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SECTION
16.
FREQUENCY-STANDARD INTERFACE BOARD (A16)
16.1 DESCRIPTION
The
Frequency-Standard Interface
board provides
the
10-MHz
time
base
for the System
Analyzer and the
interface for
an
external
10-MHz input. A
standard
temperature-compensated crystal
oscillator
(TCXO)
provides stability
of
±
1 PPM.
An optional
oven-com-
pensated
crystal
oscillator (OCXO)
provides
stability
of
±
0.05
PPM
over
temperature variation.
A block diagram
of the 10-MHz
Frequency-Stan-
dard
In
terface
board is
shown
in Figure
16-1, a
sche-
matic in Figure 16-2,
the
printed wiring board
assembly
and
the parts list in Figure 16-3, and the
assembly
and parts list in Figure 16-4.
16.2
THEORY
OF OPERATION
16.2.1
INTERNAL SOURCE
Oscillator Al6A2 generates a 10-MHz signal that is
split
to
two amplifiers. One amplifier
(Q3)
provides
the
FREQUENCY
STANDARD
SUPPLY
J16-1
10.0
MHZ
TCXO
(OR)
ovxo
U1.01
VOLTAGE
REGULATOR
AND
OVEN
IND
DRIVER
R18
GAIN
ADJUST
internal
timebase;
the
other
amplifier
(Q2) provides
the
external
timebase.
The
two
identical
amplifiers
provide
a
peak-to-peak
signal of
1
to 2V
(as
adjusted
by Rl8) into
a 50-ohm
load.
A
voltage
regulator
(U1 and
Q1)
on the
board
sup-
plies
the
voltage
to the oscillator and
monitors
the
supply current.
For
the ovenized
option,
at power
on,
the
oven draws
high current
at
approximately lA.
As
the oven
warms up,
the current
decreases,
reaching
some
low
value
when
the operating
temperature
is
reached.
A current detector
turns the oven-ready indi
-
cator
on
when the
current has
decreased
to the stabi-
lized
value of approximately
325
rnA.
16.2.2
EXTERNAL SOURCE
An
external
10-MHz
timebase can
be used by
switching S1
(accessible
from the
bottom
of
the
Ana-
lyzer)
to
the
external
position. The
external
source
is
amplified
by
Q3,
which
has
an input impedance
of
!OK
ohm.
The external
source
signal must
be
between
175
mVrms
and 2.0 Vrms into
!OK
ohm.
SYNTH
10 MHz
>-...:;.....;.;.:..;.;.;.~.:.;;.;.;:~J1
EX 10
MHz
INPUT/OUTPUT
P1
O VEN LED CATHODE
J16-6
OVEN LED ANODE
J16-5
36930-80
Fi~ure
/
n
-1
.
Fn•quency-
S
landard
Interface Board (A/6) _Block Diagram
16-1
/(
16-2
blank)
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