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I have a task which is to design a circuit using microstrip transmission lines. this circuit should generate two output signals of equal power, one which is in phase with the input sine signal and the other 90 degrees out of phase with the input signal.

Given:

  • center frequency = 9.9 GHz
  • bandwidth => the wider the better
  • \$\varepsilon_r\$ for substrate = 12.5
  • characteristic impedance = 50 ohm
  • substrate thickness = 2.25mm

Any suggestions on how I can start this design? I have read some information about quadrature demodulator but I need extra help.

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  • \$\begingroup\$ At that frequency, a phase shift can be accomplished by a measured length of transmission line. Demodulator makes no sense here since you haven't told us of anything that is modulated or needs to be de-modulated. \$\endgroup\$ – Olin Lathrop Mar 15 '17 at 11:37
  • \$\begingroup\$ If you want a wide bandwidth, better to go with a quadrature coupler rather than line lengths. \$\endgroup\$ – Neil_UK Mar 15 '17 at 12:10
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Given your specifications a Lange coupler could be a good option although it's far from being the easiest one to manufacture. However, its very wide bandwidth (if that's what you need) offsets all of its disadvantages.

Lange coupler geommetry

PROS:

  • Compact layout (for its bandwidth).
  • Very wide bandwidth.
  • Low losses.
  • Strong coupling (good for power splitting).

CONS:

  • Requires wire bondings.
  • Requires a fine etching process.

This is a simulated example of the Lange coupler performance taken from an Agilent Design Note:

enter image description here

If you want to look at other options for quadrature coupling, take a glance at this as an starting point for further research on your own.

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