Theory Of Operation - HP 5065A Manual

Model 5065A
Circuit Diagrams, Theory, and Maintenance
SECTION IV
THEORY OF OPERATION
circuit theory on individual assemblies, refer
,sc~hetm!ltic
fold-out pages at the rear of this
simplified block diagram of Figure 5-6 shows
~qu.enc;y-<;tat>iliz
ng feedback loop. The 5 MHz
osc~lll!ltor
output is stabilized, first by comparing
output in a frequency-synthesizing and
process with the resonant frequency of
·then translating the difference frequency into
voltage which corrects the quartz oscillator
~il'•OS<cill•ator
Assembly A10 generates the 5 MHz for
Mu>ltip•lier where 5 MHz is: (1) phase modulated at
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
freque"ncy-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.
Tha
harmonic generator/step-recovery diode couples to
the
Rb
17
.absorption cell which
iS
housed in a microwave
cavity
.tuned to 6.834685 GHz, the Rb"' resonant fre-
quency.
In the harmonic generator/step-recovery diode,
15.315.:::.
MHz phase-modulates the 114th harmonic of
produce the 6.834685 ... GHz lower sideband
the microwave cavity resonance and
energy level transitions in the Rb" gas.
4-6.
Figure 4-3 shows the Rb" absorption cell which
CO!Italns the Rb" gas. A 100-MHz oscillator in A12
RVFR Assy drives the lamp filled with Rb" gas. The
1'8eulting light output passes through the RbBS filter cell
lnd
the Rb" absorption cell. The light output of the
absorption cell is monitored by a photodiode.
excitation is removed in the
Rbes
filter cell
undesired transitions. When the Rb" gas is
excited by the 6.834685 ... GHz microwave field at its
resonant frequency, it increases in opacity to reduce
light transmission about 'h% as illustrated in Figure 4-1.
This phenomenon permits using Rb" 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" 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" 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
lo
CURRENT
' - - - -
-----,------'
,,
6.834685GHl
4-1