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Section
IV
Model 5254C
selected
mixing
frequency.
The
filter
has
a
50-ohm
termination
at
both
the input
and
output.
The
output
signal
from
the
filter
(A4)
is
applied
to
one
of
the in-
puts
of
the
mixer assembly
(A3).
4-11.
MIXER
ASSEMBLY
A3.
(See
Figure
4-3).
4-12.
The mixer assembly
uses two diodes
in
a bal-
anced mixer
circuit
in
order
to
minimize
the
genera-
tion
of
even order
harmonics
of
both the
input signal
and
the selected
mixing
frequency.
The
combination
of
the
terminating resistor
A3R1
and
the
shunting
ef-
fect
of
the
diodes gives
an
input
impedance
of
50
ohms
and provides
a
low standing
wave
ratio (typically
be-
low
1.
5
up
to
3.
0
GHz.
Capacitor
A3C1
provides dc
isolation
up
to
100
volts.
A5R1, A5R2,
and
A5R3
control the bias
currents
of
the
mixer
diodes
(A3CR1
and
A3CR2)
and are
adjusted
for
maximum common
mode
rejectionwithinthe video
band
(1
MHz
to
53
MHz).
The
non-linearity
of
the
diodes
(A3CR1
and
A3CR2)
gives rise
to
signals
with frequencies
which
are
the
sum
and
difference
of
the
two mixing frequencies
Figure
4-2.
Multiplier
Amplifier
Assembly
Al,
Harmonic
Generator
Assembly
A2,
and
Harmonic
Selector
Cavity,
Block
Diagram
(5254C)
INPUT
SIG
Jl
Y
MIXING
FREQ
FROM HAR
SELECTOR
CAVITY
Figure
4-3.
Low-Pass
Filter
Assembly
A4,
Mixer
Assembly
A3,
and
Video Amplifier
Assembly
A5,
Block
Diagram
(5254C)
4-2
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