Built-In Frequency Measurement on Oscilloscope?

[digi001]

How does a modern scope with automatic Frequency detection work?

FFT? PLL?

 

[FvM]

It's more trivial. My Agilent 600 manual clarifies:

Frequency is defined as 1/Period. Period is defined as the time between the middle threshold crossings of two consecutive, like-polarity edges. A middle threshold crossing must also travel through the lower and upper threshold levels which eliminates runt pulses.

 

[digi001]

I wrote a matlab script which uses local maxima that seem to work pretty good.



%Script to Find Frequency
%V1 is the Signal

Fs=1e-6; % Sampling Frequency

V1_abs=abs(V1); %rectify
V1_abs_inv=V1_abs*-1; %invert

[pks,locs]=findpeaks(V1_abs_inv,'MINPEAKHEIGHT',-1.5,'MINPEAKDISTANCE',5); %find peaks

figure(1)
plot(time,V1);hold on;
plot(time(locs),0,'k^','markerfacecolor',[1 0 0]);
hold off;


for i=1:length(locs)-1
freq(i)=1/(2*(locs(i+1)-locs(i))*Fs);
end

figure(2)
plot(freq)

min(freq)
max(freq)
mean(freq)

 

[ninju]

It uses a frequency counter.As FVM said, f= no of oscillations per second. So, what it does is, it converts the incoming sine into pulses and gives them as input to a counter and resets the counter every second. If the freq being measured is 10kHz, then it results in 10k pulses per second so the counter will read 10k.