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#39291 - 06/16/04 09:39 AM "Frequency responce" technical project.  
Ali  Offline
Joined: Oct 2003
Posts: 12
I have this technical project which requires an exprimental investigation of the frequency responce of the circuit shown below ( checkout the link plz)

I want to know if u were me which equipments would u choose for this investigation?
and how would u connect them? (a daigram would be great)
and berief detail of the methalogy for the investigation.

That would be wicked and it would give me some idea to continue on.

Thanks in advance for your help [Linked Image]


Work Gear for Electricians and the Trades

#39292 - 06/16/04 10:10 AM Re: "Frequency responce" technical project.  
Radar  Offline
Joined: Apr 2004
Posts: 349
Los Angeles, CA
Due to brain damage, I am unable to dredge up the frequency formula for RC circuits from my 3 remaining memory cells. However, what I do recall to figure frequency is that you need to know the values of resistance and capacitance. Voltage is not important (I think - someone correct me if I'm wrong).

I don't know if these values can be measured in an operating circuit. Maybe there is another way, measuring circuit voltage (Vi) and current, then using them to figure impedance, and then working backwards from there.

Best way I think is to measure the resistance and capacitance of the components outside of the circuit, then apply the formula (which I still cannot recall).

Sorry if this is toooo dumb,

There are 10 types of people. Those who know binary, and those who don't.

#39293 - 06/16/04 10:52 AM Re: "Frequency responce" technical project.  
NJwirenut  Offline
Joined: Sep 2001
Posts: 806
Bergen County, NJ
Are you sure you should'nt be doing your homework by yourself?

Anyway, the equipment I would use for checking the frequency response of a network would be a signal generator and an oscilloscope. Connect the generator to the input, hang the scope on the output, and sweep the generator frequency across the frequency band of interest, while watching the response on the scope.

The circuit you show is a simple RC low-pass network, whose time constant (in seconds)is given by RC, with R in ohms and C in Farads.

As a lowpass filter, it's 3dB point (in Hertz) is given by the formula:


again, with R in Ohms, and C in Farads.

[This message has been edited by NJwirenut (edited 06-16-2004).]

#39294 - 06/16/04 11:36 AM Re: "Frequency responce" technical project.  
Ali  Offline
Joined: Oct 2003
Posts: 12
Thanks Guys;
NJ thanks for your concern but to be honest I chose Electronics as an extra lesson to learn something extra but it appear to be extra hard for me as I dont have any background at all and I really dont have any interest so Im just looking forward to pass it with minimum mark.
I'm studying "Aerospace system engineering" in Coventry, England.

BTW; just out of curiosity what can be possible source of error in this investigation, I thought of resistance in cable? is that right? any more idea ( Don't worry it just my barin going crazy its not the question)


#39295 - 06/16/04 02:01 PM Re: "Frequency responce" technical project.  
pauluk  Offline
Joined: Aug 2001
Posts: 7,520
Norfolk, England
You might want to monitor the input level as well if you're using a cheap'n'cheerful generator. Some of the cheap ones have an output which varies quite a bit in amplitude as you adjust the frequency. Not a problem if you have half-decent lab equipment, of course. [Linked Image]

As for errors, the cable resistance is unlikely to be a problem, but capacitance could be, depending upon the frequencies involved.

If this is a filter for audio frequencies where the value of C is relatively high then it's unlikely to be a problem.

If C is down in the picofarad range, however, such as would be found in some RF filters, then the lead and 'scope input capacitance could become significant.

An alternative to using a 'scope would be a VTVM or its modern transistor equivalent. Whatever you use as the measuring instrument, you need to make sure that the frequency response of the measuring equipment itself is satisfactory. This is another possible source of error. It's no good using a 10MHz bandwidth oscilloscope if you're trying to measure the response of a filter with a 3dB point set at 30MHz, for example.

P.S. Don't mean to be picky, and I know that pointing out spelling mistakes can be annoying, but it's "response" not "responce." No point losing marks unnecessarily, eh? [Linked Image]

[This message has been edited by pauluk (edited 06-16-2004).]

#39296 - 06/16/04 03:05 PM Re: "Frequency responce" technical project.  
Ali  Offline
Joined: Oct 2003
Posts: 12
Thanks alot pual,
Nahh, you are 100% right Ive lost alot of marks in past 12-13 years becuase of my rubbish spelling but I cant do anything about it. [Linked Image]

Thanks again for those info, much appreciated.


#39297 - 06/16/04 04:41 PM Re: "Frequency responce" technical project.  
hbiss  Offline
Joined: Dec 2003
Posts: 886
Hawthorne, NY USA
...but I cant do anything about it.

Sure you can. At least spell check. When you do that you tend to learn some of your more frequent misspellings. Put a little effort into it and you will see a difference.


#39298 - 06/16/04 07:13 PM Re: "Frequency responce" technical project.  
Ali  Offline
Joined: Oct 2003
Posts: 12
Thanks hbiss, I do my best [Linked Image]

quick question;

Don't we use oscilloscope to calculate the frequency of an oscillating signal? as it said here
So why should we use f=1/2(pi)RC equation?

Sorry If its a dumb question [Linked Image]

[This message has been edited by Ali (edited 06-16-2004).]

[This message has been edited by Ali (edited 06-16-2004).]


#39299 - 06/16/04 07:32 PM Re: "Frequency responce" technical project.  
NJwirenut  Offline
Joined: Sep 2001
Posts: 806
Bergen County, NJ
Determining the frequency of a signal is only one of MANY things you can do with an oscilloscope. Determining the passband of a filter is another.

The cutoff frequency of a lowpass filter is usually defined as the frequency where the output falls off by 3dB for a constant amplitude input. You can use an oscilloscope to determine this point by watching the vertical deflection of the waveform on the screen. You could also use an AC voltmeter, if it can handle the frequency being used.

The formula will give you the THEORETICAL 3dB point. Actually sweeping the circuit with a signal generator and a scope will find the ACTUAL cutoff point, which can differ from the theoretical one due to component tolerances, lead inductances, etc.

[This message has been edited by NJwirenut (edited 06-16-2004).]

#39300 - 06/17/04 04:17 AM Re: "Frequency responce" technical project.  
Ali  Offline
Joined: Oct 2003
Posts: 12
thanks alot NJ;
I got it [Linked Image]


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