High / RF filter design - RF Cafe Forums
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Post subject: High / RF filter design
Unread postPosted: Wed Feb 18, 2004 9:02 am
can anyione help im trying to design a filter to accept 900mega hz to 1200 mega hz and scan
inbetween to pic up a specific signal.
i have been looking at bandpass filters but working it out the componets
come out at rediculouse values.
i have seen butters filters etc but i need some advice as this area is very
new to me and ive been thrown in at the deep end.
i need to be able to show worked out calculations (so i
can understand it) and have a circuit design.
can any one help?? please any little thing even any other
sites to reserch as im running short!
please email me direct with any info
Post subject: Some help
postPosted: Wed Feb 18, 2004 4:10 pm
You should have a design software to design filters.
Nowadays engineers almost never use numaclatures or tables to design filters (well only the oldies, and I am not
one of them :D ). One professional tool I recommend is Eagleware with the most used common filters. There you can
synthesize your desired filter based on a given set of parameters as: Topology, number of poles, cutoff
frequencies, insertion loss etc. after the initial values of synthesis based on the polynom of required topology,
you will fine tune the values until you get the desired filter's shape.
If you want some theoretical help
you can begin the analysis from the basic LPF filter (Prototype Filter), where based on the impedance of generator
and other parameters you can derive any kind of other filter with the required parameters based on your
I remember that in RFCAfe there was an article a couple of months ago regarding butterworth
and chebuchev filters. scroll back and you will find it.
I hope this helps,
Unread postPosted: Fri Mar 05, 2004 1:06 am
Agreed. The fastest way
is to use Eagleware and refer to Randall Rhea's book on HF filter design. The other common reference's (Zverev
etal.) will take far longer to comprehend and apply, although they are more general and comprehensive in nature.
You can probably find lumped element filter design software on the web.
Things to consider
this a lumped or cavity design?
your values are probably off because of the internal impedance of your
filter, remember that it can be adjusted and then use a transformer at the input and output (commonly a plain old
inductor or capacitor)
if your design is lumped then do not pick an architecture with "floating" inductors
if possible. always keep one end terminated to ground if you can because of parasitics. capacitors are generally
more ideal in their performance
don't expect the software to give you a functioning filter at turn on.
almost all filters all require tuning and value adjustments to make them work. it's a bit of an artform to tune a
filter so I suggest that if you know someone who's done it before, go find them
Unread postPosted: Fri Mar 05, 2004 1:10 am
Here's a good page:
Post subject: Pay attention
to the effect of the traces in the design
Unread postPosted: Fri Mar 05, 2004 10:57 am
One more thing
to remember is the effect of the traces on the design, meaning after you arrive to the desired values you will
have to connect between the componenets with 50 ohm microstrip lines. They have an influence on the overall filter
performance: They add insertion loss. In addition the vias to the GND add inductance and the substrate itself add
You should define the substrate in the simulation tool you are using: Dielectric coefficient,
Loss Tangent, Width of the substrate etc, and then add the length and shapes of your traces to the design. It will
simulate the reality in the best way.
subject: Pay attention to the effect of the traces in the design
Unread postPosted: Fri Mar 05, 2004 8:56 pm
Welcome to forum!
First of all, I need to know what is your applications, desired power level,
rejection requirements, vswr and insertion loss (i.e. full electrical specs). Can you share? I’m sorry can not
guess! Secondly, some people may get you confused when it comes to LC or cavity filters. Cavity filter design can
be lumped and non-lumped element. It all depends on what is for (we design them everyday). Is your filter for an
experimental purpose? You didn’t make any inquiry, but I am taking the liberty to ask you. Are you looking to buy?
We are not cheap, but pretty good RF & MW filtering products. It appears that you will need a BP Filter and you
can design it. But, keep in mind no filter is good for use until tuned, tested and worked as expected. There are
several books in market (you can find them on web sites as well) to study (not only reading) to get a basic
understand of electrical, electronics, rf & microwave and mechanical disciplines before get to software. Software
design tools program are good for advanced professionals. But, in your case you will need foundation first.
Your questions, comments and answers are welcome.