Block Diagram - HP 204B Operating And Service Manual

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-
FEEDBACK AMPLIFIERS EMITTER
,
RC
20
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ATTENUATOR
- -
I
FOLLOWERS
BRIDGE
(RIOA,B,C2,C71
Q I
,
Q2 0 3 ,
04
S E C T I O N I V
I
R 4 0 A , B h
I
I
I
P R I N C I P L E S OF O P E R A T I O N
I
FEEDBACK
Section
P a r a g r a p h s 4-1 t o 4-10
4 - 1 . I N T R O D U C T I O N .
4-2. T h e Model 204B c o n s i s t s of a n RC b r i d g e o s c i l -
lator c i r c u i t ,
a
peak d e t e c t o r c i r c u i t , and a n output
attenuator (bridged-T type). T h e s e c i r c u i t s and t h e
front panel c o n t r o l s a s s o c i a t e d with t h e m
are
shown
in the block d i a g r a m , f i g u r e 4-1.
4-3. T h e RC b r i d g e o s c i l l a t o r c o n s i s t s of a n RC
b r i d g e , a t w o - s t a g e a m p l i f i e r , and two e m i t t e r follow
-
e r s . T h e output of t h i s c i r c u i t
is a sine-wave signal
which is 1) r e t u r n e d t o t h e RC b r i d g e
as
feedback,
and
2)
applied t o t h e output a t t e n u a t o r .
4-4.
T h e RC b r i d g e c o n s i s t s of a n RC frequency-
s e l e c t i v e networkand a r e s i s t i v e voltage divider net
-
work. T h e RC frequency-selective network supplies
positive feedback t o t h e a m p l i f i e r and d e t e r m i n e s t h e
frequency of oscillation. T h e r e s i s t i v e voltage divid-
e r network s u p p l i e s negative feedback t o t h e a m p l i f i e r .
T h e output of t h e a m p l i f i e r is proportional t o t h e
difference between t h e feedback s i g n a l s .
4-5. T h e peak d e t e c t o r d e t e c t s changes in t h e RC
b r i d g e o s c i l l a t o r output voltage and changes t h e divi-
s i o n r a t i o of t h e r e s i s t i v e voltage divider network,
t h e r e b y changing t h e amount of negative feedback.
T h e peak d e t e c t o r with t h e divider n e t w o r k m a i n t a i n s
t h e RC o s c i l l a t o r output
at a
constant level.
4-6. T h e attenuator is
a
bridged-T attenuator which
provides continuous control of t h e oscillator output
voltage while maintaining constant output impedance.
4 - 7 . OSCILLATOR C I R C U I T .
4-8. T h e RC b r i d g e in t h e o s c i l l a t o r circuit c o n s i s t s
of a n RC frequency-selective networkand
a
r e s i s t i v e
voltage divider network. T h e frequency-selective
network
is
s i m i l a r t o one l e g of
a
Wien b r i d g e ; t h e
r e s i s t i v e voltage d i v i d e r , t h e other leg.
4-9.
A s
in any o s c i l l a t o r , an in-phase feedback volt-
a g e (from t h e o s c i l l a t o r circuit output) is n e c e s s a r y
t o maintain oscillations. T h e p r o p e r p h a s e r e l a t i o n -
ship
at
t h e d e s i r e d frequency is maintained b y t h e R C
components in t h e b r i d g e .
4-10. T h e frequency-selective network c o n s i s t s of
a
s e r i e s branch, C1 and R1, and
a
p a r a l l e l b r a n c h , C2
and R2, as shown in f i g u r e 4 - 2 . For t h e frequency
at
which X
=
R
in t h e s e r i e s a n d p a r a l l e l b r a n c h e s , t h e
positive f e e d b a c k voltage t o t h e amplifier
is
maximum
and is in p h a s e with the oscillator circuit output vol-
t a g e (figure 4-3). Only that frequency
at
which
X =
R w i l l b e amplified;
at frequencies w h e r e Xc does got
equal R, t h e positive feedback voltage
is
not of t h e
right p h a s e and is insufficient in amplitude t o sustain
oscillation.
Figure
4-3 shows t h e positive feedback
c u r v e and p h a s e relationship f o r f r e q u e n c i e s above
and below t h e frequency w h e r e
X
=
R.
C
01369-2
F i g u r e 4-1. Block Diagram
4-1