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UHF Duplexer
The motivation here is to save money on installing cables in the home.
When built, my house had a coaxial cable installed from the attic to the lounge, carefully hidden in the cavity wall. This cable carries DVB TV channels from the rooftop antenna to the TV in the lounge. I also have a cable box in the lounge that I want to distribute around the house, with the distribution amplifier preferably placed in the attic for easy access to all rooms. So if I choose the right frequencies for the cable TV distribution, the duplexers at both ends of the drop cable will allow it to simultaneously carry DVB-TV onto the coaxial cable and Cable TV onto the coaxial cable.
TV multiplexers start at 739MHz and extend to 800MHz. The programmable range for cable TV distribution is 471-860 MHz. So I will implement a low pass section to carry CableTV onto the coaxial cable at approximately 488MHz, and a high pass section to carry DVB-TV. The low pass section will also carry DC power to power the distribution amplifier and the magic eye remote control code in the attic and back to the cable box.
Summarizing the above, the specs are as follows:
*DC - 530MHz (covers UHF ch21-ch28) < 2dB insertion loss
*710- 860MHz (covers UHF ch50-ch68) < 2dB insertion loss
*488MHz rejection >40dB for high pass section
*Return loss >10dB for All ports
Equations exist for the calculation of the duplexer poles, but for this basic application, the design process can be simplified.
1) Determine the number of filter poles to ensure we exceed the rejection requirements.
2) Calculate the ideal filter using the Low Pass and High Pass Filter Designer on the RF Tools tab.
3) Using the simulator, connect them together and adjust the first few poles on either side of the common port to tune the duplexer.
4) Add microstrip lines/vias and re-tune.
5) Build the PCB.
6) Populate the PCB and test, re-tune if necessary.
The degree of accuracy actually achieved in the simulation depends on the application, frequency, components, etc. As a rule of thumb, the simulation should be sufficient to establish a fairly accurate model that requires little adjustment once established.
The simulation results show that the insertion loss, rejection, and return loss specifications are met.
The PCB was exported in Gerber format and milled using a bench milling machine and then populated by hand.
Measurement results of the built circuit show slight deviations from the simulation, but not enough to warrant re-tuning. All design targets were achieved with adequate margins.
The Cable TV output and DVB multiplex frequencies are indicated by the purple vertical lines.
Note: The measured data is for the finished boxed circuit, connected to an F receptacle.
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