Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
The resistor in combiners is there for safety reasons. If one amplifier fails it absorbs part of the power from the other so that it has a proper load. If the resistor is gone, the second amplifier will overheat and fail.
It is possible to combine two or more fm transmitters without using absorber. Each transmitter is connected to bandpass filter (50ohm-50ohm BW aprox. 300kHz) and outputs are joined to output connector. Cables from filter to star point are different lengths to compensate each other regarding return loss. Frequency spacing is aprox. 1.2MHz minimum.
Filters are coax type with trimming capacitor at the end. Each filter has two resonators with propriate coupling. Such combiners are used for 10kW transmitters
I would have some questions as to the combiner described above:
1- If you parallel two filters with 50Ohms you fet 25Ohms, what implies in a strong mismatch with the antenna and an unacceptable power loss.
2- You described a filter with two resonators. Regarding the PAs used in FM are biased in class C or at least in a strong Class AB, the rejection that one filter presented to the other carrier will not be enough to avoid a generation of a high level of IMD, what again is an acceptable situation.
By those reasons I can't see how the combiner described can work if for a 1Watt transmitter.
I think BORBER is speaking about an application similar the use of a diplexer on two transmitters, due to the fact the two frequencies must be 1,2 MHz apart minimum.
This system works, but you have to think the fact you cannot add the power of two transmitters on the same frequency o something like a system " fault resistent " but only to add the signal of two transmitters, as for video and audio carrier on HI-POWER TV transmitters.
No way. Combining like proposed don't work. If what was addressed before is a diplex the description is totally wrong.
In a diplexer you have to have a circulator and one bandpass filter, or one bandpass and one notch filter.
In this arrange one transmitter without the bandpass (or that one connected to the notch filter centered at the frequency of the second transmitter) is connected to the port one of a circulator and the second transmitter with the bandpass filter (centered at its own frequency) is connected to the second port of the circulator. The antenna is connected at the third port of the circulator.
When the power from the first transmitter (without BPF) goes into the circulation and find a high rejection due to the BPF, it is reflected back to continue its path to the antenna. As the circulator is a non-reciprocal device, the reflection back to the first transmitter almost null. The second transmitter finds the antenna at the next circulator node and all its power is absorbed. Note that all ports are matched at 50Ohms and any spurs leaking from the reverse path of the circulator is blocked by the Notch or BPF, avoiding IMD distortions.
That is how a diplex works.
Of course diplexes are used to combine different frequencies. When I started in my last post I said: If what was addressed before is a diplex the description is totally wrong.
I was just refearing to your post where you mentioned signal with different frequencies.