Homebrew SDR HF Transceiver - the Si5351

The Si5351 is "...an I2C configurable clock generator that is ideally suited for replacing crystals, crystal oscillators, VCXOs..." That is to say that it can generate frequencies from 2.5 kHz up to 200 MHz. Adafruit Industries produce a breakout board with the Si5351 chip and other required components installed for under $8, this allows us to interface with a microcontroller, e.g. Arduino, and control the frequencies produced very easily. The breakout board allows us to generate three frequencies but for this project we will only be using one of the outputs. We will produce a frequency that is four times the LO frequency we require, when the signal goes through the quadrature divider (74AC74 flip-flop) it will be divided by four to produce the LO frequency.

Adafruit Si5351 breakout board on the left, Arduino Uno on the right.

The connections are simple: +5V and GND from the 7805 regulator, pin A4 from the Arduino Uno connected to pin SDA on the breakout board and pin A5 connected to pin SCL. 

I have hacked the example program below, it sets the Si5351 to a default frequency of 28.4 MHz, i.e. 7.1 MHz after the divide by 4, and by using the serial monitor window in the Arduino IDE you can change frequency in 1 kHz steps up (1) or down (2), or 100 Hz steps up (+) or down (-). This frequency is output on CLK0 - the next step is to connect CLK0 to the 74AC74 flip flop.

You will need to install the Si5351 library by following the instructions here.

/*
 * si5351example.ino - Simple example of using Si5351Arduino library
 *
 * Copyright (C) 2015 Jason Milldrum <milldrum@gmail.com>
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */

#include "si5351.h"
#include "Wire.h"
Si5351 si5351;
char incoming_char;
long serial_input_number, serial_input_number1;
void setup()
{
  // Start serial and initialize the Si5351
  Serial.begin(57600);
  si5351.init(SI5351_CRYSTAL_LOAD_8PF, 0);
  // Set CLK0 to output 7.1 MHz with a fixed PLL frequency
  si5351.set_pll(SI5351_PLL_FIXED, SI5351_PLLA);
  si5351.set_freq(2840000000ULL, SI5351_PLL_FIXED, SI5351_CLK0);
  serial_input_number1 = 7100000;
}
void loop()
{
 if(Serial.available()>0){
    incoming_char = Serial.read();
    switch(incoming_char){
    case '1': //up by 1kHz
      serial_input_number1 = serial_input_number1 + 1000;
      serial_input_number = serial_input_number1 * 4;
      si5351.set_freq((serial_input_number) * 100ULL, SI5351_PLL_FIXED, SI5351_CLK0);
      Serial.println(serial_input_number1);
      break;
    case '2': //down by 1kHz
      serial_input_number1 = serial_input_number1 - 1000;
      serial_input_number = serial_input_number1 * 4;
      si5351.set_freq((serial_input_number) * 100ULL, SI5351_PLL_FIXED, SI5351_CLK0);
      Serial.println(serial_input_number1);
      break;
     case '+': //up by 100Hz
      serial_input_number1 = serial_input_number1 + 100;
      serial_input_number = serial_input_number1 * 4;
      si5351.set_freq((serial_input_number) * 100ULL, SI5351_PLL_FIXED, SI5351_CLK0);
      Serial.println(serial_input_number1);
      break;
     case '-': //down by 100Hz
      serial_input_number1 = serial_input_number1 - 100;
      serial_input_number = serial_input_number1 * 4;
      si5351.set_freq((serial_input_number) * 100ULL, SI5351_PLL_FIXED, SI5351_CLK0);
      Serial.println(serial_input_number1);
      break;
    }
    Serial.flush();   
  }
}

A 7805 regulator circuit used to convert ~12V to 5V is here.

When you run the Arduino sketch you should be able to hear the output from the Si5351 on an HF receiver at 28.4 MHz

Next post: Quadrature Divider