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ponenti~l~~rve.
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c~nstantvC?ltage"ls~aintained,'
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the attenuation rises .with increasing temperature. ' . "
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"With a' constant current, ,the atten:uation" drops! '"
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with increasing temperature. The fact. that theat-
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" 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,
2.a 3.0
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,FRE'QUENCY (GHz) ,
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6.
Typical 8WR'
of
a,'PIN
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Modulator(87a2B) ' ,',
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CLOSED LOOP LEVELING'
The PIN, Modulator can be used i.n a closed 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 a voltage proportional
I
to.
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 a con-
stant R}'power level. .
'Leveling Capab.il·ity.
In a closed-loop leveling sys-'
tern. involving a
P~N'
Modulator,
Ieveling capability
.depends entirely
'upon
loop equipment.' Hence,
\ . GEI\JERAL'. OPERATION
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,
, '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
a current equal to
the-value
'stamped
on'
the instru-
ment label (at, about 0.7 volts
de)
'at the
BIAS
) ,
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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.
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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.
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"-0~4
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BIAS VOLTAGE
'(VOLT~)
21-,t--+----1"
24
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·BIAS CU'RRENT (mA)
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181-t---t--.,._._f---:.f'7-·¥-''-''-=-t----,-t----i
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' , " Figure
7.
Typica'l Attenuation Variation 'at two
,
D'ifferentAmbii!nt Temperatures'
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