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No you cant.
A filters insertion loss, attenuation and etc. is determined usually with a 50 ohm impedence. If the filter is not having 50 ohm termination in the entire band, you cannot count on the specifications. You can use attenuators on either side to be sure to have a certain 50 ohm load.
tyassin has answered the question in the previous post. You need 50 Ohm source and load impedance to make the filter working properly. If you simply two filter together (I guess you want to put them in parallel), the source/load impedance will be not 50 Ohm anymore. The in-band impedance of a filter may be close to 50 Ohm, but the out-of-band impedance should be far away from 50 Ohm to get the out-of-band rejection. So you need a sort of isolation between those two filters, passive attenuator may be a solution if you don't mind the power loss. Another way is to put filter on the input/output path separately.
From a practical viewpoint, I would answer the question slightly differently.
Yes you can make a diplexer by connecting two bandpass filters, but the filter parameters have to be adjusted to take account of the different termination. A series LC circuit may be added at the common port for impedance correction.
Filter design tools like Nuhertz Filter Solutions can calculate it.
This sounds a new solution for me. It is always good to learn something new. Would you please make a simple diagram to explain this with more details? Thanks!
This sounds a new solution for me. It is always good to learn something new. Would you please make a simple diagram to explain this with more details? Thanks!
Not really need as long as Tx and Rx can be isolated.
I have a few doubts that need explanation from everyone.
My Tx and Rx frequency are quite close about 85MHz apart, then low and high pass filter still can be use?
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Hi FvM and all,
Thanks for sharing the information.
As Filter can be design in 50 ohm impedance configuration, what if the result found out it is not match to 50 ohm, this could be due to input and output source is not match to 50 ohm?
Thus, before designing a filter, it is a must to check the input and output source impedance ?
Can I said that for every -3dB, the signal will reduce half of its power?
A frequency band of 85 MHz apart, is it necessary to isolation the Tx and Rx frequency using low and high pass filter?
Please advise.
You can use an isolator for isolation between Rx and Tx. 85MHz at 2.15Ghz is quite close imo. Depending on type of isolator they can have 20/40 db attenuation.
To answer to the initial post, to make a duplexer using two independent Band Pass Filters, you can use a Wilkinson divider (about 30dB isolation between ports).
There is an unwritten rule designing high isolation duplexers (or diplexers) that never place a shunt inductor or a shunt capacitor at the common port.
Using a Wilkinson to connect two BPF it will help somehow to avoid this situation.
It all depends on HOW FAR APART the passbands are. If the passbands are very far apart, you can just connect the two filters with a minimum of line length to a T junction. (assumes that out of band, the bandpass looks like an open circuit)
IF the passband frequencies are close together, you instead need to redesign the two filters to be "singly terminated" types so that you can connect them together in shunt and they compliment each other.
In some cases you can connect the two filters together, and either tune out the junction VSWR (with a tuning screw in waveguide, or some shunt or series element in microstrip), Or accept some mismatch in the junction and use something like a magic tee to reduce input vswr into a load
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