HP 8731B Service manual

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MANUAL
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8731B~'
87328
87338
87348
87358,
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HEWlIE~T jPACKARD
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8731A
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,'8732A
8733A
8734A
8735A
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PIN MODULATORS
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IModel 8730A & H' Series'
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,PERMANENT M1JUNTING ""': '' "I
FO~6-32tapp"d,mounting holes have been pro-
vided for mounting purposes. The location' of these ,
holes and
the I~VeraII external.:dimensions 'of:)the 1
, PIN,Modulator l,are illustrated in Figure' 2. ,\ ' '
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, Also on tt,e PIN Modu lator underside are,
, the foufco~er retaining screws and the
PIN diode retaining screws. Any moue- J'
ment olr adjustment of, diode retaining
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screuie ioill cause permanent damage. "
PRINClfLES OF ~PERATION
, ! The PIN,Modulator is a high-speed, current-con-
trolled 'absorption type attenuator. Asimplified
,illustration of the modulator is shown in Figure 3."
Each PIN unit includes alow-passfilter, .two high-
pass' filters, a number of PIN diodes" and a 50~ohm
'strip transmission, line (ridged waveguide' in the
,', higher frequency M,odeI8733, 8734, and 8735
units).
The PIN diode is a silicon junction diode whose P
and Ntrace regions are separated by a layer of in-
trinsic (I) semiconductor (silicon). Thus the name
PIN diode.'
At frequencies below about 100 MHz
the PIN, diode rectifies as, a junction diode. How-
,
ever at ,frequencies above 100 MHz, 'rectification
,,' ceases-due to stored,charge in the iritrinsicIl) layer'
and 'the diode :;~cts like aresistor by conducting
,
current in" both directions. 'Thi,s equivalent resis-
tance is inversely proportional,
to the amount of
, charge in the I' layer. An increase in forward bias
"current, (current' 'at a negative voltage) increases
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the stored charge and decreases the equivalent re-
sistance of the PIN diodes. When reverse bias is
applied,reverse current flows 'until stored charce.is
depleted at which time equivalent resistance be-
comes amaximum in the order of 'thousands of
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a. Maximum temperature: 149°F(+65°C)'.
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b. Minimum temperature: -40
o'F
(~~(fC).
'c. Humidity Maximum: 95%.
Environment.
Temperature during storage or ship-
ment should belimited as follows:
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Pin Diodes in Shunt. To understand how a PIN
Mo~ulator works, considerthe following: the PIN '
,di()l\~es are' mounted as shunt elements between the
"'UJl1':l'\~;ransmissiollpa~h and ground. The transmis- '
,::sfQ~(\',I!path hasu-eharacteristicimpedance of,50ohms.
, ,:,' \Vhe:n" the PIN'dinges are forward-biased, the equiv-
c. Wrap
instrumentin heavy protective paper" alent: diode resistance is about 30 ohms and most
to avoid abrasion. ' , of; Ifhe ,Hi" energy':' is absorbed by the diodes in-
.stead of propagating down the 50-ohm transmis-
sion p,ath.
'However, when the diodes are 'reverse-
biased
'the equivalent diode resistance is in the '
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order of thousandsof 'ohms and the microwave,
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b. If the' original carton is n~t usable, a do~ble.:
walled carton 0(1.4 MPa (200 lb/sq inch) bursting'
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strength should be used. ' ' '
STOR,AGE AND SHI,PMENT
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INTRODUCYI'PN "
The He~lett.Pa~kard '8730 series of 'PIN, modula-
tors' are 'variable RF attenuators whose input and
output impedance is nearly constant for all levels
of attenuation. The 8730, PIN Modulators cover
the coaxial range from 0.8 to 12.4 ,GHz in four
overlapping bandsin addition to 'X-band ill wave-
guide. Two models are, available' in, each band;
an '~A" model 'which'provides at least 35 dB of .at-
tenuation range" arid a. "B'tmodel which .provides
at ,least 80 dB,~ ,Typbal uses include pulsing" switch-
ing, leveling, and amplitude, modulating of RF sig- ,
naIs. Table 1lists complete PIN Modulator speci-
fications.
,
Maximum RF Power., The maximum.Input rating
(peak
or CW),', for the PIN Modulators is'1 watt.
Excessive RF power levels will cause internal
damage.
Maximum Bias Input. DC voltage applied to the
BIAS input must not exceed +20 volts or -10 volts. '
Bxcessive de voltages may cause internal damage; ,
Packaging; \To'
protect the 8730 PIN Modulators
during 'storage
or shipmentv always use the best
packaging' methods available. The following sug-
gested steps:
can be used asa guide for packaging.
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a. The ~:i'liginal' carton, if undamaged, can be
'reused. \ ''
"d.Place in carton,surrounded by at least. four
inches of tightly packed shock absorbing''material.
e.
Sealcarton with strongtape .or metal bands.
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PRECAUTIONS
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Model' 87301\. &B 'S~rie~
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DIMU'SIO~~ I~ INtMU .110 IMILLIIII£TEIISI
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NOTE
DllIIlNSIOliIB IN INtHIS AIID IMILLIIllIoTE"SI ,
MODEL 8732A
NOTE . .
DIMINSIOIlS IN INtlllS AND IMIi.LIIlItTIIIS,
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OIIllIIlSIONS III lilt HIS ANDIMILLIIIII H"S I
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MODEL B735A
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Figure 1. , General Outside Dimensions
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+20, -10
100.
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'30
20
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2.0
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2.5
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+20,~10
300
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1.8, from 1.8 to 4.0 GHz
2.0, from 4.0 to 4.5 GHz
Model 8730A& BSeries
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+20,,-10'
1,00
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8734~, . " 87348 ,".' 8735A 87'35,IS'
7.0-12.4 \7.0-.12.4 8:2-12.4," 8.2-12.4
35 I
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<4.. (}, <5.0 <4.0 <5.0
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30 ' 30:
20 20
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2.5
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8733A 87~3B
3 .. 7-8.:1 ,,3;7-8.3
35 . 80
<2.0' <3.0
30 30
20" 20 '
1.8 2.0
a.o 2.2
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5.5
2.5 "
'87328
1.8~4.5
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,80
2
<3.5
30
20' "
1.6
4
,2.0
1
8732A
1.8":4.5
35
< 2.0,
40
30
1.5
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, Table 1. .Speciftcations
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Negative. voltage,applieli forwardbias to diodes
,6. At attenuation levels
01; 10 dB or more, ' "
7. Fit~ 1 x 0:5 inches (WFL 90) waveS:;..tid~',',
8. Standard female type N " ,
30
20
1.6
'2.0 '
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87318
o..8~,2.4
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+5 Vdc bias
1.
Frequency Range (GH z,) 0.8-2.4
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0111:1 mic: Range (dB) " 3~
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Mh" InserttonLoss (dB) <1.5
Typical Rise Time (n8)3 40,
Typical DecayTi~'~ (DI)S ;30
SWR, Minimum:,Atte auatton l . 1.5
I SWP., Maximum Atten iatlon t..8
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Maximum'Input Power, 1 "
'~:::'~:~~~~::~:I~:s ;11~ +2;O~1
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+2~~~1 O+2~~~ o +2~'O-OI
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Resistance (ohms)6 ' pu\ "
, 'i 8 ' 88
R, Fconnector. Type' " ,\~N I, N N
Weig~t{lbS~ 3 5.5 a
!,(kg) 1.4 02.5' I1.4
Dimenstons 'UU8trated in Figure 1. '
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2. <4.0 dB, 4.0 to 4.Ej GHz \ .,
a.Driven by HP 8403A Modul \tor
, , " 4.' 210, from 4.0 to 4;5 GHz "
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.FREQUENCY· (GH1)
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. F;gure,'3. Typical Insertion Loss Curves ".
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1 dB'irripro.~ement in insertion.loss over zero bias.
Atypical plot. of the maximum values of minimum'
insertion 'loss is shown in Figure 3. Note: a.typical
,plot fcir the "A'" models would be the same except '
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that maximum losses are less, ...
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The attenuation increases with forward bioi cur-
rent almost linearly in dBuntil diode saturation co-
curs. The attenuation, sensitivity varies from unit to
unit but the sensitiV',ity of. 'a 'given unit is predict-
able, Therefore,.intermediete values of'attenuation
can be derived once ,the current values, for t, low
. .and high attenuation are 'established, The range ~f
expected sensitivities is shown as the shaded areas
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currents will', flow down the '~ransmissibn path be-
cause"; diode resistance camp ired to the 50-ohm
path impedance is negligible.
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,', -Figure 2. Simplified Block \?iagram ~. 'I
of PIN Modulator \:
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~IASING , • .' '" 'I.' ,:!, :0 "
Each individual PIN Modulator has' label attached
to the 'underside of its casting body Thi's label has
'a, 'dc bias currentvalue stampedjonto: it. This
,stamped current, is' the' forward-blas current re-
quiredto produce rated" RF attenua ion across the
'" ,frequency band, For "A" models, the .bias current I •
.for atleast 35 dB is typically 3~5 to' milliamperea'
(about ...:.0.7 volts 'de at .the BIAS in ut), For '''B'' i ,
)models; the bias current for at,:le~st8(), dB istypi-
cally 6 to 7milliamperes (about "--0.1' volts de at
. the, BIAS input). ~or applications. Wiere a .mini-
.',l,nup} insertion loss is imporl~nt, a re rerse bias of
about +5.'to 7(.vo~tsd~may be applied, 0 the BIAS'
, ,in~ut. ,R~verse"biaSin~can,result jn '~s much as a
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Figure,5 .. Typical Frequency Response
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1.0 2.0 3.0 4.0
\ . 'CURRENT (mA) .
.il/kure 4. Typical Sensitivity .
(Plotted with frequency constant)
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in the plots of typical 35·dB and BO-dB attenuation
chsracteristicsin Figure 4. "
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The attenuation with a constant forward-bias var-
ieswith frequency. 'This variation is small at low
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levels of attenuation and more noticeable'at higher
.values of attenuation (see Figure 5). In addition.Jt
'should be noted that attenuation is.' aminimum at
. each end bf the'frequency band. Thus, the fre-
.quency band canbe exceeded provided a dp~)a.da·
tion in attenuation range as well as other stecif'ica-
tions is acceptable. ,
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. Impedance Ma·tch .
The PIN Modulator has a good impedance match
, at. all levels of attenuation over Its frequency range.
This match is illustrated in Figure 6which shows
the SWR of atypical PIN Modulator measured
.. under both zero bias' and maximum forward bias
conditions. Hence, with this constant match, fre- .'
quency pulli~g. effects due' to the PIN'Modulator
are negligible, If the PIN Modulator is used outside"
specified frequency range, SWR chara~t;e.ristics will
be degraded in addition to other' 'ope~l~ting specifi-
cations. .However, spurious responses can result
when, exceeding specified frequency r$l1g
e.
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Temperature Stability ,I
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'Ihe attenuation variationiwith applied bias current
.. and bias voltage at different temperatures is' illus-
.• ' tt!ate~, dOBver,~ l,ima1it0d't~anl;: 'e, lin. F~tg1·,I~e ,7 ~ ,Atttenh'~la
\ Ion m.' . vanes mos •. In.ar y ~1 ~/. curren ,W. 1e
variation '\VitI, voltage follo s an alri;kost perfect ex-
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, ponenti~l~~rve. Ifa c~nstant vC?ltage"ls~aintained,' ",'. ',.a
,I' the attenuation rises .with increasing temperature. '. "
"'11"With a' constant current, ,the atten:uation" drops! '" ,I , : '
with increasing temperature. The fact. that theat- 'i ' /.",
"tenuation varies oppositely ,with temperature for
constant-voltage and vconstant-current operation
suggests that an optimum voltage with aselected
series resistor added between the ..ource and the
PIN Modulatorwould give temperatu~ecomp'etisa~
tion over ,a' liInited;\;,H~rating range. This compensa-
tion is not built into the PIN Modulator,
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,FRE'QUENCY (GHz) , ,
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, " Modulator(87a2B) ' ,',
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CLOSED LOOP LEVELING'
The PIN,Modulator can be used i.n aclosed loop
leveling system as the 'power limiting device for'
maintaining RF power levels constant. A simplified',
block diagram 'of a closed loop leveling system em- i
ployinga PIN modulator,is shown in Figure 8. The .
system consists of some sampling device which
samples the main line RF. power level: a detecting
,device .provides avoltage proportional Ito. th~ .
sampledvslgnak some comparison device which
compares the detected signal to what it should' he
when main line power is at its lowest 'point:',apd
then provides 'a bias current sufficient to Increase
PINl\'lod\llator attenuation and maintain acon-
stant R}'power level. .
'Leveling Capab.il·ity. In aclosed-loop leveling sys-'
tern. involving a P~N' Modulator, Ieveling capability
.depends entirely 'upon loop equipment.' Hence,
\. GEI\JERAL'. OPERATION
J'1'" I"~ " ,
, 'I'hePtN Modulators are three-port.devices ,whieh";,'':',:' '"
." accept RF power ~G e,ither end-port and, depending
upon the. BIAS signal applied, provide a.modulated
RF output' at the ,,'other end-port. 'As, an operating ,"
device" the; PINM'odulat<;>~ should be 'thought ofas :
.a, variableRF attenuator whose attenuationlevel is, .,
controlled by decurrent and voltage. By applyinrt", .
, a+5 volt de potential to/the,;BIAS inI;>ut connector, .
RF signals will be passed through the PIN)\1odula-
tor with' only minimum insertion loss;By applying
acurrent equal to the-value'stamped on' the instru-
ment label (at, about 0.7 volts de) 'at the BIAS
), '~ "
input, at least rated 'attenuation IS provided atall
points. across' the frequency band (i.e.,' for "A"
models at least '35 dB and for "B" modelsat least
80 dB)~ Direct cUITentat"a'neg~ti~'e~oltag'e' is' foro.
"'I' ward-bias' arid typically is about 3 to 5 rnA for, "A"
models and 6 to 7'mA for "B" models. By varying·
the forward bias current between' the stamped
value and zero, any level below maximum rated
attern .1 can be established. !\'j'
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--+ .......... .....,,..,.,~, ---+,....---+---+-----1
- 0.3 "-0~4 ,.',-0.5 -0.6 - OJ ,.. 0.8
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BIAS VOLTAGE '(VOLT~)
21-,t--+----1"
24 -+-.;... ...... -t----t---J'--:Ir--~,t---_t
6i+--+...,.q.- ,,---+---+-~.t----+---i
,0 ',0.30.8 0.'" ,,1.2 ,i.5 I.a '
·BIAS CU'RRENT (mA) 1130A-I-;
iii 181-t---t--.,._._f---:.f'7-·¥-''-''-=-t----,-t----i
'0,
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~ 151'1" --+-----I--#--+--+_._ .-t-----1
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, D'ifferentAmbii!nt Temperatures'
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. "oe 'POWER
SUPPLY
~6216A
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RF
SIGNAL
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detector should be used at !~h~ te~e~ed RF' output,
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':'., ,.<~7,9~n:, J " ~FOR' .P~CSE MOOULATION: ~MODEL.~ ~I~A,' t'.'
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, Pigure' 9. '.,Typical Clqsed L~)'op t~veling S;istem', .,' Figutlj"lO)Typical,Genera/Modulation ~etup
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For'faste~ leveled swept frequency rates, l~~gative
,output detector-and directional couplercombina-
tions such 'as .the HP Model 7'86D and 78·7D Direc-
.tional Detectors can be used insteadof the power
.: meter' and ,"thermistor mount combination .de-,
scribedabove.'Typically,' for the directional detec-
, '.,i' tor and. leveler campJifier combination, leveled
,
loop equip~nen(should be selected. onthebasis of. ' power can be held constant within ±0.2'idB (plus
.main"line R.F· power. .levels and' leveling: .require- fr~quency '.' response hf directional detector).
ments. III the closed-loop system. shown in-Figure 9Note:Ithe,' f~quency response of the directional
:the".~atnes~o..~ lev~1iIlg'i3 deteririi'n~d primarily by detector includes coupling ;:variation,a~d, RF re-
,,'.:' the,',co.\1plirig' variation of the directional coupler spouse of the crystal. For the ~~~~Jel 7,'86D (0.96.
,," :used. l'he powermeteris usetr,Q'ecause it provides to 2.11 GHz) this ,is I±O,;2"dB. ,,' ". i ,
extra 190P gain. In' addition, with output coupling
,'l(nQwl\;"'t,he' main line'.output.power can be read
directly on the powermeter allowing for coupling' AMP~ITUDE MO'DULATI'ON .
'variation. The 'leveler ~pIU~.er used has a, 0to, 'The PIN' Modulator can be, used to amplit~de-'
-27, Vdc output..c~p~l,)i1ity WIth output impedance modulate the RF' signal with .almost any 'time-
."" of about 20 'kn, hence' the 20 k,Q "series resistor 'varYing signal. Modulation '~~accompl,ished with a"
• ::'" 'i, makes it 'a biascurrentsource providing a 'maxi-dcblas' 'current. to obtain a, specific attenuation
mum, L3S' rnA' decurrent (referring to ~igltr.e 4, "1' "level and thensuperimposi~ga .time varying cur-
.- '. this means tha,t'the maximum control range po~: .' rent upon the bias current, Th~ specific attenua~
" '. ,sible using 'an' "A" model 8730 is'between l~'and tion level ~.tpon~"which the 'modulating signal is .
.. ", 20 dB depending upon the individualB'[Sf) used).. '. "sllperHnp6$~d' must be equal to or 'greater.. than the
Typically, with the power meter and leveler ampli-' peak amplitude of the, modulating signal, or peak
fier combination, leveled
power can be, held con-, ' 'clipping will QC,<;\1r ."A '¢ontroi instrument designed:
stant with ±0.2 dB,(plus coupler variation) for R'F fQr, I use '. 'tn modulating applications with ~he PH~"
swept 'frequency rates equal to orlonger .than 30' Modulators is the' HP Model 8403A M,odulator
',seconds/o,ctave. 'In practice, slowest swept' fre- which provides' frequency compensation' for ex- ..
.' ;'. tending ·the.PIN Modulator AM frequency response .
. ) ':':", tolD MHz. However, atypical setup for modula- '
J; , , ting applications, without. the use or the HJi' Model'
~1*P3A Modulator.Isillustrated in Figure 10.'
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slarr'bY,4Pplying ground to the, input,
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,:i! ", NOTES ';, .' .. " " '"
P ALL ,~PN~"TR'ANS;IStCR~'ld'54~000'5
2,PNP TRAN'siSTORS 185~'·'OQ09 ',\ '
:~" ,'[>tOCES 1901 ~,0627. I,' , ,',' .':" ,',
4;CIRC,l,lIT GROUND \'t1UST:'!Be;CO~MON '
WmiplN: MOQULATOR' .. ' :: I,) "
",' , 5, :RE~hsT~NC~ "~OHl'S;">CAPA~'!iT~~C~
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, ",1)'~!,: I\" ::' I:,"~', OTH,ERWISf. "INOICA:r.~":.',,,< "
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,/ I' '" \'1:, '"I~,.' ':'I:~tw/c'ir~,~i't\ il~l~s,tl'ateit ~b?\re ,Will typically meet I' 2.,1 .. CttcuitArlju,GtrrtP.llt;'h:'"
I "\ .•. :>: .;th~(~:~~~~l~g ~~~f~~~,a~te feat~~s: .. : ' " .. " .' .. ">',~~\l~~~~r:~~,:'~~~ft~e~~~:~~i~~t~n~:l~~~
,,<,':;) "!, ) ,:,,(~) rrhc"~«;Ji&hs(er cheraeteristlc is within 2(r,j:"or ',' ' ~:F':,~igQal::f~eq~~~~cy of interest. / '.' i
. '\ i'. .' "so ~t,/lh~~iir: ~'t ,1.S'OHz. Slncethe PIN Modl.d~tor:,,' I.: ~~)iroadju~t~' apply de voltage ~o the input .and
,~! :"atten;6ati~n'i Ibias I curr~nt ' chararit~rbtic, variJs ':'",,' adjusit~e'alipropriate potentiometer. in the input
"""', " , "','\vl'dely.::,:II," ' " '\I,~! '," \ ;:/ ' " divider as' irJ,dlcated for' the "initi~l 'setup volt-
:,V " ./," '\ ,I, " I,'r,', ", , ,,"'" 'I' , "ages' (s6~'Fig",ure' 12A). '.
,':(b), The,;, ban'~~id,t~" '~,f t,~e circu~t is ,limited' to .," :'
t ~~out' /4 MH,~ '.<~odulating signal, frequency). I
': (c )':J'~~a1. ha~ti,~nic'd isto~iionshould be gre~~~
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,,(d) . ~~rp,,()p~r:,'drr.uit opera~ion" it:is nec~ssarY \':;
. i"to.' have an 8~v,o~,t signal on ,the base ()f th~,input\ '
:,,~'~ansist?r',l8 ~?!~ts',p,OS~~i;ve re~ferenced:to g!~~fl,~r '
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Figure 13. Dynamic Bias Switching Waveform ..
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level with the .same PIN Modulator. This can be'
,accomplished by simply setting up aclosed loop
leveling system ~~d adjusting the reference atten-
uation level £0 a point equal to the peak amplitude
of the d~sir~d modulating signal. For .this type of .
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MUL TIPLEFUNCTION"
FQr" c~rtain applic;.ations, /it .may be desirable to
level and amplitude. modulate a given RF power
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tenuation .level is established,' the RFpowe,r may
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.. :be' pulsed "on" by applying .aconstant +5 to ,6 .,' '
'::volts' to the "BIAS input (voltage' ,ntist' b~ refer,; , :
".' e~ce(r>to'PIN,Moduiator ground).At~the·~~l(i 'o'f", .
.desired pulsewi,d,th, the ,+5 to 6, voltdepotential '
must beswitchedto.a -0.8 volt levelwithratedv
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.bias current to switch the RF off. 'i :
SrGNALSEPARATION (GATING)
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;'\ Figure 14! illustrates u~, of the rI~{ .Modulator as'
...' an· RF gating device for eliminetlng' spurious or'
""gh'ost"'signals,in antenna range receivers.' ", 'r ,~ .
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order of 15 'to 40 nanoseconds, the PIN Modulator
rnust be biased with the' HP 'Model 8403A or from, :
,", ,;!,,' ,I~ specially shaped' impulse waveform' sho~n" in','
",F~gure -13. However, if rise ,and fall times in the
, 'I. ' .order of 100 to 300 nanoseconds aresatisfactory ,a'
, " \ 'setup such as is shown in Figure 10 may be used.
;i:', ~l'Q'te: in any modulating system,'modulating capa-
bility depends upon the modulating waveform' at
the BIAS 'input ·cQnnectQf. Hence, lead, lengths
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should-be as short .aspossibleto avoid capacitive
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