Multiplier; Harmonic; Filter; Amplifier - HP 5254C Operating And Service Manual

Model 5254C
Section IV
SECTION
IV
PRINCIPLES
OF
OPERATION
4-1.
GENERAL.
4-2,
The Model 5254C
is
a
heterodyne frequency con-
verter designed
to
extend the
range
of
frequency
meas-
urement
of
an Electronic Counter
to
,
15
GHz
through
3
GHz
(150
MHz
through 3000
MHz).
output
of the
video amplifier
is
monitored
by a
meter
circuit
which
indicates
when
difference frequency
out-
put
amplitude
is
greater than
the
minimum
signal re-
quired by
the
counter
input circuit.
Note
4-3.
The Converter
contains
five
basic functional
sections: multiplier amplifier,
harmonic
generator
and harmonic
selector
cavity,
mixer,
filter,
and
video
amplifier. (See
Figure
4-1,
and
for circuit details re-
fer
to
the
schematic diagrams, Figures 8-2 and
8-3.
)
10
MHz
SIGNAL
FROM
COUNTER
DIFFERENCE
FREQUENCY
AUX TO
OUT
COUNTER
Figure 4-1.
Model 5254C
Block
Diagram
4-4.
In
normal
operation
the
harmonic generator pro-
duces
all of
the
harmonics
of
50
MHz
between 150
MHz
and 3000
MHz. The harmonic
selector cavity
is
tuned
to
select
one
of
these
harmonics
to
be supplied through
the
low-pass
filter to
the
mixer.
The mixer
output
is
the
difference frequency
produced by mixing
of the in-
put frequency and
the
frequency
supplied
by
the
harm-
onic selector
cavity.
This difference frequency
is
amp-
lified
by
the video amplifier
and
supplied
to the
counter
input
circuit.
A
low-pass
filter
within the video
amp-
lifier
prevents difference frequency
signals
above
53
MHz
from
reaching
the
counter
input
circuit.
The
In
the following discussion complete reference
designations are used
to identify
components.
This
is to
prevent confusion between reference
designations
of
components
located
on
the
chas-
sis
and components
located on an assembly.
For
example,
"RF* would
refer
to
a
component
located on
the chassis,
while
"A1R1"
would
re-
fer
to
a component
located on the multiplier
amplifier
assembly A1
(see
Table
5-2 for
as-
sembly
designations).
4-5.
MULTIPLIER AMPLIFIER
Al.
(see
Figure
4-2).
4-6.
A
10-megahertz
signal
from
the counter
is
ap-
plied
to
buffer amplifier
A1Q1. The
buffer amplifier
A1Q1
is
a tuned amplifier providing a constant ampli-
tude 10
MHz
signal
to
the multiplier
A1Q2, and
pro-
vides
isolation
of
converter
from
counter.
The
x
5
multiplier
A1Q2
is
a tuned class
C
amplifier with
the
input tuned
to
10
MHz
and
the
resonant
output tuned
to the
fifth
multiple
of the input signal providing a 50
MHz
output
signal.
The
output
of
the multiplier
is
amplified by
A1Q3
and
applied
to
the crystal
filter.
The
half lattice crystal
filter
(AlYl and A1C13)
is
a
50
megahertz band-bass
filter.
A1C13
is
used
to
bal-
ance out crystal capacitance.
A
series
of
tuned
power
amplifiers
A1Q4, A1Q5, and
A1Q6
amplify
the signal
from
the
crystal
filter to
drive the
harmonic
generator.
4-7.
HARMONIC GENERATOR
A2
AND
HARMONIC
SELECTOR CAVITY.
(See
Figure
4-2).
4-8.
The harmonic
generator
consists
of
a
50-MHz
tuned
circuit, driving the step
recovery
diode,
A2CR1.
The
step
recovery diode takes energy from
the tuned
circuit during a portion
of
each
cycle
of
the 50
MHz
oscillation
and produces a sharp
step
in the
current
flowing through
the diode.
The
diode
forms
a loop
in-
put coupling
to the
harmonic
selector cavity and
the
step in the current through
the diode
makes
available,
inside
the cavity, the
harmonics
of
50
MHz
from
150
MHz
(third harmonic)
to
3000
MHz
(sixtieth
harmonic).
The
probe tunes
the cavity
to
select the desired har-
monic and
provides coupling
from
the
harmonic
selec-
tor cavity through
the
filter
assembly A4
to
one
of the
two
inputs of the
mixer assembly A3.
4-9. FILTER
ASSEMBLY A4.
(See Figure 4-3)
4-10.
The
output signal
of
the
harmonic
selector
cavity
is
applied
to
a Tschebyscheff low -pass
filter
(A4),
with
a cut-off
frequency
of
3
GHz,
to
limit
4-1