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Transmission Line Designer >
Transmission Lines >
Offset-Coupled Stripline Transmission Lines

Offset-Coupled Stripline Transmission Lines

Keyword	Default	Unit	Description
W	Req (Phys)	m	Conductor width
S	Req (Phys)	m	Spacing between conductors
WC	0	m	Conductor offset
P	0	m	Physical length of transmission line
Z0	Req (Elec, Opt 1)	ohm	Impedance of the coupled lines
K	Req (Elec, Opt 1)	dB	Coupling coefficient (positive)
Zo	Req (Elec, Opt 2)	ohm	Odd-mode impedance of the coupled lines
Ze	Req (Elec, Opt 2)	ohm	Even-mode impedance of the coupled lines
E	0	deg	Electrical length of the coupled lines
Frequency	0	Hz	Analysis frequency
ER	Required		Relative dielectric constant
B	Required	m	Substrate thickness
TAND	0		Dielectric loss tangent
RGH	0	m	Rms surface roughness

Synthesis and Analysis

Two options exist for synthesis of offset-coupled stripline transmission lines:

— Specify impedance, Z0, and the coupling coefficient, K.

— Specify even-mode impedance, Ze, and odd-mode impedance, Zo.

• The conductor spacing, S, and substrate parameters B, and ER must be entered prior to clicking the Synthesis button. The width of the striplines, W, and the offset between the lines, WC, will be computed. Also, the alternate electrical option set will be computed (e.g., if Z0 and K are entered, Ze and Zo will be computed).

• For analysis, the parameters W, S, B, and ER must be entered prior to clicking the Analysis button. WC can be optionally entered, but will default to 0. The electrical properties, Z0, K, Zo, and Ze will be computed. The frequency will be used if entered; otherwise, 0 Hz will be used in the calculation.

• Conversion from electrical length, E, to physical length, P, can be performed by entering values for E and Frequency. Click Synthesis to compute P. Similarly, to convert from physical length to electrical length, enter values for P and Frequency and click Analysis to compute E.

Dielectric Substrates

A dielectric substrate is defined by the parameters B, ER, and TAND. The substrate is assumed to be lossless unless TAND is specified and greater than zero.

Conductor Metallization

• Conductor specification is performed in the Metallization control group. If the conductor is not specified, the conductor loss is zero and no thickness corrections are made to the structure’s propagation characteristics. Up to three conductors of different metals and thickness can be specified.

• Metallization rms surface roughness, RGH, can be specified for additional conductor losses due to imperfect metal surfaces. RGH is specified in terms of rms variation from an ideal flat surface.

Sweep Options

Parameters that can be swept for coupled offset stripline are:

Frequency
Width, W
Spacing, S
Offset, WC

The order shown is the order used to generate output data when multiple parameters are swept simultaneously. Refer to the Sweep Entries section for further information.

Limitations

The approximate limitations on strip dimensions are:

(1) Tight coupling case (i.e., when the two strips overlap, WC > 0):

and

(2) Loose coupling case (i.e., when the two strips do not overlap, WC < 0):

and

Analyzing very loose coupling (e.g., coupling < 36 dB) will require a large number of iterations and the results may not be accurate. Synthesis, however, will be accurate.

Example

To select the offset-coupled stripline medium, select TRL on the Product menu, click Stripline, and click OCL. Set the units to mm and GHz. We will use the following parameters to synthesize a 6-dB coupler at 10 GHz:

Impedance, Z0: 50 ohms

Coupling, K: 6 dB

Frequency: 10 GHz

Substrate thickness, B: 1.27 mm

Dielectric constant, ER: 2.2

Metal: Cu 0.01 mm

Spacing, S: 0.2 mm

Click the Synthesis button to determine the coupler’s parameters:

Width, W: 0.728 mm

Offset, WC: 0.324 mm

Odd-mode impedance, Zo: 28.82 ohms

Even-mode impedance, Ze: 86.74 ohms

To specify a 1/4 wavelength line, enter:

Electrical length, E: 90

Click Synthesis again to determine the corresponding physical length:

Physical length, P: 5.05 mm

A sweep analysis can be performed on W, S, WC, or Frequency. To perform a sweep analysis on W, type 1,1.5,0.05 in the W field to specify a sweep from 1.0 mm to 1.5 mm in steps of 0.05 mm. Clicking Analysis then yields following results:

Offset Parallel Coupled Strip Lines

B = 1.270mm ER = 2.20

Freq WC WC/B S S/B W W/B ZE ZO

ghz mm mm mm Ohms Ohms

10.0 0.324 0.255 0.200 0.157 1.000 0.787 67.08 25.93

10.0 0.324 0.255 0.200 0.157 1.051 0.828 64.65 25.16

10.0 0.324 0.255 0.200 0.157 1.102 0.868 62.38 24.44

10.0 0.324 0.255 0.200 0.157 1.153 0.908 60.59 23.68

10.0 0.324 0.255 0.200 0.157 1.204 0.948 58.60 23.04

10.0 0.324 0.255 0.200 0.157 1.255 0.988 56.73 22.44

10.0 0.324 0.255 0.200 0.157 1.306 1.028 54.98 21.86

10.0 0.324 0.255 0.200 0.157 1.357 1.069 53.33 21.32