NATIONAL RADIO ASTRONOMY OBSERVATORY

GREEN BANK, WEST VIRGINIA 24944

ELECTRONICS DIVISION INTERNAL REPORT No. 181

CRYOGENICALLY-COOLED C-BAND PIN DIODE SWITCH

GEORGE H. BEHRENS, JR,

OCTOBER 1977

NUMBER OF COPIES: 150

CRYOGENICALLY-COOLED C-BAND PIN DIODE SWITCH

George H. Behrens, Jr.

Introduction:

This report deals with a cryogenically-cooled, C-band, PIN diode switch

developed for radio astronomy applications. The various switching modes

available, principles of operation, operating characteristics and construction

details are discussed.

The switch was developed to satisfy the Dicke switching requirements for

a new law noise, dual band (25 cm and 6 cm), dual channel, cryogenic radiometer

under construction at NRAO. The design goal was to achieve a switch with mini-

mum loss, VSWR and noise contribution which could operate at cryogenic tempera-

tures over the frequency band from 4.5 - 5.1 GHz and provide the necessary

Dicke switching functions as dictated by the various types of astronomical

observations.

[1]The Dicke Radiometer:

In radio astronomy the signal levels received are of such law levels that

receiver gain instabilities can seriously limit the effectiveisensitivity of

the radiometer. This effect can be minimized by using the Dicke radiometer

as shown in Figure 1A. The receiver input is continuously switched between

the antenna feed horn and a comparison noise source at a frequency high enough

so that the gain has no time to change during one cycle.

The detected output V o of the Dicke radiometer is proportional to the

difference between the antenna noise temperature TA and the noise temperature

of the comparison noise source T :

V o = K (TA - Tc), (1)

[1] J. D. Kraus, Radio Astronomy. New York: McGraw-Hill, 1966.

2

and the minimum detectable signal is given by the following expression if

TA = T

C

AT

min = 2 T

RA/P.T(2)

where:

TR = receiver noise temperature, °K.

= predetection bandwidth, Hz.

T = integration time, sec.

In practice the comparison noise source is usually either a microwave

termination held at a fixed physical temperature or another antenna feed horn.

When a cryogenic receiver is used, the termination is usually held at the op-

erating temperature of the refrigerator (e.g., 20°K). Antenna temperatures

are usually around 20°K also, so TA T.21; T If there is a significant unbalance

C

between TA and TC' noise can be injected via a directional coupler, into the

colder channel to balance the system or the receiver gain can be reduced dur-

ing the higher temperature half of the switch cycle.

When a feed horn is used as the comparison noise source, it is usually

identical to the signal feed horn. Both horns are then offset laterally by

equal amounts from the focal point of the reflector insuring equal noise

temperatures for both feeds. This method has the advantage that fluctuations

in antenna temperature caused by atmospheric conditions are effectively can-

celled since the beams of both feeds see essentially the same area of the

atmosphere because their beams are usually only on the order of 2-3 half power

beam widths apart. However, when observations are made of extended sources,

it is advantageous to use the switch load mode of operation; otherwise, both

RF

IF

0SE

CTI

ON

SQU

ARE

LAW

DET

ECTO

R

TA

vIN=

K ( T

A ---

Tc

SYN

CH

RO

NO

US

DET

ECTO

RIN

TEG

RAT

OR

• .1■

1111111111w

wim

•■■■■

■■•)

AN

TEN

NA

FEE

DH

OR

N

CO

MPA

RIS

ON

NO

ISE

IS

OU

RC

E L

SW

ITC

H I

__D

RIV

ER

FIG

. lA

SIN

GLE

CH

AN

NEL

DIC

KE R

AD

IOM

ETER

INTE

GRA

TOR

ps..

..■4

1111

1

AD

DER

1V:D

EO•

AM

P

TRAN

SFE

RSW

ITCH

1.1■1.1

11110

SYN

CHRO

NO

US

DET

ECTO

R.1■

0411

10RF &

IF

SECT

ION

SIG

NAL

AN

T.

FEED

HO

RN

TA

Tc

SQU

ARE

LAW

DET

ECTO

R

SWIT

CHD

RIV

ER

SQU

ARE

LAW

DET

ECTO

R

SYN

CHRO

NO

US

DET

ECTO

R

VID

EOAM

Prim

emen

nem

s111011

CO

MPA

RIS

ON

AN

T.FE

ED

HO

RN

INTE

GRAT

OR

RF8II

FSE

CTIO

N

FIG

. 1B

DU

AL C

HAN

NEL D

ICKE R

ECEIV

ER U

SIN

G T

HE S

WIT

CH

BEAM

CO

NFIG

URATIO

N

beams will be on source and the output will be distorted. It is therefore

desirable in the design of a radiometer to incorporate a means to provide

both a beam switching mode and a load switching mode.

Dual Channel Operation:

In the single channel Dicke radiometer the signal power is observed only

half of the time. However, by switching the signal feed horn between two re-

ceivers and adding their outputs as shown in Figure 1B, the efficiency of the

radiometer is increased. It can be shown that the minimum detectable signal

noise temperature of a dual channel Dicke radiometer is reduced by a factor of

over a single channel radiometer. In order to achieve the necessary switch

action for a dual channel Dicke receiver using the beam switching mode a trans-

fer switch is required.

Dicke Switch Design Considerations:

One of the major disadvantages of a Dicke radiometer is the degradation

of system noise temperature due to the noise temperature of the Dicke switch.

The amount of noise added to the system by the loss mechanism of the switch is:

ATs

= (TP + T

R) - 1) (3)

where:

AT

•

noise added to system due to switch.

• physical temperature of switch.

= loss of switch.

TR = receiver input noise temperature.

11•11•11111,1

10

fD

rive

Mot

or, lo

ft'

hcin

eP

Gra

rnps

Cha

nnel

tipco

nver

ter

Ing1N

RC

EE

ME

NIE

MM

O:,

,,:,

,

$0

01

11

6::

.

Noi

seC

ontr

ibut

ion

Mea

sure

men

tsF

IG.

83A13038

CIV

O1

01

w39 /

w3g

Z

dad H

aLIM

S U

N3 IN

O8A

191,,d

C133d01

nis

ON

INill

3A110110N

1enis o

Nm

ini

3A1110V

ci VO

1100 SV

19Atig,

S4dVO

ssvdm

i dd

1:13111:1N

OIS

S3eiddnsS

IN 3

3AIIIT

MV

O

d IN V

Vd

01

830,180:4XU

ft

S 3-111GO

IN'M

S 3

001

0N

id 1.012dHSSN

O:18

aitild

01

00

39V3 1-1011PA

S

1104ei3dd00 "zo

03.1.V1d 0100

S4 dV

0 ON

1>4001930 Ad 2'2

01810313100

99

gacm

Anoo/n01

a J 3d01

afn01

8

Therefore, to minimize AT s , both the physical temperature T p and the loss L

of the switch should be kept to a minimum. In the 25/6 cm radiometer, -Co

minimize Tp the Dicke switch is mounted directly to the 20°K station of the

refrigerator as shown in Figure 2.

In planning the 25/6 cm radiometer, it was decided that a dual channel

Dicke radiometer be implemented. It was also decided that the Dicke switch,

besides providing the transfer switching function needed for dual channel op-

eration, should also incorporate a means to permit either load switching or

beam switching for either channel. Further, it was decided that the switch

be used to connect the L-band upconverters to the parametric amplifiers dur-

ing 25 cm operation. Dicke switching is not needed during 25 cm operation

because only line observations are performed and they are relatively un-

affected by gain instabilities.

TABLE 1

RF Paths thru Switch for Various Switching Modes

No. Switching Mode

Connections Made DuringSwitching Cycle

First HalfSwitching Cycle

Second HalfSwitching Cycle

,

I. Both Channels 2 4- A 1 4. ALoad Switching 5 ÷ B 3 ± B_

II. Both Channels 2 4- A 5 ± ABeam Switching 5 4. B 2 --->- B

III. Channel A Beam Switching 2 4- A 5 ÷ AChannel B Load Switching 5 4- B 3 .--''' B

IV. Channel A Load Switching 2 4- A 1 -÷ AChannel B Beam Switching 5 4. B 2 ÷ B

V.Both ChannelsLocked to Upconverters

6 4. A4 --->. B

No switching.

PIN

A D

IOD

ESW

ITCH

Para

mp

5

LOAD

TOM

IXER

Spa

ram

p

tro

4.5

— 4

,95

GH

zO

UTPU

T

f- L

—BAN

D.k

U/C

/\ip

conve

rters

Dua

l Mod

eA

1.0

ToT

rans

duce

r1.

40 G

Hz >

I

Ho V

ER

T,

IFE

ED

Pola

riza

tion

10.

HO

RIZ

e Po

lariz

atio

n

U/C

FEED

1>

A

4.4—

5,2

G H

zD

UAL

BEAM

FEED

FEED

LOAD A

L CRYO

GEN

IC D

EWAR jjl

Cor

n(Ene

nts

At 1

5°K

)

FIG

.4 B

lock

Dia

gram

Of T

he F

ront

End

Sec

tion

Of T

he 2

5/6c

m C

ryog

enic

Rad

iom

eter

10

The switch is a combination transfer switch and two SP4T switches which

provides various switching modes best described by referrring to Table 1 and

Figure 4. As shown in Table 1 there are five different switching modes avail-

able. The table shows the connections for the various modes for each half of

the switching cycle, e.g., in mode I (both channels load switching), Feed A

is connected to Paramp A and Feed B is connected to Paramp B during the first

half switching cycle. During the second half of the switching cycle, loads A

and B are connected to Paramps A and B, respectively.

Principles of Operation:

Referring to Figure 5, the operation of the switch in the Beam Switch Mode

can be explained as follows. Assume RF energy is propagating from Feeds A and

B towards junctions Jl an J2, respectively, and diodes D1 thru D8 are biased

as shown during the first half switching cycle. Next consider how the energy

from Feed A divides at Jl due to the impedance conditions at Jl as seen look-

ing towards JA and JB. Since D3 is forward biased, its low impedance (see Fig-

ure 6A) is transformed to a high impedance at Jl by the quarter wavelength

50 ohm transmission line according to the usual quarter wave transformer

equation:

.2/ (4)

ZINZo Zd

where:

0 characteristic impedance of line.

Zd = forward biased diode impedance.

However, the impedance at J1, as seen looking towards JA, is close to

50 ohms since D2 is reverse biased and presents little affect on the line

since its impedance is high (see Figure 6B). Also, diodes D1, D7 and D8 are

Z O

N c

° 1

44—

X/4

ZO

FF= L

O =

50S

/ -_-.

3-1

5012

ON

, O

FF

6cm

FEED

A

20°K

LOAD

A

25cm

FEED

25cm

FEED

HOR.

POL.

VER.

POL.

NO

TE:

1 )

Bia

s Con

ditio

ns

For

Sw

itch

ing D

iodes

Are

Encl

osed

By

Rec

tangle

s. B

ias

Conditio

ns

Of

Oth

er D

iodes

Rem

uin

s The

Sam

e Fo

r Both

Hal

ves

Of

Sw

itch

Cycl

e.

2)

Solid

And D

ott

ed R

ect

angle

s In

dic

ate

Bia

s Conditio

ns

During F

irst

And S

eco

nd H

alv

es

Of Sw

itch

Cycl

e,

Resp

ect

ively

.

3)

Solid

And D

ott

ed S

ignal Path

s In

dic

ate

Sig

nal Path

s D

uring F

irst

And S

eco

nd H

alv

es

Of Sw

itch

Cycl

e,

Resp

ect

ively

.

FEED

B6c

m

FIG

. 5

Sim

pli

fied

Sch

emat

ic O

f F

ront

End S

wit

ch O

per

atin

g I

n T

he

Bea

m S

wit

chin

g M

ode.

12

Ii I I

/ ii !

/

14‘' 3.8mm-01.- 3.8Mnrel L p Rp R p Lp 1 1,,e‘____4_4::,I I

150pH On On 150pH

Zo =50aRs 0.841

Zo= 50aL I 20pH

(a) Foward Bias

l it '

I 0 1

2 I I /1-2 1I.*. 4.4m m -01 14- 4.4mm-01I I l_p Rp Rp Lp

1 1

o 0-1-YY1 (YY1-----<) o200pH On OA 200pH

R 2 oicaL2CT T,14pF

0

( b) Reverse Bias

FIG.6 Room Temperature Equivalent Circuit OfHP5082-3141 PIN Diode

Zo= 501/ opH Zo= 50D,

Mode OOOOOOO .

1stHalf of sw. cycle

D1 .. FWD Fwd Fwd

Fwd FwdREV

Fwd FwdFwdREV

FWD REV FWD REV

Fwd Fwd FWD

D2 ..

D3 . •

D4 • • FWD Fwd Rod Fwd

D5 • • Fwd Fwd Fwd Fwd

Fwd FwdFWDREV FWD REV FWD REV FWD REVD6 • •

Fwd

Fwd Fwd Fwd

Fwd Fwd Fwd

Fwd

D7 • •

D8 • •

IVIII

1st 2nd

13

forward biased causing the impedance looking into each of these arms from JA

to be high. Therefore, all the energy entering Jl from Feed A is directed to

Paramp P.A. "A!' except for the relatively small losses absorbed in the high

impedance arms of junctions JA and Jl.

The energy arriving at J2 from Feed B can similarly be analyzed to show

that it is directed to P.A. "B". During the next half switching cycle the

bias conditions of D2, D3, D6 and D7 are reversed causing the RF energy to be

directed from Feed A and B to P.A. "B" and "A", respectively. The operation

of the switch in the other modes can be explained similarly by referring to

Table 2 which gives the bias conditions for the various switching modes.

TABLE 2

Bias Condition of Diodes for Different Switching Modes

Capital letters are used to indicate bias condition of diodes switching in that

particular mode. Bias conditions of diodes in a fixed bias state are italicized.

BIA

S 2

:CT1

C2

Sym

etri

c A

xes

Are

Sam

e A

sA

xis

Sho

wn

NO

TE:

BIA

S 8

All

Tra

nsm

issi

on L

ines

Are

Bal

ance

dS

oli

d D

iele

ctri

c S

trip

line

Const

ruct

edF

rom

lo

z. C

op

per

Cla

d D

uro

id 5

88

0

FIG

S?

Dia

gra

mati

c L

ay

ou

t O

f S

wit

ch

15

The complete schematic of the switch is shown in Figure 7. Table 4

gives the values of the various components and the manufacturers' part number.

The 50 ohm transmission line system used is balanced stripline con-

structed from photographically etched, copper clad, .0625" thick, teflon/

fiberglass. The ground planes are formed by the gold plated brass case (see

Figure 3). Channels 0.300" wide by .062" deep were milled in the switch case

to accommodate the stripline material. This type of construction was used to

maximize isolation between the various arms and achieve good mechanical sta-

bility. The ground plane spacing is 0.125" and the gold plated copper (1 oz)

center conductor is 0•100" wide for Zo = 50 ohms. A small lip was left along

the edge of all channels to improve mating surfaces and minimize radiation

leakage.

The switching diodes, HP 5082-3141, are silicon PIN diodes incased in a

50 ohm hermetic package which permits a continuous transition in the 50 strip-

line circuit. This stripline package concept, according to Hewlett-Packard,

overcomes the limitations in insertion loss, isolation and bandwidth that are

imposed by the package parasitics of other discrete devices.

The photograph in Figure 3 shows the physical layout of the various

components.

16

TABLE 4

Component Data

Components Value Function Manufacturer's P/N

C i 3.3 pF Blocking capacitor ATC-100A-3R3-P-50

C 2 56 pF Bypass capacitor ATC-100A-560-K-P-50

C 3 0.05" x .075" Capacitive tap ••••• •■■■•

L ___ Bias Injection coil Pionics 9 1/2 T-47

LPF ___ EMI suppression filter Erie #1250-003

Dl-D8 ___ PIN diode HP 5082-3141

Striplinematerial c = 2.23 R/T Duroid 5880

2, 1 Zo = 50 0 Transmission line56 = .27"

162 Zo = 50 0 Transmission line= .573

2, 3 Zo = 200 0 Inductive tuning stub ••••

= .700"

2'4 Zo = 200 Q Capacitive tuning stub •■•■■

56 = 0.195

2, 5 Zo = 41 Q Impedance transformer56 = .36"

Zo = 41 Q

Impedance transformer= .55"

1•11■1

FEED A

FEED BLOAD A

.. aft

.0•1111..4•■••

.0 ammo. • awrow■ II0

...................a • • IP

•■■

• • •

U/C A• asies:a •

• •

•••■ • ...40 ••• • r ..jr•.. • .• •

RECEIVER PASS BAND

1111■10

LOAD B FEED BU/C B

• • • 111, IP • • • a. I • • IP • •Er •

6---RECEIVER PASSBAND-1 FEED A

17

4.0 4.2 4.4 4,6 4.8 5,0 5.2 5.4 5.6 5.8 6.0

FREQ.— GHzFIG. 8 I NS E RTION LOSS PARAMP "A"—÷ INDIVIDUAL PORTS T= 300°K , I=20mA/DIODE

-0 0.5

.„„

°N.

111111...1

6.02,0- I I I I I I I I I

4.0 4.2 4.4 4.6 4.8 5.0 5.2 5,4 5.6 5.8FREQ.— GHz

FIG. 9 INSERTION LOSS PARAMP "B" INDIVIDUAL PORTS T=300°K, I=20mA/DIODE

_.-_—_—__. .......

LOAD A

I ....- ..10-17 • l'—f—dr.Z.

41\_ FEED Al

_

U /C A NZ-FEED B

•

18

1.5 -4.0

14—RECEIVER PASSBAND—olI1I I 1 I I I I I I I

4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6

FIG. 10 INSERTION LOSS PARAMP "A" INDIVIDUAL PORTST= 18°K, I= 150mA/DIODE

5.8 6.0

03

cocn0 0.5-J

0

cr I .0

cn

1.54.0

U/C BLOAD A

01111•1•11

1lo--RECEIVER PASSBAND---.1II

I I1 I I I 1 I I I 4.2 4.4 4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0

• dir . .... .... .....auld "I'd dr. ar.

r

— • FIE E D A

FEED B

FIG. II INSERTION LOSS PARAMP "B" INDIVIDUAL PORTST =18°K, I = 150mA / DIODE

19

TABLE 5

Measured Noise Contribution due to Dicke Switch

Feed A Feed B°K °K

Channel A

Continuum, 4.245-5.040 12.8 14.9

4.6 GHz, 3% bandwidth 10.2 13.3

4.75 Gllz, 3% bandwidth 15.2 16.9

4.870 GHz, 3% bandwidth 11.8 13.0

4.920 GHz, 3% bandwidth 13.0 10.0

Average .................. 12.55..... 13.5

Channel B

4.245 - 5.040 17.4 14.3

4.6 GHz, 3% bandwidth 13.7 9.6

4.75 GHz, 3% bandwidth 18.9 16.76

4.870 GHz, 3% bandwidth 17.8 12.2

4.920 GHz, 3% bandwidth 17.5 14.7

Average . • OOOOOOOOOO • 17.0 13.3

A summary of the measured switch characteristics are given in Tables 6 and 7.

20

TABLE 6

Switch Characteristics over 4.4 - 5.1 GHz Band

Channel A Channel B

Maximum insertion loss (dB) 0.75 0.75

Maximum noise contribution (°K) 16.9 18.9

Average noise contribution (°K) 13.0 15.2

Maximum VSWR 1.5:1 1.5:1

Minimum isolation (dB) 30 30

TABLE 7

Switch Characteristics over 4.0 - 6.0 GHz Band

Channel A Channel B

Maximum insertion loss (dB) 1.1 2.0

Noise contribution Not measured Not measured

Maximum VSWR < 2.0:1 < 2.0:1

Minimum isolation (dB) 30 30

21

Performance:

Measurements were made to determine switch insertion loss, isolation,

reflection coefficient and noise contribution at both 18°K and room tem-

perature conditions. Results of the insertion loss measurements are shown

in Figures 8 thru 11. At 18°K the switch has a maximum loss of 0.75 dB over

the operating band of the receiver. Isolation measurements, made in the 4.0-

6.0 GHz band, revealed greater than 30 dB isolation between ports. VSWR mea-

surements indicate a maximum VSWR of 1.5:1 in the pass band of the receiver

and a maximum of 2.0:1 from 4.0 - 6.0 GHz. Noise contribution due to the

switch was measured at various frequencies across the band and are as shown

in Table 5.

Discussion of Results:

The theoretical noise contribution AT of the switch can be calculated

from equation (3). Inserting the measured value of switch loss, 0.75 dB, and

an assumed physical temperature of 18°K (temperature at which refrigerator

was operating) into equation (3) gives a AT s = 7.8°K. However, the average

measured value of AT is 14.1°K (average of values given in Table 5) and is

6.3°K greater than that calculated by equation (3). This discrepancy can be

attributed to three known sources of error: (1) impedance mismatch, (2) shot

noise, and (3) uncertainty in the value of physical temperature used in equa-

tion (2). The temperature could be significantly higher than 18°K due to

diode junction heating caused by the bias current.

The first source of error, impedance mismatch, could account for 0.7°K

of the 6.3°K discrepancy. This comes about due to the termination on the par-

amp input circulator which can be considered an 18°K noise source. Assuming

22

the switch has a VSWR of 1.5:1, as measured, 0.7°K of the 18°K can be re-

flected at the switch and returned to the paramp as an increase in noise

temperature.

The remaining 5.6°K of noise can be attributed to short noise and

inaccuracy in the known physical temperature Tp , however, an analysis to

determine the magnitude of these two sources of error has not been made.