Microchip Technology SA5 Series, User Manual

Page: 45 / 63

MAC-SA5X USER’S GUIDE
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2019 Microchip Technology Inc. DS50002938A-page 45
Appendix A. Principle of Operation
The MAC is a passive atomic clock, incorporating the interrogation technique of Coher-
ent Population Trapping (CPT) and operating upon the D1 optical resonance of atomic
87Rb to control the frequency of a quartz crystal oscillator via a frequency-locked loop
(FLL). The FLL function block is shown in Figure
A-1. The principal RF output from the
MAC is provided by a Temperature-Compensated Crystal Oscillator (TCXO) and pro-
vided on the MAC output pin 3. The laser is mounted on a Thermal-Electric Cooler
(TEC) for precise temperature control and is modulated by the microwave synthesizer
at half the hyperfine frequency to produce sidebands in the laser spectrum. The light
travels through the temperature-controlled Rb cell exposed to a magnetic “C-field”
before hitting the photodetector that detects the transmitted CPT resonance signal. The
photodetector current is used to generate a control signal with phase and amplitude
information that permits continuous regulation of the TCXO frequency. Servos control
frequency synthesis chain, laser current, TEC setpoint, C-field, and cell heater. Addi-
tionally, the microprocessor and FPGA provide precise 1 pulse per second functional-
ity.
The main advantage of employing CPT is it allows the use of low power lasers allowing
to construct low power, small size atomic clocks.
Rb Gas Cell Rb Gas Cell Rb Gas Cell DDS
Laser/Micro-TEC
Photodiode
Filters/
Bu
ī
ers
Cell Heater
Cell Heater Control
TCXO
CPU/FPGA
DAC
Microwave
Synthesizer
La
se
r C
u
rre
n
t
C-F
ield
u
W
a
v
e
P
o
w
e
r
C
o
n
tro
l
Si
gna
l
TE
C
S
e
t
TEC Control
DAC DAC DAC DAC DAC ADC
Fr
e
q
u
e
n
c
y
Co
n
tr
o
l
FIGURE A-1: Block Diagram.