RF Low Pass Filter

Thread Starter

Mazaag

Joined Oct 23, 2004
255
Hi guys,

Just wanted to ask if an RC Low Pass filter (100Hz cut-off) is sufficient to filter out high frequency components of a signal coming in at 460KHz without having any inductive effects come into play ? Or are there special "RF" filters that should be used in this case ?

Thanks
 

nanovate

Joined May 7, 2007
666
What is the application? You could use a ferrite bead in series in addition for the >460KHz. What type of capacitors do you intend to use. Use a few in parallel -- tantalum and ceramics (X7R min, NPO preferred).
 

Thread Starter

Mazaag

Joined Oct 23, 2004
255
Thats the question, i'm not entirely sure what to use ..

I have a signal which is a sinusoid running at 460KHz and I need to check for low frequency components..(below 100Hz) so Ineed to filter at that frequency...
 

Thread Starter

Mazaag

Joined Oct 23, 2004
255
I jsut care about the stuff below 100Hz... this filter is basically an Anti-Aliasing filter preceeding a Digital Oscilloscope..
 

SgtWookie

Joined Jul 17, 2007
22,220
Take a look here:
http://www.arrl.org/tis/info/pdf/9902044.pdf

Low-pass passive RF filter, 50 Ohm input/output, cutoff freq around 18 MHz, down 60 dBm by 28 MHz. You could probably improve on that by adding shielding between the stages, but that will change your coupling between the stages.

You'll need to tune it, preferably using a network analyzer with an S-parameter test set. You COULD use a spectrum analyzer and a frequency sweeping signal generator; for that matter, it's been done with a manual-cranked sig gen and a VOM. You have to be somewhat of a masochist though ;)

Yes, the 18 MHz band edge is quite a bit higher freq than you were asking for - but if you really want to filter that low, you're going to need some REALLY large inductors. There are values supplied in Table 1 to get the roll-off to start @ 1 MHz.

Use high-Q caps. Watch the Q on your inductors - if you try to over-squeeze the coils, you'll kill their Q and wind up with lousy insertion loss. :( It can be tough to get good toroids - they can be pretty inconsistent. Baking them for 24 hours @ 100°C just prior to tuning the filter will ensure they're pretty dry. Yes, moisture will get into the toroids if you scratch them, and yes, it will throw your filter out of tune.

You COULD build an active Sallen-Key filter using an op amp and some components - but then you'd need power supplies, have to deal with S/N ratios and all that other mess. A well-designed and properly tuned passive LC filter should do the trick for you quite nicely.
 

nanovate

Joined May 7, 2007
666
Down at 100Hz you are probably better off with an RC filter augmented with a balun or ferrite bead. I am assuming that the 3dB point is at 100Hz. What is your stopband desired attenuation? What does the 100Hz signal look like?
 
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