Please Help High Pass RC Filter - Page 1
Forum section: electronics / beginners

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 12, 2011, 07:18:54 pm
content: this is a continuation fromhttps://www.eevblog.com/forum/index.php?topic=1869.0but most people like a new page, so...i tried to make and learn some setup to find capacitance of a capacitor. it is the mother of all High Pass Filter ie a basic single C and R circuit.now i have a problem...R = 7.7OhmC = 1uFthe cutoff frequency should be 20KHz as hand written down below.above than that, there should be no attenuation. below that, signal is cut.well, thats the theory saying, but my practical saying no.the problem is, when i inject high frequency (a lot higher than cutoff) signal, its still being blocked at around -3db. the theory said, it should be 0db right?as indicated in the pictures, so this one made me like hitting a brick. so, SOS, please explain master.

author: Simon
author_rank: Unknown
timestamp: April 12, 2011, 07:25:03 pm
content: err 7.7 ohms ? that is probably shorting your output stage pulling the voltage down, check your output capability

author: tecman
author_rank: Frequent Contributor
timestamp: April 12, 2011, 07:39:24 pm
content: A good rule of thumb is that your R in the RC should be 10 times higher than your source impedance. This results in the source impedance having a small, although possibly significant, effect on the RC network. You will often see in multi-order RC filters that each stage is 10 times the previous stage in terms of R values.For your example, you should have no less than 500 ohms for the R value.paul

author: Simon
author_rank: Unknown
timestamp: April 12, 2011, 07:44:55 pm
content: Quote from: tecman on April 12, 2011, 07:39:24 pmor have an opamp stage in between filters ?

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 12, 2011, 07:46:27 pm
content: ok yes its pulling the voltage (Vi) down (probably damaging the FG). but what i was measuring is at node ch1 and node ch2, ie there only is a cap in between, why its blocking the voltage coming through it?or maybe it has significant internal resistant? does it answer my own question?

author: Simon
author_rank: Unknown
timestamp: April 12, 2011, 07:57:34 pm
content: your whole filter has a low impedance, the cap is virtually out of the equation (a short circuit) at higher frequencies than the cut off, the point at which the signal generator goes into the filter is the mid point of the potential divider formed by the generators internal impedance and the filters (resistors) impedence

author: jahonen
author_rank: Super Contributor
timestamp: April 12, 2011, 08:11:05 pm
content: Maybe that electrolytic capacitor has just high ESR, in order of ohm? That would explain why the signal fails to rise to full height.Regards,Janne

author: Simon
author_rank: Unknown
timestamp: April 12, 2011, 08:18:07 pm
content: yea that's a possibility

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 12, 2011, 08:45:04 pm
content: Quote from: jahonen on April 12, 2011, 08:11:05 pmthis explains it. it creates a voltage divider between node ch1 and node ch2. cheapo cap with high esr i think. anyway, i changed to 600++ ohms and it works ok now. at least the esr divider is put to minimal. i was checking 110uF earlier with the 7.7ohm to get F in range (not too low) and decided to leave the resistor there to check lower capacitance later. Mr Murphy caught me this time. thanx guys for advices.ps: using my simple math, esr maybe around 4-5ohms!