High / RF Filter Design - RF Cafe Forums
Post subject: High / RF filter design
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
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
Post subject: Some help
Unread 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 requirements.
that in RFCAfe there was an article a couple of months ago regarding
butterworth and chebuchev filters. scroll back and you will find
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
is this a lumped or
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
Unread postPosted: Fri Mar 05, 2004 1:10 am
Here's a good
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 capacitance.
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.
Post subject: Pay
attention to the effect of the traces in the design
Fri Mar 05, 2004 8:56 pm
Welcome to forum!
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