Differential Bandpass Filter Design - RF Cafe Forums
Post subject: Differential Bandpass Filter Design
Unread 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?
Can some 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 a HPF.
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) = 1.37nH
C=1/(pi*Z0*Fc) = 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
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
I suggest Eagleware: This is a great design tool
for distributed and lumped filters with synthesis and simulation
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
Unread 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.
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 stuff..