# Decreasing of $V_{in}$ in a high pass filter due to the type of capacitor used

I observed a strange fact while using a simple RC high pass filter. Here is the circuit I used. On $CH1$ I measured $V_{out}$, while on $CH2$ I observed the value of $V_{in}$.

I kept the function generator at $V_0=20V$ pk-pk (sinusoidal), and I increased the frequency from $10 Hz$ to $10^4 Hz$ in steps.

The strange thing is that voltage on $CH2$ (which is $V_{in}$) decreased while the frequency was increased! That is, at low frequency $V_{in} =V_0=20 V$ (pk-pk), but then it decreased to $4.5V$ at $10^4 Hz$!

Nevertheless the filter seemed to work ok because if I plot the ratios $\frac{Vout}{Vin}$ (whith $V_{in}$ measured with oscilloscpe, hence decreasing) vs frequency (on log scale) I get the right curve, corresponding to my cutoff frequency $f=\frac{1}{2\pi RC}$.

Obviously doing a simulation on Multisim or other simulators I get that $V_{in}= V_{out}$ (besides little variations).

Then I tried to build another filter with a different capacitor and I did not observe this strange behaviour, which makes me think that this was probably caused by the capacitor I used.

What I would like to know is if there can be any explanation for this behaviour of the filter wich is due to the characteristics of the capacitor used.

• that's a very low impedance filter you have there. What's the output impedance of the generator? Was the 'other filter' built with a different value capacitor? Feb 9 '17 at 17:43
• It was a normal $50 \Omega$ output impedance. I tried to put a different capacitor (0.1 $\mu F$) (the function generator was exactly the same) and I observed that $V_{in}$ was approximately constant increasing the frequency Feb 9 '17 at 17:52

• Thanks a lot for this answer! Since I do not know exactly how output impedance work in my function generator: isn't the function generator supposed to provide the voltage difference choosen "beyond" its output impedance? In other words, if I set voltage to 20Vpp isn't there a voltage of 20Vpp at the terminals of the function generator? Or should I really subtract $|Z_{out} I|$ if I want the voltage actually delivered to the load? (if it is useful I used a AimTTi TG2000 and I selected 20Vpp on the menu). Feb 11 '17 at 23:19
• Doing again the calculations this really seems to explain the data I got! So I must consider a $50 \Omega$ resistance in series! If I may ask one last thing, when I did the measurements I connected to the function generator a BNC coaxial cable with characteristic impedance of $50 \Omega$. Must the cable impedance be added to the output impedance of the function generator ($50 \Omega$ too)?(that would give a total of $100 \Omega$, which would be strange) Or is the impedance of cable already taken into account in the output impedance of function generator? Feb 19 '17 at 18:30