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Synthesizing Diplexers Using GENESYS

Question asked by ROB_LEFEBVRE on May 27, 2003
Latest reply on May 27, 2003 by ROB_LEFEBVRE
A diplex filter is a three-port device with one common port and two output (or input) ports. Incident signals at the common port are delivered to one of the output ports depending on frequency. There are a number of uses for these devices such as using one antenna for both transmit and receive but at different frequencies (satellite earth stations) or launching and receiving signals onto one coaxial cable with forward and return paths at different frequencies (cable TV systems).

A triplexer has one common port and three output ports. A multiplexer has one common port and N output ports. A diplexer may separate the ports using a highpass and lowpass section, or a lowpass and a bandpass section, or two bandpass sections, etc. An infinite variety of combinations are available for diplex, triplex and multiplex filters.

Typical design goals for a diplexer might be:
1. Good return loss at the common port, either over the two desired passbands or perhaps over the entire frequency range of the device.
2. Low insertion loss from the common port to one output over its frequency range and to the second port over its frequency range.
3. High insertion loss (isolation) between the two output ports at all frequencies.
4. Other typical filter specifications such as group delay/phase linearity, temperature stability, power handling, etc.

Setting up a synthesis program for any combination is more trouble than it is worth. But it is easy to design multiplexers in GENESYS by simply designing the individual filters and connecting them together at the common port in SCHEMAX. By adhering to the following guidelines, the perturbation of these sections on each other are eliminated or at least minimized.

CONTIGUOUS VS. NONCONTIGUOUS
Consider a typical CATV application. The reverse direction subsplit frequency band might be 5 to 30 MHz. The forward direction frequency band might be 50 to 500 MHz. The diplexer is formed by connecting together a lowpass filter and a highpass filter at the common port. With a lowpass cutoff at 30 MHz and a highpass cutoff at 50 MHz, there is a 20 MHz wide guardband. These are noncontiguous passbands. If there were no guardband, i.e. if the 3dB cutoff frequencies are the same, the diplexer is contiguous.

Contiguous Butterworth multiplexers using ideal components have perfect return loss at the common port at all frequencies! Chebyshev multiplexers have good return loss at the common port at all frequencies when the contiguous cutoffs are near 3 dB attenuation. Noncontiguous multiplexers have good return loss in their respective passbands but poor return loss in the guard bands.

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When passbands are noncontiguous, then design the individual filter sections using conventional doubly-terminated techniques. If the passbands are contiguous, then design the individual filter sections singly-terminated. In this case the filters have a normal specified termination like 50 or 75 ohms at the output port but the ends of the filters which will be connected at the common port are designed for 0 ohms termination impedance. That this is the correct design procedure is certainly not intuitive but it works! Singly terminated filters are designed by FILTER in GENESYS.
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IMPEDANCE PROTECTION
Consider the two possible topologies at the common port of a lowpass/highpass diplexer. The first element may be series or shunt.
For normal filters these two topologies are identical and have the same response. However, consider what happens when filters with shunt elements first are connected together. Low frequencies signals which would be routed to the lowpass filter and out the top right port are shorted to ground by the highpass filter. It won’t work! The alternative topology using shunt elements first does not have this problem. A similar state of affairs exists for bandpass filters as well. Select bandpass sections with series resonators at the common port.  

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