Minimal WSPR transmit with Arduino and Si5351

Intrigued by the ZachTek pocket WSPR transmitter Peter, VK3YE, has reviewed, I wondered if I could get a simple beacon going using just an Arduino, an Si5351 clock generator and (you'll be pleased to hear) a low pass filter.

Here's the rig. The low pass filter is from the VK3YE Beach 40 design which I like because it uses off the shelf inductors.

The waveform isn't perfect but it's certainly a long way from the square wave the comes out of the clock generator.

I've taken this sketch, ripped out the lovely GPS time synchronisation, and turned it into a 1 shot beacon. You power on, or press reset, it transmits one message and then turns off RF. Of course you need to push the button at the beginning of a two minute cycle.

Here's how it decodes locally on my receiver with no antenna connected.

You can see that big signal drift earlier where I powered up with the antenna disconnected to let it get up to temperature. In the actual slot it reports a drift of -1 and decodes nicely (well... twice).

Hooked it up to the antenna just now and the little Arduino/sSi5351 transmitter is being heard quite widely.

The software uses some very handy modules:

• JTEncode from etherkit
• Si5351 from etherkit

Both very easy to use. Here's my hack of the code, I'm afraid Blogger doesn't like parts of it like angle brackets so no doubt there will be some things to fix up.

/*
 * Minimal WSPR beacon using Si5351Arduino library
 *
 * Based on code:
 * Copyright (C) 2015 - 2016 Jason Milldrum 
 * 
 *
 * 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 .
 */
 // https://gist.github.com/NT7S/2b5555aa28622c1b3fcbc4d7c74ad926

#include "si5351.h"
#include "Wire.h"
#include <JTEncode.h>
#include <int.h>

#include "Wire.h"

#define TONE_SPACING            146           // ~1.46 Hz
#define WSPR_CTC                10672         // CTC value for WSPR
#define SYMBOL_COUNT            WSPR_SYMBOL_COUNT
#define CORRECTION              0             // Change this for your ref osc

#define TIME_HEADER             "T"           // Header tag for serial time sync message
#define TIME_REQUEST            7             // ASCII bell character requests a time sync message 

#define TX_LED_PIN              13
//#define SYNC_LED_PIN            13

Si5351 si5351;

JTEncode jtencode;
unsigned long freq = 7040500UL;                // Change this
char call[7] = "VK2TPM";                        // Change this
char loc[5] = "QF56";                           // Change this
uint8_t dbm = 10;
uint8_t tx_buffer[SYMBOL_COUNT];

// Global variables used in ISRs
volatile bool proceed = false;

// Timer interrupt vector.  This toggles the variable we use to gate
// each column of output to ensure accurate timing.  Called whenever
// Timer1 hits the count set below in setup().
ISR(TIMER1_COMPA_vect)
{
    proceed = true;
    // Serial.println("timer fired");
}

// Loop through the string, transmitting one character at a time.
void encode()
{
    uint8_t i;
  Serial.println("encode()");
    jtencode.wspr_encode(call, loc, dbm, tx_buffer);

    // Reset the tone to 0 and turn on the output
    si5351.set_clock_pwr(SI5351_CLK0, 1);
    digitalWrite(TX_LED_PIN, HIGH);

    // Now do the rest of the message
    for(i = 0; i < SYMBOL_COUNT; i++)
    {
      uint64_t frequency = (freq * 100) + (tx_buffer[i] * TONE_SPACING);
        si5351.set_freq(frequency, SI5351_CLK0);
        Serial.print("freq = ");
        Serial.println(tx_buffer[i]);
        proceed = false;
        while(!proceed);
    }
    Serial.println("message done");
    // Turn off the output
    si5351.set_clock_pwr(SI5351_CLK0, 0);
    digitalWrite(TX_LED_PIN, LOW);
}

void setup()
{
   Serial.begin(9600);
   Serial.println("setup");
  // Use the Arduino's on-board LED as a keying indicator.
  pinMode(TX_LED_PIN, OUTPUT);

  digitalWrite(TX_LED_PIN, LOW);
  Serial.begin(9600);

  // Set time sync provider
  //setSyncProvider(requestSync);  //set function to call when sync required

  // Initialize the Si5351
  // Change the 2nd parameter in init if using a ref osc other
  // than 25 MHz
  si5351.init(SI5351_CRYSTAL_LOAD_8PF, 0, CORRECTION);

  // Set CLK0 output
  si5351.set_freq(freq * 100, SI5351_CLK0);
  si5351.drive_strength(SI5351_CLK0, SI5351_DRIVE_8MA); // Set for max power
  si5351.set_clock_pwr(SI5351_CLK0, 0); // Disable the clock initially

  // Set up Timer1 for interrupts every symbol period.
  noInterrupts();          // Turn off interrupts.
  TCCR1A = 0;              // Set entire TCCR1A register to 0; disconnects
                           //   interrupt output pins, sets normal waveform
                           //   mode.  We're just using Timer1 as a counter.
  TCNT1  = 0;              // Initialize counter value to 0.
  TCCR1B = (1 << CS12) |   // Set CS12 and CS10 bit to set prescale
    (1 << CS10) |          //   to /1024
    (1 << WGM12);          //   turn on CTC
                           //   which gives, 64 us ticks
  TIMSK1 = (1 << OCIE1A);  // Enable timer compare interrupt.
  OCR1A = WSPR_CTC;       // Set up interrupt trigger count;
  interrupts();            // Re-enable interrupts.

  encode(); // transmit once and stop
}


void loop()
{
   // blink LED when we've finished the transmit
  digitalWrite(TX_LED_PIN, HIGH);
  delay(500);
  digitalWrite(TX_LED_PIN, LOW);
  delay(500);
}