Toggle sidebar Toggle sidebar
  • You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
  • The Retevis Holidays giveaway winner has been selected! Check Here to see who won!

Low Pass filter design?

Moleculo

Moleculo

Ham Radio Nerd
Apr 14, 2002
9,264
1,851
283
I want to build a simple low pass filter with a cut off frequency of 30Mhz that can handle about 500 watts. I've downloaded a couple of applications to help calculate the design specs and also used this site:
http://www.wa4dsy.net/filter/hp_lp_filter.html

I'm not sure where to start on the inductor design. I know you can do this with a simple coil, but how do I figure out the dimension/turns of the coil to achieve the recomended inductance value? Also, I have to take the necessary power handling capabilities of the caps and inductor into consideration.

I'm sure this is simpler than I'm making it to be, so if someone can point me in the right direction, I'd appreciate it.
 

OK, I am admittedly L. A. M. E. when it comes to circuit design. I was hoping that I could find a simple tool to help design a 5 or 7 pole filter. Hell, I'd even start with a 3 pole filter and then go from there. Using any of the tools I've found so far, I can get various designs fairly easily. However, they always come out with capacitance values that you'll never be able to find. It also seems like you have to use trial and error with the inductance calculators to come up with the right size air wound coil. Is there a tool somewhere that can make this a little simpler (especially since I'm math lazy)? I'm not even trying to worry about impedance matching at this point...i'm just assuming 50ohms in and 50ohms out.
 
Usually when odd capacitor values are needed, two smaller-valued capacitors are put in parallel. Typically the value needed can be within +/-10%, because it varies anyways due to inductor tolerance and cavity effects of the enclosure.

If you're really into such things like filters, or anything RF really, you need to look into getting a vector network analyzer. A VNA allows you to measure the loss of the filter, the SWR of the filter, etc. It sweeps frequency and gives you a plot.
One popular VNA is the N2PK VNA. Unfortunately, its not sold as a completed unit as far as i know. Only the PCB is available, and it has to be built like a kit.

Cool thing about the VNA is that its a do-everything RF test instrument.
 
Yeah, I know I can buy one...I actually have a Bencher already. I'm hoping I can learn something and also since a low pass is just a handful of capacitors and wire, it seems a shame to pay for something that can be built for only a couple of bucks.

I've attached a couple of screen prints from SVC Filter Design used to design a 5 pole Chebyshev low pass filter. Perhaps someone can comment on if it looks like I did this properly. If I'm reading this right, it looks like i can use 130pf caps and make/buy a couple of 200nH and one 430nH inductors to build this which will result in approx -14db at 50mhz?
 

Attachments

  • filterdesign1.jpg
    filterdesign1.jpg
    68.1 KB · Views: 187
  • 5polelowpass.jpg
    5polelowpass.jpg
    102.9 KB · Views: 195
Essentially the right idea.

I always study other's designs, but you are going to have to get over the trial-and-error and math phobias to come up with something better.

Try running some of these designs through your filter designer:

low-pass%20filter_schem.jpg


filter.gif


50tlpfschm.gif


Harmonic suppression
 
Oh oh yeah, don't forget to isolate each inductor from capacitive coupling, not really shown in the link, but it is important.
 
I put that first one into Elsie and it didn't look very impressive.

Unless I did it wrong.


Isolating each inductor is just a matter of getting them spaced a litte ways apart, right? If I used toroid wrapped inductors and placed them side by side (not physically parallel), that wouldn't be as big of a problem, would it?
 

Attachments

You might have done that correctly. I would admit that the first design does not look like one I would use and, IIRC, was designed for higher frequency.

The last link I think has a better design, and I would probably add more elements to whatever I was building since performance/cost is high.

I also am a fan of open air coil inductors, maybe just for their cost and simplicity, but they do present other types of difficulties.

Orienting the inductors will certainly help, or will it? It would be worth looking at the EMF, but I'd still want to shield each inductor.
 
Try a 7th order filter designed for 30 MHz, C-input Lowpass, Chebyshev style

Then you can add the open air inductor Q from the calc. I posted earlier.
 

dxChat
Help Users
  • No one is chatting at the moment.


      Top Bottom