building Passive LC Bandpass filters? - Page 1
Forum section: electronics / projects

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: July 05, 2014, 08:36:16 pm
content: I have recently gotten in to Software Defined Radio via a TV dongle and LNA. Id like to take the radio a step further though, I plan to put the dongle and preamp in a shielded box with a USB isolator. I would also like to add a frequency preselector, I went to radioshack and bought a bunch of magnet wire to wrap inductors, and built a little arduino circuit to help me measure them.I was under the assumption you could just use an Lowpass back to back with a Hipass LC filter. I ripped open a TV set Rabbit ear antenna and it is set up this way with open air inductors. The problem im having is when I try to calculate an LC filter above 100mhz either the inductor or cap is ridiculously big! It is even worse when I get in to the GHz range! The thing is the rabit ears has pretty small caps and coild in it and it is set up for VHF/UHF!?Can someone show me how to build a simple 1090mhz Hipass/Lowpass LC filter, withought needing a 100,000 wrap inductor?

author: PA0PBZ
author_rank: Super Contributor
timestamp: July 05, 2014, 08:53:55 pm
content: Just a guess, but when you really try 1090 mhz I guess you can expect values like that, try 1090 MHz...

author: KJDS
author_rank: Super Contributor
timestamp: July 05, 2014, 08:55:22 pm
content: Quote from: rwgast_lowlevellogicdesin on July 05, 2014, 08:36:16 pmNo, because you're far better off using a bandpass filter. What bandwidth do you need?

author: cyr
author_rank: Frequent Contributor
timestamp: July 05, 2014, 08:56:37 pm
content: How are you calculating the values? You should end up with values in the pF and nH range. Maybe tricky to make, but because they are so small values - not large.

author: JackOfVA
author_rank: Unknown
timestamp: July 05, 2014, 10:39:49 pm
content: Yes, it is possible to cascade a LP and HP filter to form a bandpass. If the fractional bandwidth of the filter is large, then a cascaded HP & LP is a good choice, e.g., HP at 1 MHz, LP at 100 MHz.If the fractional bandwidth is smaller, I would look for a different solution.At 1 GHz, it's possible to use air core inductors and chip capacitors, but it's on the cusp of a strip line design being a better choice.Frankly, as a first experiment in building filters, I would look for a simpler design at a lower frequency. Why not start with something as simple such as a 2 MHz high pass filter to roll off AM broadcast band (medium wave) signals.And, mHz = milli-Hz, i.e., 0.001 Hz -- there are some programs that are picky about capitalization, MHz = mega-Hz, i.e., 1,000,000 Hz. (commas are thousands separators). LTspice will get you every time with this, MHz is read as milli-Hz; for megaHz, you need to use meg. There may be others as well.

author: nctnico
author_rank: Super Contributor
timestamp: July 05, 2014, 11:25:46 pm
content: How about a simple LC parallel resonant circuit to begin with?

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: July 06, 2014, 12:40:23 am
content: Im not even sure what an LC parallel resonant circuit is..... Im pretty new to this stuff, that is RF, the only place I have ever used an inductor before this is with a switching regulator.My problem was actually the mhz Mhz thing... I was using an Android app called electrodroid to do the calculations for a LP and a HP apparently it has a mhz and Mhz also.I know cascading a lowpass and hi pass isnt the best way of doing things, but Im pretty knew to this and figured it is better than nothing?

author: nctnico
author_rank: Super Contributor
timestamp: July 06, 2014, 01:22:51 am
content: Look here for the basics:http://en.wikipedia.org/wiki/LC_circuitAnd here for more practical filter applications:http://en.wikipedia.org/wiki/RLC_circuit

author: mikerj
author_rank: Super Contributor
timestamp: July 06, 2014, 09:03:14 am
content: Quote from: rwgast_lowlevellogicdesin on July 06, 2014, 12:40:23 amThe problem you are going to face is making something that works as you'd expect at 1 GHz. Just a tiny amount of stray capacitance or inductance will be enough to drastically alter the behaviour of your circuit at these frequencies, and without suitable test equipment you'll never know why.

author: kalhana
author_rank: Contributor
timestamp: July 06, 2014, 08:30:43 pm
content: I don't have much experience in RF, but I can tell you about filters for general use.You have to be careful with cascading 2 filters, better to check the frequency response (bode plot, which basically tells you how much attenuation you get at different frequencies) by multiplying the two transfer functions and plotting.There are different types of bandpass filters you can use.Depends on your pass band, how sharp you want the roll off, phase response (delay) etc.I would look at this website which offers a wide range of filter design tools:http://sim.okawa-denshi.jp/en/You can also try MATLAB (free trial available) to plot filter responses and also LTSPICE (free) for circuit simulation in terms of frequency response etc.Then when you practically make the filter, you have to keep parasitics to a low by careful layout and component selection.Don't forget that the inductors you use will have equivalent series resistance (ESR) and the capacitors will have ESR+ESL. These thing may or may not affect the filter performance and should be considered.Good luck!