Demo 4

Download MATLAB files: demo4.zip

This MATLAB script controls the function generator and oscilloscope to measure the response of an RC circuit to an input sine wave as a function of frequency.

The circuit under test is shown below.

The routine get_data.m uses the oscilloscope to measure the amplitudes of the input and output signals and the phase difference between input and output.

The autoscale function of the oscilloscope does not always work properly at low frequencies. The MATLAB code could be improved to detect and correct that situation.

Open function generator and scope
take data
show data
look for 45-degree phase shift
compare time constant to RC
Close all instruments

Open function generator and scope

fg = gpib('agilent',7,10);
os = gpib('agilent',7,7);
fopen(fg)
fopen(os)
fprintf(fg,'*IDN?');
idn = fscanf(fg);
fprintf('function generator: %s',idn);
fprintf(os,'*IDN?');
idn = fscanf(os);
fprintf('oscilloscope: %s',idn);
clear idn;

% Set fg output load
fprintf(fg,'Output:Load INF');

function generator: HEWLETT-PACKARD,33120A,0,10.0-5.0-1.0
oscilloscope: AGILENT TECHNOLOGIES,54622D,MY40004759,A.01.50

take data

npts = 20;
freq = logspace(1,3,npts)';
y = zeros(npts,5);
for k=1:npts
    fprintf(fg,'FREQ %g',freq(k));
    y(k,:) = get_data(os);
end

show data

subplot(2,1,1);
yratio = y(:,4);
yphase = y(:,5);
plot(freq,yratio,'ko-','LineWidth',2);
ylabel('voltage ratio');
subplot(2,1,2);
plot(freq,yphase,'ko-','LineWidth',2);
xlabel('frequency (Hz)');
ylabel('phase (degrees)');

look for 45-degree phase shift

idx = find(yphase>30 & yphase<60);
fz = interp1(yphase(idx),freq(idx),45);
fprintf(fg,'FREQ %g',fz);
get_data(os);

freq 42.37 Hz vratio 0.703514 phase 45 deg

compare time constant to RC

fprintf('time constant %g msec\n',1e3/(2*pi*fz));
R = 1000;
C = 3.55e-6;
tau = R*C;
fprintf('R %g ohms C %g uF tau %g msec.\n',R,C*1e6,tau*1e3);

time constant 3.75269 msec
R 1000 ohms C 3.55 uF tau 3.55 msec.

Close all instruments

g = instrfind;
for k=1:length(g)
    fclose(g(k));
    delete(g(k));
end
clear
close all