Arduino/Genuino DUAL UHF POWER METER

Shield NRVD, using two ADL5513 (Log Detect/Control.) boards

✈ Vout and Log Conformance vs. Input Power

The datasheet of the ADL5513 offers various graphs for output voltages vs. input power at different frequencies. From the graph on the left we can derive a formula to display the power in dBm as a function of the measured voltage.

P[dBm] = 50 * U[V] - 90


It even offers the possibility to set a voltage which compensates for a given frequency. From the table to the left, it can be seen, that a formula is not that trivial. We would rather suggest to use a look-up table, if this compensation feature is to be used over a wider frequency range.

Both drawings courtesy of Analog Devices.

✈ The Detection Circuit

✈ Downloads

✈ Test Sketch for Arduino/Genuino UNO

/* ///////////////////////////////////////////////////////////// 
ARDUINO/Genuino (UNO) DUAL UHF POWERMETER
BOARD "NRVD"
Software Version 1.0, 
13.10.2017, Alexander C. Frank
//////////////////////////////////////////////////////////////*/

const int beep = 6 ;
const int alarm = 7 ;
const int comp = A0 ;
const int SWITCH = 4 ;
const int data = A1 ;
const int clck = A2 ;
const int CS1 = 2 ;
const int CS2 = 3 ;
const float GAIN1 = 0.92893637 ;
const float GAIN2 = 0.93589236 ;
const float OFFSET1 = -2.40037 ;
const float OFFSET2 = -2.99111 ;

float Power1, Power2 ;

unsigned int TadjCompValue = 255 ;
float TadjCompVoltage = 1.0 ;

#include <Wire.h>
const byte MAX5381 = 0x64 ;
// MAX 5381 HAS A 4.0 V REFERENCE
// MARKING - ADDRESS
// ADMV   0x60
// ADNB   0x62
// ADNH   0x64
// ADMR   0x66

#include <LiquidCrystal.h>
LiquidCrystal lcd(13, 12, 8, 11, 9, 10);

void setup() 
{
  pinMode(SWITCH, OUTPUT);
  pinMode(data, INPUT_PULLUP);
  pinMode(clck, OUTPUT);
  pinMode(CS1, OUTPUT);
  pinMode(CS2, OUTPUT);
  pinMode(comp, INPUT);
  pinMode(beep, OUTPUT);
  pinMode(alarm, OUTPUT);
  digitalWrite(beep, HIGH);
  digitalWrite(alarm, HIGH);
  delay(200);
  digitalWrite(beep, LOW);
  digitalWrite(alarm, LOW);

  Serial.begin(9600);
  Wire.begin();
  
  lcd.begin(16, 2);
  lcd.setCursor(0, 0); lcd.print("NRVD DUAL UHF");
  lcd.setCursor(0, 1); lcd.print("POWER METER");

  // APPLY Tadj FACTOR

  Write_DC(0, TadjCompValue);
  Write_DC(1, TadjCompValue);
  TadjCompVoltage = float(TadjCompValue / 64.0);

  delay(2000);

  lcd.clear();

  
}

void loop() 
{
  digitalWrite(SWITCH, 0);
  Power2 = GAIN2 * Read_RF(0) + OFFSET2 ;
  lcd.setCursor(0, 1); lcd.print("CH2: ");
  if (abs(Power2) < 10) lcd.print(" ");
  if (Power2 > 0.0) lcd.print("+");
  lcd.print(Power2,2);
  lcd.print(" dBm ");
  Serial.print(Power1,2);Serial.print(",");
  delay(250);
  
  digitalWrite(SWITCH, 1);
  Power1 = GAIN1 * Read_RF(1) + OFFSET1;
  lcd.setCursor(0, 0); lcd.print("CH1: ");
  if (abs(Power1) < 10) lcd.print(" ");
  if (Power2 > 0.0) lcd.print("+");
  lcd.print(Power1,2);
  lcd.print(" dBm ");
  Serial.println(Power2,2);
  delay(250);
}



float Read_RF(unsigned int port)
{
  long RawData = 0x00000000 ;
  float power = -99.0 ;
  long a,b,c,d ;
  // CHIP-SELECT
  if (port == 0){digitalWrite(CS1, LOW); digitalWrite(CS2, HIGH);}
  if (port == 1){digitalWrite(CS2, LOW); digitalWrite(CS1, HIGH);}
  // CHECK IF DATA IS READY - DATA GOES LOW
  // YES WE KNOW, THIS IS DANGEROUS :-)
  while (digitalRead(data) == HIGH ) { delayMicroseconds(10); }
  digitalWrite(clck, LOW);
  a = shiftIn(data, clck, MSBFIRST);
  // Serial.print("a:");Serial.println(a,DEC);
  b = shiftIn(data, clck, MSBFIRST);
  // Serial.print("b:");Serial.println(b,DEC);
  c = shiftIn(data, clck, MSBFIRST);
  // Serial.print("c:");Serial.println(c,DEC);
  d = shiftIn(data, clck, MSBFIRST);
  // Serial.print("d:");Serial.println(d,DEC);
  RawData = (a << 24) | (b << 16) | (c << 8) | d ;
  // BITS 31-28 ARE NOT USED HERE
  RawData = RawData & 0x0FFFFFFF ;
  // BITS 3-0 ARE NOT USED HERE
  RawData = RawData >> 4 ;
  // WE USE A 2.5 V REFERENCE
  // WE USE ONLY 16 BITS
  RawData = RawData >> 8 ;
  // THEREFORE 0xFFFF == 2.5000 V
  power = ( 50 * 2.5000 * RawData / 65535.0 ) - 90.0 ;
  return power ;   
}


float Write_DC(unsigned int port, byte value)
{
  digitalWrite(SWITCH, port);
  Wire.beginTransmission(MAX5381);  
  Wire.write(value);               
  Wire.endTransmission();     
  // HINT: THE BOARD HAS A VOLTAGE DIVIDER
  // U = 560 / (1k6 + 560) = 0.259259
  // for a value of 255, this results in 
  // a voltage of 1.037 V
}

// ///////////////////////////////////////////////////////////// 
// END OF FILE.
// ///////////////////////////////////////////////////////////// 

✈ The Formulae

The Power in dBm as of the voltage reading is :

This formula is valid if you use the ADL5513 as shown here.

Converted to mW this is :

And with that, we can calculate the reflection coefficient:

And finally the SWR or the VSWR is:

E.g. with a Γ of 0.5 we would have a VSWR of ( 1 + 0.5 ) / ( 1 - 0.5 ) = 3 : 1
(Remember these three values .:. open : Γ = 1, short : Γ = -1 and loaded with 50 Ω : Γ = 0)


The Return Loss of a load is the reflection coefficient expressed in dB:

Note: You should always try to have a Return Loss better than 10 dB. Everything above 20 dB is considered "beautiful". And if you have more than 40 dB - we suggest you apply for a job at Hewlett Packard :-)

✈ Share your thoughts