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Differential Bandpass Filter Design - RF Cafe Forums
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Below are the old forum threads, including responses to the original posts.
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Post subject: Differential Bandpass Filter Design
postPosted: Sat Apr 10, 2004 12:02 pm
Joined: Sat Apr 10, 2004 11:35 am
I am tring to
design a differential BPF to be used in a grad school project. It's
suppose to have a bandwidth of 5.2Ghz to 5.9Ghz, less than 1dB of loss,
Input/output match less than -10dB and have more than 30dB's of rejection
for the bands 2Ghz-2.5Ghz and 10Ghz-12Ghz.
I used the basic "T"
equations to get my L and C values, athen built a Low-pass and a High-pass
and cascaded them. To get the 30dB rejection I have to add additional
stages on the Low-pass and High-pass filters. However, when I do this
the bandwidth widens out to be greater than 5.2Ghz-5.9Ghz. Does anyone
know why this is happening? And how can I correct this?
please help or advise.
Also, here the equaltions that I used.
They are for a "T" section, but I think they should work for a differential
with some changes to the cap (take half the value) between the inductors
for a LPF or to the inductor (double the value) between the caps on
Fh=5.8Ghz, Fc=11.6Ghz, Z0=50Ohms
C=1/(pi*Z0*Fc) = 1/(3.14*50*11.6e9) = 0.55pF
L=(Z0)^2 * C = (50^2)(0.55pF)
Fh=5.2Ghz, Fc=10.4Ghz, Z0=50Ohms
= 1/(3.14*50*10.4e9) = 0.612pF
L=(Z0)^2 * C = (50^2)(0.612pF) = 1.53nH
Does anyone know if these equations are correct for a Differential
BPF? And is the assumption of Fc = 2 Fh correct for a Differntial Circuit
or just for the "T" model?
Unread postPosted: Sat Apr
10, 2004 2:53 pm
You will of course have to realize
your filter with distributed elements. There are equations for transforming
the capacitance and inductance values to physical dimensions of transmission
lines. You can easily find these equations in a text book :idea:
The bandwidth can change because you change the characteristics
of the filter when you add aditional sections. Do you take the Q element
under consideration? You should use a design tool to synthesize your
filter and by this you can save time and iterations.
Eagleware: This is a great design tool for distributed and lumped filters
with synthesis and simulation capabilities.
The equations for the
'T' sections are:
When you realize
the filter with transmission lines (micro-strip or another), you will
have to define the substrate and from that to derive few properties:
Er (the dielectric coefficient), Loss tangent, resisitivity etc... you
will have to define those and consider them in your final stage of design
in order to match to the filter's requirements.
I suggest you
will use Rogers laminates 4350 or similar as the substrate due to its
relatively low Er and stable characteristics.
Post subject: check out this web site
Tue Apr 20, 2004 4:38 pm
postPosted: Wed May 05, 2004 3:40 pm
there is an furmula. You
can find out how many resonators you need for defined bandwith. What
i can see these days, the engineers use software as crutches. Take a
tea, switch off the computer, and start thinking. Purely theoretically.
In my times we asked professor for consultation.
If you add additional
50 resonators, it might be even wider.
Unread postPosted: Wed May 05, 2004 3:50 pm
some Maxim's appnotes are good. But only some. Maxim never was
wireless company and therefore the datasheets are full of mistakes.
e.g. The don't know location of minimum noise figure impedance and s-parameters.
They measure only IP2 to come up with +56dBm IP2 for LNAs and similar