Step one
- The sketch
Step Two
- Test Amp
  - Grabber results at 1W
  - Grabber Results at 200mW
Stretch Goals
Further Information
Discussion

QRSS MEPT

I started work on my small QRSS MEPT idea with a simple Arduino / SI5351 / 16×2 LCD arrangement. The LCD is only there because I found this breadboard setup already with Seeeduino Nano, SI5351 and LCD in my box of “Arduino Project bits” - from a previous experiment - and I thought I'd keep it for the time being as a display to help in debugging (?). Once the MEPT is functional and ready for operation I'll remove the LCD (and its code from the sketch)

Step one

modify an old sketch & strip out un-needed code until I had a simple carrier generator
add function to vary the frequency in a stepped-ramp sequence to check operation
add functions to make dits and dahs and spaces of appropriate lengths
add function to send a callsign/message explicitly entered as a series of calls to dah(), dit() and space() and run it from loop()
add code to light an onboard LED during “key-down”
add code to show operating frequency and the letters of the callsign as they're sent on the LCD

I monitored the SI5351 output on my normal 30m grabber - there's enough radiation direct from the SI5351 board to make a trace on the waterfall.

The sketch

Click to read / hide

#include <rgb_lcd.h>
#include <si5351.h>
 
char version[] = "v0.1";
 
 
 
volatile long shift_hz = 0;
int count = 0;
int dit_time = 6000;
 
volatile long freq = 10139780;  //      
 
int digit0 = 0;
int digit1 = 0;
int digit2 = 0;
int digit3 = 0;
int digit4 = 0;   
int digit5 = 0;
int digit6 = 0;
int digit7 = 0;
 
 
 
// Instantiate the Objects
rgb_lcd lcd;
Si5351 si5351;
 
void setup()
{
 
  // Initialize the display
  lcd.begin(16, 2);
 
  lcd.cursor();
 
  // Initialize the Si5351
  si5351.init(SI5351_CRYSTAL_LOAD_8PF, 0, 0);
  si5351.set_correction(-37500, SI5351_PLL_INPUT_XO);
  si5351.set_pll(SI5351_PLL_FIXED, SI5351_PLLA);
  si5351.drive_strength(SI5351_CLK0, SI5351_DRIVE_8MA);
 
  pinMode(13, OUTPUT);
  // Update display and send start frequency
 
    SendFrequency();    
    digit0 = (freq%10);
    digit1 = ((freq/10)%10);
    digit2 = ((freq/100)%10);
    digit3 = ((freq/1000)%10);   
    digit4 = ((freq/10000)%10);
    digit5 = ((freq/100000)%10);
    digit6 = ((freq/1000000)%10);
    digit7 = ((freq/10000000)%10);
 
    UpdateDisplay();
 
}
 
 
void loop()
{
 
// SendRamp();
SendCall();
// SendRamp();
 
}
 
 
void SendCall() {
 
  space();
 
  dah();
  dah();
  dit();
  lcd.setCursor(5,1);
  lcd.print("G");
  space();
 
 
  dah();
  dah();
  lcd.setCursor(6,1);
  lcd.print("M");
  space();
 
 
  dit();
  dit();
  dit();
  dit();
  dah();
  lcd.setCursor(7,1);
  lcd.print("4");
  space();
 
  dit();
  dit();
  dit();
  lcd.setCursor(8,1);
  lcd.print("S");
  space();
 
  dit();
  dah();
  dit();
  dit();
  lcd.setCursor(9,1);
  lcd.print("L");
  space();
 
  dit();
  dit();
  dit();
  dah();
  lcd.setCursor(10,1);
  lcd.print("V");
  space();
 
  space();
 
 
}
 
 
void space() {
  shift_hz = 0;
  SendFrequency();
  delay(dit_time * 2);
}
 
void dah() {
  shift_hz = 4;
  digitalWrite(13, HIGH);
  SendFrequency();
  delay(dit_time * 3);
  shift_hz = 0;
  SendFrequency();
  digitalWrite(13, LOW);
  delay(dit_time);
 
}
 
void dit() {
  shift_hz = 4;
  digitalWrite(13, HIGH);
  SendFrequency();
  delay(dit_time);
  shift_hz = 0;
  SendFrequency();
  digitalWrite(13, LOW);
  delay(dit_time);
 
}
 
void SendRamp() {
 
 shift_hz = 0;
 while ( shift_hz < 6 ) {
  digitalWrite(13, HIGH);
 SendFrequency();
   shift_hz++;
   delay(2000);
   SendFrequency();
 }
   shift_hz = 0;
   SendFrequency(); 
   delay (dit_time * 2);
 
   digitalWrite(13, LOW);
}
 
void UpdateDisplay()
{
 
 
 
  lcd.setCursor(0,0);
  lcd.print(digit7);
  lcd.setCursor(1,0);
  lcd.print(digit6);
  lcd.setCursor(2,0);
  lcd.print(",");
  lcd.setCursor(3,0);
  lcd.print(digit5);
  lcd.setCursor(4,0);
  lcd.print(digit4);
  lcd.setCursor(5,0);
  lcd.print(digit3);
  lcd.setCursor(6,0);
  lcd.print(".");
  lcd.setCursor(7,0);  
  lcd.print(digit2);
  lcd.setCursor(8,0);
  lcd.print(digit1);
  lcd.setCursor(9,0);
  lcd.print(digit0);
  lcd.setCursor(10,0);
  lcd.print("Hz");
 
  lcd.setCursor(0,1);
 
  lcd.print("QRSS");
 
  lcd.setCursor(12,1);
  lcd.print(version);
 
}
 
void SendFrequency()
{
 
  si5351.set_freq(((freq + shift_hz) * 100ULL), SI5351_CLK0);     // VFO
 
}

Step Two

The next step will be to build a small TX PA to boost the SI5351 output up to around 1W and a LPF. I'll add a PTT line to key the PA, driven from the Arduino so that the sketch can have periods of carrier off between cycles if necessary. 1W should be enough power and should be simple enough to generate. I might have a go at a Class-E MOSFET amplifier.

Test Amp

I found a 30m PA strip from a previous project that produces about 6W from < 10mV drive, with 3 class-A bipolar driver stages feeding an IRF510 MOSFET - originally intended as a linear amplifier.

I've played with the supply volts and the MOSFET bias and can comfortably get up to 3W carrier with 8V supply. It's not terribly efficient but at QRP levels that's hardly relevant.

I've got it connected to the Ardiuno/Si5351 and it's on the air running 1W.

I'll use this amp as a blueprint for a new PA for the MEPT, perhaps with fewer stages as I don't need so much gain or linearity.

I've seen it on grabbers around Europe

Grabber results at 1W

At 1533z I reduced power to around 200mW

Grabber Results at 200mW

Stretch Goals

Add a function to generate “Slow FSK Hell” to make waterfall-readable text - see the image of GM4GHK below
Add DFCW
Add a “normal-speed CW ident” for cases where it might make sense (?)

Further Information

Page created : 12/07/26 07:40 BST

Page updated : 01/01/70 01:00 BST

radio, qrss, arduino, mept

Table of Contents