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Old 2nd December 2014, 06:49 PM   #1
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Default Cutoff frequency for RF rejection?

What -3db frequency do you guys use to suppress RF? 100MHz? Even lower?
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Old 2nd December 2014, 07:05 PM   #2
Bone is offline Bone  United Kingdom
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50kHz
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Old 2nd December 2014, 07:24 PM   #3
rayma is offline rayma  United States
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Quote:
Originally Posted by Fusion916 View Post
What -3db frequency do you guys use to suppress RF? 100MHz? Even lower?
As much as the frequency, it matters also how the suppression is executed in the particular hardware.
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Old 2nd December 2014, 07:30 PM   #4
RJM1 is offline RJM1  United States
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Just my opinion, but I go about 300Khz to stay away from the 100KW AM transmitters ( 540 to 1700Khz). I had a problem in the 1970's with a CB radio (27Mhz) over comming my HK Citation I's phono preamp section only rated at 80Khz.
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Old 3rd December 2014, 09:15 AM   #5
AndrewT is offline AndrewT  Scotland
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F-3dB between 100kHz and 300kHz, passive at the input.
I use ~700us for my passive RC.
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Old 3rd December 2014, 09:35 AM   #6
mcd99uk is offline mcd99uk  United Kingdom
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I use around 300kHz. 2nd order filter.
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Old 3rd December 2014, 09:41 AM   #7
AndrewT is offline AndrewT  Scotland
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Might be worth mentioning that these "filters" attenuate the RF. They don't remove, nor eliminate it.
A filter eventually stops filtering at some highish frequency.
Two filters staggered by a couple of decades, may perform better than one "super filter".
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Old 3rd December 2014, 09:42 AM   #8
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by mcd99uk View Post
I use around 300kHz. 2nd order filter.
How do you implement your 2nd order filter?

What Q does it achieve?
Does it have any "ringing" as a result of 2pole characteristic?

I use a 47oF at the input socket and then my ~700us RC at the PCB.
Strictly this is a staggered frequency 2pole, or maybe a 3pole if cable inductance has a significant effect.
I don't know how to analyse it for Q, nor ringing.
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Last edited by AndrewT; 3rd December 2014 at 09:44 AM.
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Old 3rd December 2014, 09:51 AM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by AndrewT
A filter eventually stops filtering at some highish frequency.
Two filters staggered by a couple of decades, may perform better than one "super filter".
Yes. Don't expect a filter to work from 50kHz to 2.4GHz.
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Old 3rd December 2014, 01:50 PM   #10
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by RJM1 View Post
Just my opinion, but I go about 300Khz to stay away from the 100KW AM transmitters ( 540 to 1700Khz). I had a problem in the 1970's with a CB radio (27Mhz) over comming my HK Citation I's phono preamp section only rated at 80Khz.
If you get enough MHz or whatever signal coming in, it can overload a stage and force it into nonlinearity. Then you hear the modulation signal. This could even happen in a power supply component (and your electrolytics are no good at these frequencies). The RF level required to do this may not even be very high as you don't have much if any distortion-reducing feedback at RF.
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