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Diplexer calculator

Why terminate what you do not want

A mixer only behaves well when it sees a resistive termination at all frequencies, not merely in the passband. An ordinary filter does not give it one: outside the passband it reflects everything back, and the mixing products return to the mixer to breed new ones. A diplexer splits the flow in two and terminates both halves — one into the wanted load, the other into a resistor.

from the mixerto the IFload50 Ωsplit at 9 MHz
The unwanted branch is loaded into a resistor — and that is what eats the products you do not want
System impedance (Ω)
Crossover frequency (MHz)
Order

📐 The formulas​

First order (two components per branch):

low-pass branch:  L = R/ω₀,   C = 1/(R·ω₀)
high-pass branch: the mirror image

Both branches are terminated in R, and the input impedance equals R at every frequency.

Third order — from a Butterworth prototype split across the two branches:

low-pass:  L1 = g₁·R/ω₀,  C2 = g₂/(R·ω₀),  L3 = g₃·R/ω₀
high-pass: C1 = 1/(g₁·R·ω₀),  L2 = R/(g₂·ω₀),  C3 = 1/(g₃·R·ω₀)

A check point:​

R = 50 Ω, f₀ = 9 MHz → L = 884 nH, C = 354 pF.

Where this calculator stops:
  • The unwanted branch must be terminated in a real resistor. Leave it open and the whole point of a diplexer is lost
  • That resistor dissipates real power: at the output of a transmit mixer this has to be allowed for
  • Coil Q is not accounted for: in a real circuit the input impedance is rather less constant
  • The model is ideal — in practice |Z| = R holds over roughly a decade either side of f₀

🎯 In practice​

  • After a diode mixer — the classic and most important use.
  • Between a mixer and a crystal filter: outside its passband the filter reflects everything, and the diplexer makes up for that.
  • Separating two bands in one signal path.
Practical advice:
  1. Checking a diplexer is easy: look at |Z| at the input on an analyser over a wide sweep. A flat line means it is right.
  2. Use a non-inductive resistor: an ordinary wirewound one will ruin it.
  3. Set the crossover at the working IF, so the wanted signal goes into the right branch.
  4. A diplexer noticeably improves a mixer's IP3 — you can see it in a two-tone measurement.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

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UR3PKI. «Diplexer calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/diplexer (licence CC BY-NC-SA 4.0).

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