by GPSDO stability!

By other words you can simply made "frequency reference" of any frequency you need. I tested it and it really works! Yes, it is just testing board and maybe I could use some output buffer but output signal even now is "nice looking" and board is usable asi it is. This is why I published it.

Schematic is simple. It is just Si5351C with required low-noise power supply such LP5907 (Changes 5V from Arduino to needed 3.3V). Blocking capacitors on the back side of PCB are needed for Si5351C correct work. Then there are 3 pieces of "Logic level shifts" needed to correct communication. More about it on: https://www.digikey.com/en/blog/logic-level-shifting-basics.

Board allows you to set two independent output frequencies. Third output frequency have to be an integer multiple of one of the inner PPLs frequency.

Supplies

All components I used is possible to buy on DigyKey site

1x Si5351C

3x BSS138

6x R 10K (size 0805)

6x C 10000PF (size 0805, I recommend type C0G ), P/N GRM2165C2A103JA01D

2x C 1uF

1x C 100 to 470uF - for example P/N APXJ160ARA271MF80J

1x Linear reg LP5907MFX-3.3, P/N296-38557-1-ND

1 x crystal 27MHz (or 25MHz), P/N1908-q270-jxs11-8-10/10 (or P/N 1908-q250-jxs11-8-10/10) / Not necessary when external clock only will be used.

2 or4 SMA connectors

Notes:

C1(1uF) and C2 (10nF) are 1206 size but not problem to solder 0805 size to its place as I also did. Of course C1 range is 1uF to 22uF. 100nF instead of 10nF will also work as blocking capacitors.

Board thickness 1.6mm due to used SMA connectors. (I personally used 1mm :-) )

Step 1: Assembling

  • For using Eagle and possibility to edit project or just for easier work, download attached files. Then rename file "SI5351C_testboardV1.1.sch.brd" to "SI5351C_testboardV1.1.sch" (Not possible upload .sch file here)
  • Build the PCB according to the schematic. I ordered the PCB from https://jlcpcb.com/. It took about a week for them to arrive. (I have 1 more if anyone would like. - without components.)

Note:

It is not easy to solder small Si5351C and crystal. See size comparison picture. Do not try if you are not enough experienced!

Attachments

Step 2: Frequency Setting

Connect Arduino UNO and Si5351C board. 5V to 5V, GND to GND, SDA to SDA and SCL to SCL.

Run attached file "si5351C_example.ino" on Arduino board!

Important note: All commands are in "void setup()" loop as code is written for tests only!

Set wanted frequency in line 50: si5351.set_freq(1400000000ULL, SI5351_CLK0);

where "1400000000ULL" means 14MHz

More about library and used commands: https://github.com/etherkit/Si5351Arduino



Note:

It is possible to use Arduino, display (as ST7735) and one rotary encoder for frequency setting (as on picture) but it requires more hardware and it's a bit beyond the scope of this simple project.

Oscilloscope was not connected to 10MHz reference. Only signal shape, not displayed frequency is important here.

Attachments