Frequency meter, inaccurate at Teensy ++2.0

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BenJo

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Hello Board,

hope you can help!

I try to build up an frequency meter but I have problems to reach the needed accuracy.
Whats important to me:
-Frequency range from 50Hz to 500 Hz
-Accuracy in 0,1 Hz

Currently I have the following results with the Frequency Generator at F0:
Test 1:
The generator delivers 110 Hz to the teensy, the serial monitor shows 109,89 Hz and 110,21 Hz, but nothing in between.

Test 2:
The generator delivers 330 Hz to the teensy, the serial monitor shows 328,74 Hz and 331,57 Hz, but nothing in between.


Here is my current code:

//----------------------------------------------
boolean clipping = 0;

//data storage variables
byte newData = 0;
byte prevData = 0;
unsigned int time = 0;//keeps time and sends vales to store in timer[] occasionally
int timer[10];//storage for timing of events
int slope[10];//storage for slope of events
unsigned int totalTimer;//used to calculate period
unsigned int period;//storage for period of wave
byte index = 0;//current storage index
float frequency;//storage for frequency calculations
int maxSlope = 0;//used to calculate max slope as trigger point
int newSlope;//storage for incoming slope data

//variables for decided whether you have a match
byte noMatch = 0;//counts how many non-matches you've received to reset variables if it's been too long
byte slopeTol = 10;//slope tolerance- adjust this if you need
int timerTol = 3;//timer tolerance- adjust this if you need

//variables for amp detection
unsigned int ampTimer = 0;
byte maxAmp = 0;
byte checkMaxAmp;
byte ampThreshold = 30;//raise if you have a very noisy signal

void setup(){

Serial.begin(9600);



cli();//diable interrupts

//set up continuous sampling of analog pin 0 at 38.5kHz

//clear ADCSRA and ADCSRB registers
ADCSRA = 0;
ADCSRB = 0;

ADMUX |= (1 << REFS0); //set reference voltage
ADMUX |= (1 << ADLAR); //left align the ADC value- so we can read highest 8 bits from ADCH register only

ADCSRA |= (1 << ADPS2) | (1 << ADPS0); //set ADC clock with 32 prescaler- 16mHz/32=500kHz
ADCSRA |= (1 << ADATE); //enabble auto trigger
ADCSRA |= (1 << ADIE); //enable interrupts when measurement complete
ADCSRA |= (1 << ADEN); //enable ADC
ADCSRA |= (1 << ADSC); //start ADC measurements

sei();//enable interrupts
}

ISR(ADC_vect) {//when new ADC value ready

PORTB &= B11101111;//set pin 12 low
prevData = newData;//store previous value
newData = ADCH;//get value from A0
if (prevData < 127 && newData >=127){//if increasing and crossing midpoint
newSlope = newData - prevData;//calculate slope
if (abs(newSlope-maxSlope)<slopeTol){//if slopes are ==
//record new data and reset time
slope[index] = newSlope;
timer[index] = time;
time = 0;
if (index == 0){//new max slope just reset
PORTB |= B00010000;//set pin 12 high
noMatch = 0;
index++;//increment index
}
else if (abs(timer[0]-timer[index])<timerTol && abs(slope[0]-newSlope)<slopeTol){//if timer duration and slopes match
//sum timer values
totalTimer = 0;
for (byte i=0;i<index;i++){
totalTimer+=timer;
}
period = totalTimer;//set period
//reset new zero index values to compare with
timer[0] = timer[index];
slope[0] = slope[index];
index = 1;//set index to 1
PORTB |= B00010000;//set pin 12 high
noMatch = 0;
}
else{//crossing midpoint but not match
index++;//increment index
if (index > 9){
reset();
}
}
}
else if (newSlope>maxSlope){//if new slope is much larger than max slope
maxSlope = newSlope;
time = 0;//reset clock
noMatch = 0;
index = 0;//reset index
}
else{//slope not steep enough
noMatch++;//increment no match counter
if (noMatch>9){
reset();
}
}
}

if (newData == 0 || newData == 1023){//if clipping
clipping = 1;//currently clipping
Serial.println("clipping");
}

time++;//increment timer at rate of 38.5kHz

ampTimer++;//increment amplitude timer
if (abs(127-ADCH)>maxAmp){
maxAmp = abs(127-ADCH);
}
if (ampTimer==1000){
ampTimer = 0;
checkMaxAmp = maxAmp;
maxAmp = 0;
}

}

void reset(){//clean out some variables
index = 0;//reset index
noMatch = 0;//reset match couner
maxSlope = 0;//reset slope
}


void checkClipping(){//manage clipping indication
if (clipping){//if currently clipping
clipping = 0;
}
}


void loop(){

checkClipping();


if (checkMaxAmp>ampThreshold){
frequency = 38462/float(period);//calculate frequency timer rate/period

//print results
Serial.print(frequency);
Serial.println(" hz");
}

delay(100);

}

//----------------------------------------------

Hope you guys can help, thanks in advance!

Best Regards!
 
With a sampling clock of 38.5 kHz then the period values have a time granularity of 25.97 uS.
Measuring a 110Hz signal, then gives an uncertainty of 0.3 Hz,

deltaf = 110 - 1/(1/110 + 1/38500) = 0.313

similarly for 330 Hz :

deltaf = 330 - 1/(1/330 + 1/38500) = 2.804
 
Understand, thanks a lot!
Is there a chance to become more accurate by software optimization or is it a hardware limitation?
 
Maybe try the FreqMeasure library?

https://www.pjrc.com/teensy/td_libs_FreqMeasure.html

If you already have a signal generator set up, just connect it to pin 17 on Teensy++ 2.0 and open the example in Arduino with File > Examples > FreqMeasure > Serial_Output. Very easy to just run the example and see if it does any better.

This libary works very accurate, I can say it`s around only +/- 0,01 Hz deviation!
Do you see any chance to combine this libary with my program? Because normally I need a analog (ADC) input with the checkMaxAmp function to seperate the usefull signal from any other "noise" like overtones.

I already tryed to built up an analog circuite with frequency filter compression and comerator in the end, but even this works not very well. Unfortunatly im more in analog engineering and not very deep in programming, so it's quite hard to find a solution... .

Thank you in advance!
 
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