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Duplexer 400Mhz - RF Cafe Forums
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romainp
Post subject: Duplexer 400Mhz
Unread postPosted: Thu
Mar 25, 2004 5:00 am
Hi,
i have to disign a rf duplexer about
400Mhz for a full duplex communication at 1Mb/s.
I havn't find a
duplexer with the different manufatcurers
How can i make it myself
?
Thank u
sory for my poor english (i am french)
Romain
Top
JGP
Post subject: Duplexer 400Mhz
Unread
postPosted: Tue Mar 30, 2004 8:22 pm
Welcome to Forum,
You can make it if you put all the efforts into it! Following are
some tips for you:
First off, you will need to select your Tx
frequency and then Rx frequency. After, you will need to be certain
you have a split frequency (SF) between Tx & Rx and the greater
separation the better isolation (isolation is very important in Duplexer
applications and full radio operation) between Tx & Rx. For instance,
let’s say you have 400MHz on Tx branch and 500MHz on Rx branch and the
frequency separation (split) is 100MHz (i.e. Tx-Rx = Split Frequency
or SF). As you can see in my example above, the band is very narrow
in frequency and the split of course, it is narrow as well. Therefore,
the cavity Duplexer has high Q and it will be into one Omnidirectional
antenna (full duplex operation). In addition, you must select VSWR,
Return Loss and maximum Tx RF power level. You could make the cavity
Duplexer with Aluminum or Brass. It depends on how much money you want
to spend and how easy for you to machine the metals. Chebychev will
do, but Butterworth is better (no ripple). Again, it will depend on
your electrical and mechanical specifications and ultimate Duplexer
performance. The selectivity, rejection, Tx & Rx isolation and Q
performance of the cavity will depend on the following factors: Size
or number of the cavities and Fc (center of frequency). Make certain
the I.L. (insertion loss) is low (3dB is too much and you will loose
half of your Tx power) as possible and with low ripple as possible as
well (i.e. <0.5 dB), but remember that for Digital Communications
high ripple has a great negative effect and will damage BER (bit error
rate) on Receiver and also high insertion loss will destroy the NF (noise
figure) on Rx side. Read the specifications of your radio receiver manufacturer
to make sure you don’t apply too much insertion loss on your Duplexer
Rx branch. Coaxial cables to transfer RF signal from one cavity to another
cavity are very critical. Therefore, size, length and velocity must
be taking into consideration. Don’t forget, in any filter design, we
must have tradeoffs! With that approach in mind, you must select and
do the right measurement. There are several steps to go through until
this is done. I mean, from design it on the paper with sophisticated
hand calculator (an old fashion and less accurate and not reliable for
voice or data digital communications) or by software (with software
filter design tool the results are reliable and accurate), machining
your cavities (make them from scratch) to tuning process. Remember,
no filter (like any other electronic device) is good for use until tested
and met the required specifications.
I thought this might help
you to start. If you need help in order to accomplish this project,
please let me know because I will be keen to help you.
Sincerely,
John Pereira
Posted 11/12/2012
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