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Microstrip Filters - RF Cafe Forums
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Antonio
Post subject: Microstrip Filters Posted: Tue Jun 19, 2007
3:57 am
Captain
Joined: Sat Jun 16, 2007 12:26 am
Posts: 12
Hi All,
I am trying to desing a Microstrip bandpass
filter using edge coupled line filter technique.
This filter
is a narrow band filter. I did lot of trials to design it but the problem
is the Insertion Loss in the band is too high. I am not able to reduce
it.
I want to know that is it true that Edge coupled line filter
technique is not suitable for a narrowband filter?
If it is not
suitable then which technique will be suitable :
a) End coupled structure
b) Hairpin line structure
c) Comb line structure
Can anyone
help me in this problem??
Is there any book or link in the web
from which I can get useful information for designing a narrowband filter??
Top
nubbage
Post subject: Posted: Tue Jun 19, 2007
10:13 am
General
Joined: Fri Feb 17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi Antonio
For a coupled multi-element
filter of the Chebychev type, the in-band insertion loss is approximately
L = (4.34/(w*Qu)* SUM(g) ......dB
where w = bandwidth/centre
freq
Qu is the un-loaded resonator Q
and SUM(g) is the sum of
all the element values in the low-pass prototype.
Thus if w is small
(say < 0.2) then for a given Qu the loss is high.
With a strip-line
the Qu is always lower than say copper bars, due to film skin resistance
and dliectric dissipation.
The best design for really narrow band
filters will always be coupled cavities, such as used in narrow-band
diplexers.
Top
IR
Post subject: Posted: Tue Jun
19, 2007 2:30 pm
Site Admin
Joined: Mon Jun 27, 2005
2:02 pm
Posts: 373
Location: Germany
Hello nubbage,
I have one question concerning the term Sum (g) in the equation.
Can you please elaborate on that?
Thank you!
Top
Antonio
Post subject: Microstrip Filter ImplementationPosted:
Tue Jun 19, 2007 10:21 pm
Captain
Joined: Sat Jun
16, 2007 12:26 am
Posts: 12
Hello nubbage,
Thanks for
your reply. But my question is still unanswered. Due to resource constraint
I am trying to desing it as edge coupled filter. I want to know that
among the listed structure which is suitable for designing a narrow
bandpass filter.
a) Edge coupled
b) End coupled
c) Hairpin
line
d) Combline structure.
I am able to design this filter
but how to calculate the length, width and spacing for different structures
is still not clear.
Is there any book or website where I can
get detail explanation on all these structures to calculate the length,
width and spacing for diiferent structures.
Thanking you,
Antonio
Top
nubbage
Post subject: Posted:
Wed Jun 20, 2007 4:07 am
General
Joined: Fri Feb
17, 2006 12:07 pm
Posts: 218
Location: London UK
Hi IR
The SUM(g) term is the sum of all the element values of the normalised
low pass filter prototype. An N element LPF prototype will have N values
for the reactances of each L and C used.
I got the formula from
H.L.Schumacher's paper "Understanding Filters and Multiplexers" Microwave
System News Vol 13 No 12, but almost certainly it is stated in matthei
Young and Jones as well (which is more readily available).
Top
nubbage
Post subject: Posted: Wed Jun 20, 2007 4:29
am
General
Joined: Fri Feb 17, 2006 12:07 pm
Posts:
218
Location: London UK
Hi Antonio
First off, my last posting
should have said an N element LPF has N+! values of g.
Secondly,
we would need to know how "narrow" is your "narrow" when you say narrow
band?
A good source of filter design information is the Radio Society
of Great Britain (RSGB) publication "Microwave Handbook" and in particular
Volume 2 chapter 12. It is a largely non mathematical treatment and
explains the physics going on. An example is given of a coupled element
coaxial multi-cavity filter, resembling a comb-line type, at 1GHz with
a bandwidth of 10MHz (a narrow band type in my thinking) that has a
band-centre loss of 1.0dB. However, the resonator Qu is estimated to
be 1400 to achieve that loss, and you stand no chance of achieving that
with strip-line substrate, no matter whether it is edge, face or end-coupled.
The chapter also covers stripline edge coupled designs and presents
a BASIC routine for their design.
I am not familiar with hairpin
types.
The classical text book on microwave filter design is Matthei
Young and Jones without a doubt. A copy of that might help.
Posted 11/12/2012
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