NASA Technical Memorandum 102289

Channelized Coplanar Waveguide

Pin-Diode Switches

G.E. Ponchak

Lewis Research Center

Cleveland, Ohio

and

R.N. Simons

Case Western Reserve University

Cleveland, Ohio

November 1989

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CHANNELIZED COPLANAR NAVEGUIDE PIN-DIODE SNITCHES

G.E. Ponchak

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135

and

R.N. Simons*

Case Nestern Reserve University

Cleveland, Ohio 44106

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SUMMARY

Three different types of p-i-n diode, reflective CPW switches are presented. The first two switches are the series and the shunt mounted diode

switches. Each has achieved greater than 15 dB of isolation over a broad bandwidth. The third switch is a narrow band, high isolation switched filter which

has achieved 19 dB of isolation. Equivalent circuits and measured performance

for each switch is presented.

INTRODUCTION

Coplanar waveguide, CPW, on a dielectric substrate consists of a center

strip conductor with semi-infinite ground planes on either side (ref. I).

Channelized coplanar waveguide, CCPW, consists of CPW transmission llne placed

in a metal enclosure (ref. 2). Because the ground planes and the center conductor are on the same side of the substrate, shunt as well as series mounting

of circuit components can be done without the need for wraparounds or viaholes. The improvements in circuit yield and the reduction in inductance for

ground paths over microstrip based circuits should permit microwave integrated

circuits, MlC's, to be fabricated at higher frequencies and less expensively.

However, the extent of applications of CPW circuits is limited due to the

unavailability of circuit elements and models which can be incorporated into

CAD programs.

Microwave switches are a basic circuit element for phase shifters and

radiometers. A CPW switchable attenuating medium propagation, SAMP, switch

has been demonstrated by Fleming et al. (ref. 3). This device is useful for

GaAs MMIC circuits but it is not easily incorporated into MIC's on oassive substrates such as alumina or duroid. P-i-n diodes are good microwave switches

since the impedance of the diode can be changed from a very high value to

nearly zero in a short time (refs. 4 and 5).

This paper presents for the first time CPW D-i-n diode, reflective

switches. Three basic switches are oresented. The first is a shunt mounted

diode switch. This switch is similar to fin line shunt mounted diode switches

(ref. 4). The second switch is a series mounted diode across a gap in the center strip conductor. The last switch is a novel design which converts a CPW

interdigital coupler with bandpass filter characteristics into a spurline,

bandstop filter.

*NASA Resident Research Associate at Lewis Research Center.
The three swltches have been fabricated on CCPWtransmission llnes. All

of the circuits have been fabricated on RT/Duroid 5880 substrates w_th Metellcs

Corporation beamlead diodes, MBP-IO30-BII. Figure I is the equlvalent clrcuit

7 REVER FORWARD

BIAS yK-" BIAS p

TCj IrF 

TYPICAL VALUES

SUPPLIED BY

MEIELICS

Rs O,2fl

RI 6.011

Lp O.I nH

CT:Cj_ Cp O.03pF

FIGURE I. - P-I-N DIODE EQUIVALENT CIRCUIT AND TYPICAL CIRCUIT

ELEMENT VALUES.

of the diode with the circuit element values supplied by Metelics. Although no

tuning to resonate off the diode parasitlcs was done, the CPW slots were made

equal to the length of the packaged diode to minimize the package inductance,

Lp. Testlng of the switches has been done on an HP 8510 automatlc network analyzer with bias tees to supply the dc bias to the diodes. The test fixture is

comprised of a 2 in. length of CCPW with a 0.045 In. center strip and 0.010 in.

slot. Connectlon to 3.5 mm coax cables is made through a pair of coaxial connectors. Tuning notches in the ground plane have been used to improve the

coax-to-CCPW characterlstlcs over selected frequency bands. The test fixture

has a total Insertlon loss of 0.5 dB and a return loss greater than 15 dB for

the frequencles reported In this paper.

P-I-N DIODE SHUNT SWITCH

In a shunt mounted configuration, a pair of diodes are placed In parallel

across the slots of the CCPW transmission llne (fig. 2). When the diodes are

1.... _,glq--- "I

FIGURE 2. - SCIIEMATICAND EQUIVALENT CIRCUIT OF CPW P-I-N

DIODE SIIUNISWITCII.
forward biased, each slot Is loaded by the forward blas impedance of the diode;

at ]0 GHz, the impedance across the slot is approximated by Rf + jeLp = 8.7

<< Zo. The shunt impedance for CPN, with two parallel slots, is therefore

4.35 Q. This low impedance loading the slot reflects the propagating wave.

Hhen the diode is reverse biased, each slot is loaded by an impedance approximated by I/(juCt) = 530 _ >> Zo. This is an equivalent shunt impedance of

265 Q for CPH. This load results in a small attenuation. Using expressions by

Natson (ref. 6) modified for two parallel shunt elements, an isolation of 16 dB

and an insertion loss of 0.04 dB is predicted.

S 2 :]. Iog MAG

REF 0. @ dB

10. @ riB/

C +--_ ..... i

........ 1

(a) MEASURED INSERTION LOSS AND ISOLATION.

$11 Io 9 MAG

REF 0.0 dB

10.0 _B/

,Prn-ei_eEpHuszr_w__eH-P Ct! ............... i -_

= r-................ T- t.... ; ........f......... ? ---

' ' 4

L ........... ! ! +--i .............. i..... 4

i = ; i ____£ .........

_-----; .... SwITCH-0FFL - I _

I_T----,-/ -7 _ 1

, , + I i ', E SWITCH-ON r

t-----'-__}x-, (0.0 MA) 'r ............. ---;-.......... '----' --_

;........ __ ..... _.. :...... •....... : ..... t..... i--

START 8. 0000000_D GHz

STOP II.0_0000000 GHz

(b) MEASURED RETURN LOSS.

FIGURE 3. --CPW P-I-N DIODE SHUNT SWITCH WITH TWO PAIRS OF

DIODES.

3
An insertion loss less than I dB and an isolation of 15 dB has been measured over the frequency band of 8 to 11GHz. The return loss was less than

10 dB across the band. By incorporating more than one pair of diodes across

the slots with a _g/4 separation, higher isolation can be achieved. Typically, two pairs of diodes have resulted in an Isolation of 30 dB over the 8 to

11GHz frequency band with little Increase In insertion loss, this is shown in

figure 3(a). Figure 3(b) shows the return loss which is less than I0 dB across

the band.

P-I-N DIODE SERIES SWITCH

In the series mounted diode conflguration, a diode is mounted across an

0.008 in. gap in the center strip conductor of the CCPN line (fig. 4). The

J

c, Tc,

FIGURE 4. - SCtlEPLATICAND EQUIVLENT CIRCUIT OF CPWP-I-N I)IOI)E

SERIES SWITCtl.

center strip has been tapered to the width of the beam lead diode to provide a

better match to the width of the diode package. The gap appears as an equivalent capacitive _ network (ref. 7). Nhen the diode is forward biased, the

coupling capacitance is shorted by the low diode impedance and the wave is

transmitted. Reverse biasing the diode results in an impedance across the gap

which can be approximated by I/[jm(Ct + Cc)] >> Zo at I0 GHz. Therefore, the

propagating signal is reflected as if from an open circuit. A measured insertion loss of I dB and an isolation of 15 dB has been obtained from 0.045 to

8 GHz (fig. 5(a)). The return loss over this band is less than 10 dB

(fig. 5(b)). The gap impedance is not large enough at higher frequencies to

provide good isolation. The gap can be lengthened to decrease Cc but the

increase in inductance from the longer diode leads will ultimately limit the

gap separation. Resonating out these reactances is required for higher frequency operation (ref. 6).

4
$21

REF 0.0 dE)

10. @ dE)/

_IN

7

i_ i =-- r

__SWITCH-OfF

Iog NAG

rODE SERIE_ SWl rCH

I./ r

(a) I_I_ASURED INSERTION LOSS AND ISOLATION.

_____ SWITCH-ON_ __

(0.7 MA)--\ / -INSERTION LOSS-- J,, )

I't

-- ISOLATION --

S11

REF 0.0 dB

5.0 d8/

_IN D

Iog MAG

[ODE ERIE _ SWI

=SWITCH-OFFJ____

// (0.0 _)

d t

I

F SWITCH_ON

,_x0'7 _)

/ -

START 0.045000000 GHz

STOP 8.000000000 GHz

(b) _ASURED RETURN LOSS.

FIGURE 5. - CPW P-I-N DIODE SERIES SWITCH.

P-I-N DIODE SPDT SNITCH

To realize a SPDT switch, a CCPN Tee-junction with gaps in the center

strip conductor at the junction was formed (fig. 6). Diodes were mounted

across the gaps in parallel. As shown in figure 7, the measured insertion loss

is l dB and the isolation is greater than 15 dB over the octave bandwidth of

2.25 to 5.5 GHz. The return loss was less than lO dB.
PORI #2

_ PORT It_

PORT 111

FIGURE G, - SCIIEMMIC AND EQUIVALEN1 CIRCUIT

OF SERIES MOUNTED P-I-N DIODE CPW SPDT

SWI TCII.

c

$21 I o g HAG

RE_" 6'. 0 dB

5.0 dB/

SWITCH-ON

----(0.7 MA)_-_ _INSErTION lOSS ..... I---

START 2. 250@0000@ GHz

STOP 5. 580®_@®8_ GHz

FIGURE 7. - EASURED INSERTION LOSS AND ISOtATION OF SERIES

MOUNTED PIN DIODE CPW SPD[ SWITCH.

P-I-N DIODE SWITCHED-SERIES-STUB SWITCH

A diode Is mounted across the open end of a Xgl4 stub which is in series

with the center strip conductor of the CCPW as shown in figure 8, When the

diode is unbiased, the stub Is terminated in an effective open clrcuit and

therefore appears as a series short circuit. Hence, the wave propagates with

negllglble attenuation. This is the on-state of the switch. When the diode

is forward biased, the stub is terminated in an effective short circuit which

[ ;: J

F

I)IODE UNBIASED DIODE FORWARD BIASED

FIGURE 8. - SCHEMMIC AND EQUIVALENT

CIRCUIT OF CPW P-I-N DIODE SWITCIIEDSERIES-STUB SWITCH.
therefore appears as a series open circuit. The wave is therefore reflected;

this is the off-state of the switch. The diode reactances result in an effective lengthening of the stub and can easily be compensated for. A measured

insertion loss of 1.0 dB and an isolation of 19 dB has been obtained at 9 GHz

(fig. 9(a)). Figure 9(b) shows the return loss.

1;

14

S21 log MAG

REF 0.0 dB

5.0 dB/

IN DIODE _WlTCH i

i

I [SWITCH-ON

[-(o.o _>_--

I

I-INSERTION LOSS

7 /! !

---- --ISOLATION

SWITCHIOFF ____/

(0.7 jMA)_' I

(a) MEASUREDINSERTION LOSS AND ISOLATION,

SII log MAG

J

START 8.000000000 OHz

STOP 10.000000000 GHz

(b) MEASURED RETURN LOSS.

FIGURE 9. - CPW P-I-N DIODE SWITCHED-SERIES-STUB SWITCH.

CONCLUS IONS

P-I-N diode reflective switches are easily realizable on CCPW transmission

line and therefore CPN transmission line. Each of the three type of switches

presented are practical for specific applications. The performance of the
switches could be improved through typlcal tuning of the diode reactances once

the necessary CPWcircuit models becomeavailable.

REFERENCES

1. Wen, C.P., "Coplanar Waveguide, A Surface Strip Transmission Line Suitable

for Nonreciprocal Gyromagnetic Device Applications," 1969, IEEE Trans.

Microwave Theory Tech., vol. MTT-17, pp. 1087-1090.

2. Simons, R.N., Ponchak, G.E., Martzaklis, K.S., and Romanofsky, R.R.,

'IChanne]ized Coplanar _aveguide: Discontinuities, Junctions, and

Propagation Characteristics,: 1989, IEEE MTT-S International Microwave

Symposium Digest, IEEE, Piscataway, NJ, pp. 915-918.

3. Fleming, P.L., Smith, T., Carlson, H.E., and Cox, W.A., "GaAs SAMP Device

for Ku-Band Switchlng," 1979, IEEE Trans. Microwave Theory Tech.,

vol. MTT-27, pp. 1032-1035.

4. Bhat, B., and Koul, S.K., Analysis, Design and Applications of Fin Lines,

Artech House, Norwood, MA, 1987.

5. White, J.F., Microwave Semiconductor Enqineering, Van Nostrand Reinhold

Co., New York, 1982.

6. Watson, H.A., Microwave Semiconductor Devices and Their Circuit

Applications, McGraw-Hill Book Company, New York, 1969.

7. Slmons, R.N., and Ponchak, G.E., "Modeling of Some Coplanar Waveguide

Discontinuities," IEEE Trans. 1988, Microwave Theory Tech., vo1. MTT-36,

pp. 1796-1803.
Nalional Aeronauhcs and Report Documentation Page

Space Administration

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.

NASA TM- 102289

4. Title and Subtitle

Channelized Coplanar Waveguide Pin-Diode Switches

7. Author(s)

G.E. Ponchak and R.N. Simons

9. Performing Organization Name and Address

National Aeronautics and Space Administration

Lewis Research Center

Cleveland, Ohio 44135-3191

2. Sponsoring Agency Name and Address

National Aeronautics and Space Administration

Washington, D.C. 20546-0001

5. Report Date

November 1989

6. Performing Organization Code

8. Performing Organization Report No.

E-4959

10, Work Unit No.

506-44-2C

11. Contract or Grant No.

13. Type of Report and Period Covered

Technical Memorandum

14. Sponsoring Agency Code

15. Supplementary Notes

This report is an expanded version of material presented at the European Microwave Conference cosponsored

by the IEEE, IEE, URSI, and EUREL, London, England, September 8, 1989. G.E. Ponchak, NASA Lewis

Research Center; R.N. Simons, NASA Resident Research Associate from Case Western Reserve University,

Cleveland, Ohio 44106.

16. Abstract

Three different types of p-i-n diode, reflective CPW switches are presented. The first two switches are the series

and the shunt mounted diode switches. Each has achieved greater than 15 dB of isolation over a broad

bandwidth. The third switch is a narrow band, high isolation switched filter which has achieved 19 dB of

isolation. Equivalent circuits and measured performance for each switch is presented.

17. Key Words (Suggested by Author(s))

Microwave transmission lines

Diodes

Coplanar transmission line

18. Distribution Statement

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