Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

How to choose -3dB filter bandwidth for an input signal frequency?

Status
Not open for further replies.

sudeep_

Member level 1
Joined
Jul 31, 2013
Messages
40
Helped
0
Reputation
0
Reaction score
0
Trophy points
6
Activity points
261
Hi,

I have one doubt on choosing filter Bandwidth. My input signal frequency is 40 MHz for an active RC filter. I don't know how to choose value of the -3dB BW. Is there any relation between the input signal freq versus BW?



Thanks!
 

FvM

Super Moderator
Staff member
Joined
Jan 22, 2008
Messages
47,489
Helped
14,048
Reputation
28,351
Reaction score
12,706
Trophy points
1,393
Location
Bochum, Germany
Activity points
276,162
there any relation between the input signal freq versus BW?
I suppose it is, but dedepending on both, the filter and signal details. Are you talking about a low-pass filter?

To ask differently, what's the intended filter effect? What's the signal waveform, do you expect it changed or maintained after passing the filter.
 

SunnySkyguy

Advanced Member level 5
Joined
Sep 26, 2007
Messages
6,743
Helped
1,675
Reputation
3,348
Reaction score
1,643
Trophy points
1,413
Location
Richmond Hill, ON, Canada
Activity points
50,727
It depends squarely on Nth order rolloff for -3dB, ripple skirt shape etc.​

But for 1st order approximation, 10ns rise time = 30MHz

1ns = 300MHz etc.

so tR = 3/10f when defined as 10~90% ,
not 0~ (1-1/e)=63%

Thus 40MHz= 7.5ns

- - - Updated - - -

FWIW, learn to specify RF filters as bandpass ripple f and bandstop rejection point.
Eg flat to 40MHz +/- 0.x dB and reject y dB at f2, then add phase error and group delay specs as needed
 

sudeep_

Member level 1
Joined
Jul 31, 2013
Messages
40
Helped
0
Reputation
0
Reaction score
0
Trophy points
6
Activity points
261
If I set the BW at 40 MHz, the SR is dropped. If SR is setteled then BW of the Filter show at higher frequency.

Moreover, the currents at Compensation Capcitor in Opamp is very low (swing < 80uA). But Input stage and output stage currents are 300uA, 800 uA.

How to increase the Current swing at Cc in Opamp.
 

Borber

Advanced Member level 5
Joined
Dec 31, 1999
Messages
1,602
Helped
234
Reputation
470
Reaction score
111
Trophy points
1,343
Location
on third flor
Activity points
11,862
Only transfer function can give you -3dB bandwidth from allowed attenuation at max. frequency of interest.
 

RCinFLA

Advanced Member level 2
Joined
Aug 31, 2010
Messages
569
Helped
185
Reputation
374
Reaction score
182
Trophy points
1,323
Activity points
5,566
Beside knowing what you need to reject, you have to know what you need to pass.

For example, passing a bunch of narrow band RF channels below 40 MHz is a different filter then passing a video signal that is critical to group delay.

An active LPF filter with corner at 40 MHz can be very tough to keep stable.
 

Status
Not open for further replies.
Toggle Sidebar

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top