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Model 5065A
Circuit Diagrams, Theory, and Maintenance
SECTION IV
THEORY OF OPERATION
4-1. THEORY
4-2. General
4-3. For circuit theory on individual assemblies, refer
to the schematic fold-out pages at the rear of this
manual.
4-4.
The simplified block diagram of Figure 5-6 shows
the frequency-stabilizing feedback loop.
The 5 MHz
quartz oscillator output is stabilized, first by comparing
the 5 MHz output in a frequency-synthesizing and
multiplying process with the resonant frequency of
Rb
87
and then translating the difference frequency into
a control voltage which corrects the quartz oscillator
frequency.
4-5. Oscillator Assembly A10 generates the 5 MHz for
A3 Multiplier where 5 MHz is: (1) phase modulated at
137 Hz, (2) multiplied to 60 MHz and,
(3) combined
with the synthesized 5.315. . . MHz after multiplication
to 60 MHz. The 5.315. . . MHz is derived from 5 MHz in
a frequency-synthesizing process.
The combined 60
MHz and 5.315. . . MHz signal goes from A3 Multiplier
to the harmonic generator step-recovery diode in A12
RVFR (Rubidium Vapor Frequency Reference) Assy.
The harmonic generator/step-recovery diode couples to
the Rb
87
absorption cell which is housed in a microwave
cavity tuned to 6.834685 GHz, the Rb
87
resonant fre
quency. In the harmonic generator/step-recovery diode,
5.315. . . MHz phase-modulates the 114th harmonic of
60 MHz to produce the 6.834685. . . GHz lower sideband
which matches the microwave cavity resonance and
causes energy level transitions in the Rb
87
gas.
4-6. Figure 4-3 shows the Rb
87
absorption cell which
contains the Rb
87
gas.
A 100-MHz oscillator in A12
RVFR Assy drives the lamp filled with Rb
87
gas.
The
resulting light output passes through the Rb85 filter cell
and the Rb
87
absorption cell.
The light output of the
Rb
87
absorption cell is monitored by a photodiode.
Rb
85
photo excitation is removed in the Rb
85
filter cell
to remove undesired transitions. When the Rb
87
gas is
excited by the 6.834685. . . GHz microwave field at its
resonant frequency, it increases in opacity to reduce
light transmission about
1
/2% as illustrated in Figure 4-1.
This phenomenon permits using Rb
87
gas as a fre
quency reference. Phase modulation at 137 Hz (in A3
Multiplier) produces a sinusoidal scan of the excitation
frequency. As a result, 2nd harmonic 274 Hz appears
in the photodiode output when "on" frequency and
fundamental 137 Hz appears when "off" frequency, as
shown in Figure 4-2. For example, as the 6.834685 . . .
GHz excitation is steered towards the Rb
87
natural
resonance by the feedback action of the frequency-
control system, second harmonic appears in the photo
diode output is mostly 2nd harmonic 274 Hz with a
small amount of 137 Hz.
4-7.
Temperature control of the Rb
87
lamp and ab
sorption cell in the A12 RVFR Assy is accomplished
by temperature control circuits in the A11 Temperature
Control Assy working with temperature sensors and
heating elements in A12 cell and lamp ovens.
Oper
ating current for these ovens is monitored in the CELL
OVEN and LAMP OVEN positions of the CIRCUIT
CHECK switch. The A10 Oscillator Assy has its own
temperature-control circuit for the 5 MHz quartz oscil
lator.
Operating current for the oscillator oven is
monitored in the OSC OVEN position of the CIRCUIT
CHECK switch.
Figure 4-1. Rubidium Absorption Plot
I o
CURRENT
1
I
fo =
6 . 8 3 4 6 8 5 GHz
4-1
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