Wednesday, January 26, 2011

GAGA-1: Flight Computer radio's first transmission

Work is progressing on the flight computer with the first RTTY transmission. In the picture you can see the computer (the Arduino) in the foreground, connected with test leads to a small breadboard containing a voltage divider made from three resistors which then connects to the radio itself. The radio is connected to the ground plane antenna in the capsule.

The radio module is a Radiometrix NTX2 which transmits using FSK on 434.650Mhz. Its TX pin takes a voltage between 0 and 3V and converts that into a frequency shift of 0 to 5kHz. For the RTTY transmission I'll be using a shift of 425Hz between the mark and space frequencies.

That shift corresponds to a difference in voltage on the TX pin of 0.255V. The configuration I'm using is based on picoatlas where a pair of pins from the Arduino are used to power a voltage divider. The set up is as follows:
Arduino                         Radio
7 ---27K--------------\
                       ---------- TX
8 ---22K----2K2-------/
The pins 7, 8 and are switched so that one gives 5V and the other 0V. Thus the TX will be fed 2.36V or 2.64V depending on which is high and which is low. That's a difference of 0.27V. The TX pin has an input impedance of 100K and in the real measured circuit the voltages at the TX pin were 2.01V and 2.25V for a 0.24V shift. That should correspond to a shift of 400Hz. I'll measure that later.

The test code is as follows:
// These are the digital pins used to control the Radiometrix NTX2
// module.  TX0/TX1 are for the two tones used for RTTY and EN is
// connected to the Enable pin to enable or disable the module

#define EN  6
#define TX0 8
#define TX1 7

// These are the pins used to read/write two the internal and
// external temperature sensors

#define TEMP_INT
#define TEMP_EXT

void setup()
{
  // Set up the pins used to control the radio module and switch
  // it on
  
  pinMode( EN,  OUTPUT );
  pinMode( TX0, OUTPUT );
  pinMode( TX1, OUTPUT );
  digitalWrite( EN, HIGH );
  
}

void loop()
{
    char test[255];
    sprintf( test, "GAGA-1 High Altitude Balloon Test Transmission on 434.650 @ 10mW" );
    rtty_send(test);
    
    delay(5000);
}

// ---------------------------------------------------------------------------------
// RTTY Code
//
// Code to send strings via RTTY.  The RTTY parameters are defined by constants
// below.
// ---------------------------------------------------------------------------------

// The number of bits per character (7), number of start bits (1), number of stop bits (2)
// and the baud rate.

#define ASCII 7 
#define START 1
#define STOP 2
#define BAUD 50
#define INTER_BIT_DELAY (1000/BAUD)

// rtty_send: sends a null-terminated string via radio to the ground trackers
void rtty_send( char * s ) // The null-terminated string to transmit
{
  char c;
  while ( c = *s++ ) {
    int i;
    for ( i = 0; i < START; ++i ) {
      rtty_bit(0);
    }
    
    int b;   
    for ( i = 0, b = 1; i < ASCII; ++i, b *= 2 ) {
      rtty_bit(c&b);
    }
    
    for ( i = 0; i < STOP; ++i ) {
      rtty_bit(1);
    }
  }
 
  // Note that when idling RTTY specifies that it be in the 'mark' state (or 1).  This
  // is achieved by the stop bits that were sent at the end of the last character. 
}

// rtty_bit: sends a single bit via RTTY
void rtty_bit(int b) // Send 0 if b is 0, 1 if otherwise
{
  digitalWrite(TX0,(b>0)?HIGH:LOW);
  digitalWrite(TX1,(b>0)?LOW:HIGH);
  delay(INTER_BIT_DELAY);
}
Thanks to everyone on #highaltitude for tips and encouragement.

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