User's Manual

Theory
of
Operaton
-
494A/4gdAp
Service,
Vot.
1
CRl029
added
for
maximum
attenuation.
lf
Rl01
5
is
set
higher
(nearer
+i
5
V),
diode
CR3030
is forward
biased
and
starts
to conduct.
Since
the
2
mA supply
current
is
relatively
constant,
this subtracts
from
the current
through
CRl02g.
Thus,
the impedance
of
the
diodes is
relatively
constant,
which
results
in
a
good
impedance
match
over
a
broad
range.
The
RF
signal
path
is
determined
by
the
exact
amount
of
curent
through
CR3030;
part
of
the
RF
signal
path
is
through
CR3030
to
th€ output
amptifier
ind
part
is
through
R2032
and
diode
CRlO2g
to
ground.
This
results
in
reduced
signal
attenuation.
lf
R1015
is
set
to
the
positive
timit,
the
entire
2
mA
flows
through
CR3030.
This
routes
the
RF
signal
through
CR3030 (which
exhibits
titil€
resistance
with
high
current)
to
the output
amplifier
with
almost
no
att€nuation.
Cl-he
insertion
loss is
approximately
I
dB.)
_From
the adiustable
attenuator,
the
signal
is
applied
to
the linal
amplifier
09018.
This
stage
ts a
broad'-6and
feedback
amplifier
that supplies
relatively
substantial
output
current
and
exhibits
good
intermodulation
distor_
tion
performance.
This
is
provided
primarily
through
the
large
curr^ent
.capacity,
by
negative
feedback
thiough
resistor
R3014,
and
emitter
degeneration
through
resls-
tor
R4029. These
resistors
are
sized
to
provide
a
r€a-
sonably good
impedance
match
at
110
MHz.
Nominal
gain
of
the
stage
is 13
dB.
With
Gain
potentiomet€r
Ri0i5
set
for
maximum
gain
(least
attenuation),
the
gain
of
the 110MHz
tF
Amplifier
is
approximatety
26dB
to 27d8.
R.t01S
is
normally
adjusted
for
total
gain
of
21
dB.
Th€
output
signal
from
the
110
MHz
tF
Amplifier
is
applied
through
the 110
MHz
band-pass
fitter
FL36
and
low-pass
filter
FL37
to the
3rd Converter.
110
MHz
FTLTERS
(Diagram
17)
The 110
MHz
band-pass
fitter
(FLg6)
determin€s
the
widest
resolution
bandwidth
of
the analyzer,
provides
i-I1g-g-
rejection
to
prevent
the mixer
fiom
producing
10
MHz
outputs
from
input
signals
of
90 MHz,
and
also
limits
the
noise
spectrum
that appears
at
th€
1O
MHz
lF
circuits.
The low-pass
filter
(FL3Z)
that follows
turther
reduces
harmonics
and
spurs from
the
2nd
Converters.
Both filters
are
sealed
units
with
no internal
servicing.
The
band-pass
filter
consists
of
four
helical
resona-
tors that are
tuned
with
multi-turn
trimmer
capacitors.
For
purposes
of
impedance
matching,
the fittei
is
sym_
metrical.
Adjustment
of
the
filter for minimurn
attenuation
is
performed
by setting
the
trimmer
capacitors.
Insertion
loss is
approximately
4 dB to 4.5
dB. From
this
filter.
the 110 MHz signal
is applied
to the separate
low-pass
filter.
There
is no adjustment
for
the
low-pass
filter.
The signal
then
feeds
the 3rd
Converter
board.
3rd
CONVERTER
(Diagram
17)
The
3rd
Gonverter consists of
a 100
MHz
crystat
oscillator
and a
mixer. lt
outputs
th€ 3rd lF of 10
MHz,
for
the
Variable
Resolution
(VR)
circuits,
and
a
stable
100
MHz reference
for
other
circuits
within
th€
instru-
ment.
100
MHz
Oscillator
A
Colpitts
oscillator
is
formed
by
Q2038,
yg03g,
L1041,
C1038, and
related
compon€nts.
Y3038
is a
100
MHz crystal
that op€rates in a
series resonant
mode in
the
fEedback
loop
of
the
oscillator.
The oscilla-
tor output
couples
through C2042
to differential
amplitier
4204402441.
The
two
separate
outputs
of
approximately
2V
peak-to-peak
amplitude
go
to three
hybrids
(mixer
U3051,
distribution
amplifier
U30il1. and
cafibrator
U2022) on
the 3rd Converter
board.
lnductor
L3041,
varactor
diode
CR3039, and
crystal
Y3038
form
a series
resonator
that tunes th€ oscillator
approximately
*1
kHz.
The
RPL VOLTS
TUNE
tine
varies
from
0V
to +12V,
changing
CR3039's
capacL
tance
to
phase
lock
the
oscillator
to
the
Reference
Lock
source.
RPL
GND is
tied to
ground
in
the Reference
Lock module.
Mixer
At
mixEr
U3051, 100 MHz
enters
on
pin
2 and is
amplified
to
drive
a
ring diode
mixer.
110 MHz
enters
on
pin
10 and
is mixed with
the
100 MHz
to
yield
mixing
products
at
10
MHz and
90
MHz.
The 1O MHz signal
pass€s
through a low-pass
filter
and
is
sent
to
the
Vari-
able
Resolution Input
circuat,
while
the unwanted
90 MHz signal
is
terminated
within the mixer,
Distribution
Amplilier
U3031
distributes
a
100
MHz signal
to
other
modules
in
the
instrument.
The input
level on
pin
2
is
typically
2
V
peak-to-peak,
while the output level is
0 dBm into a
50
ohm
load.
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7-20