SAFRAN MRO-50 Ruggedized Rubidium Oscillator User Manual

: June 13, 2024
: Safran

Table of Contents

Introduction
mRO system description
The mRO-50 Ruggedized SPECIFICATIONS
mRO-50 Ruggedized installation and operation
References
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SAFRAN MRO-50 Ruggedized Rubidium Oscillator User Manual

Introduction

Safran Timing Technology SA used its over 25 years of experience in manufacturing ultra-stable Rubidium oscillator for ground and space applications to produce a miniaturized Rubidium Oscillator (mRO) with dimensions, packaging and pinout of crystal oscillators (OCXO), measuring 50.8 mm x 50.8 mm x 20 mm. The clock is ruggedized to operate in harsh environment (large temperature range, stringent mechanical stress). It delivers a square reference signal at 10 MHz (0 to 3V) with outstanding performances for a steady power consumption below 0.57 W at room temperature.

Such frequency references are well suited for applications demanding low power consumption, accuracy and retrace, high frequency stability, operation in harsh environment and low frequency drift such as military comms, electronic warfare or airborn & avionics.
The mRO is described in this document divided into 4 chapters:

A brief introduction
A simple description of the principle of the clock
The specifications of the system
The operation manual

mRO system description

Principle of operation and basic configuration

The mRO-RUG is a miniature rubidium clock and essentially consists of a voltage-controlled crystal oscillator (VCXO) which is locked to a highly stable atomic transition in the ground state of the 85Rb isotope. While the frequency of the VCXO is at the convenient standard frequency of 10 MHz, the rubidium clock frequency is at 3.036 GHz in the microwave range. The microwave signal is directly generated with a second voltage controlled oscillator (VCO). The phase-stable link between the two oscillators is established with a fractional-N PLL (phase-locked loop).

The rubidium atoms are confined in a vapor cell at an elevated temperature. The cell is placed inside a cylinder with a gap to which the microwave power derived from the VCO is coupled. The 85Rb atoms in the cell occur with equal probability in the two hyperfine energy levels of the ground state 5S (F=2 and F=3).

In order to detect the clock transition between these two levels, the atoms need to be manipulated in such a way that most of them occur in only one level. This is done by optical pumping via a higher lying state (5P). Figure 1 shows the atomic energy levels and transitions involved in the optical pumping process on the D1-line at 795 nm.
Principle operation configuration

The pump light comes from a VCSEL (Vertical Cavity Surface Emitting Laser) which is tuned to resonance with the transition between the ground-state F=3 level and the (un-resolved) excited states F’=2,3. The pump light excites 85Rb atoms which are in the upper hyperfine level (F=3) to the short-lived excited state 5P from which they decay to the two ground state levels (F=2,3) with equal probability. Since pumping occurs continuously out of the F=3 level, a steady-state is reached where most atoms are found in the F=2 level.

The level of the transmitted pump light is detected by a photodiode after the cell. When a microwave field resonant with the clock transition F=2*F=3 is coupled to the interaction region, the level F=3 is repopulated and light absorption is enhanced. A sweep of the microwave field over the resonance is detected as a small dip in the transmitted light level after the cell.

This signal from the photodiode is fed into a synchronous detector whose output generates an error signal which corrects the frequency of the crystal oscillator so that the (fixed) phase-locked loop keeps the microwave VCO exactly on the atomic resonance maximum.

The vapour cell is filled with metallic rubidium that contains both isotopes 85Rb (72%) and 87Rb (28%). In addition the cell is filled with a buffer gas which collides with rubidium atoms so as to keep them away from the cell walls and restrict their movement. As a result the rubidium atoms are practically “frozen in place” for the interaction time with the microwave field. In this way the Doppler-effect is essentially removed and a narrow line width results.

The cell region is also surrounded by a pair of C-field coils which generate a small axial static magnetic field to resolve the Zeeman sub-transitions of the hyperfine line and select the clock transition, i.e. the one with the least magnetic sensitivity, see Figure 2. To further reduce the magnetic sensitivity, the complete physics package is placed inside a magnetic shield.

Figure 3 gives a basic overview of the different functional blocks of the rubidium atomic clock. The rubidium clock consists of two different packages. First, the Physics Package noted PP, which includes the VCSEL, the rubidium vapor cell, the cylinder coupling the microwave to the rubidium atoms, two C-field coils and an optical filter. Second, the Electronics Package noted EP, which includes the microwave generation, the detection circuitry, temperature controllers, monitoring and signal processing.
Principle operation configuration

Physics Package (patented)

The main design characteristics of the PP are its low power consumption, small size and mass, along with minimal environmental sensitivities and mechanical ruggedness.

All parts of the PP are incorporated into a DIL-14 package hermetically sealed off under Xenon atmosphere to reduce temperature exchange by convection and minimize electrical power consumption.

The light source selected for its compactness and low power consumption is a Vertical Cavity Surface Emitting Laser (VCSEL) at 795 nm. It is coupled to a glass blown cell filled with Rubidium and buffer gas surrounded by a cavity coupled to the microwave signal.

The cavity has two purposes: 1) couple the Rubidium atoms to the microwave field as mentioned previously but also 2) transfer the heat to the cell and make a thermally stable environment around the glass cell as an oven. Both components, VCSEL and Cell, are temperature stabilized.

The design is completed with Helmholtz coils, an optical filter and the photodetector.

Electronics Package

Atomic resonance signal capture

The mRO-RUG transition is a microwave transition at 3.036 GHz.

The microwave resonance which occurs as a dip in the optical signal after transiting the cell, is detected by a photodiode.

The basic purpose of the EP is to synchronize the entering microwave frequency, derived from a temperature compensated crystal oscillator (TCXO), to this absorption dip. It is achieved by tuning the microwave frequency to maximum optical absorption.

Frequency variations of the microwave signal are transformed into DC current changes at the photodetector. The dip, visualized in the photocurrent versus microwave frequency curve of Figure 4 is very small: on the order of 1% of the total photocurrent.

Since DC detection of the dip is not feasible, an AC detection method is used for the following reasons:

The dip amplitude is very small compared to the total photocurrent.
The slope of the derivative of the dip photocurrent versus microwave frequency corresponds to roughly 100 pA/Hz. AC detection is the only solution to have a good signal/noise ratio since the photo-detector with associated amplifier are affected by flicker noise.

The AC method involves square wave frequency modulation of the microwave signal at a rate of approximately 105 Hz. As shown in Figure 4, the modulated microwave frequency flips between 2 discrete frequency valuesf1 and f2. The resulting photo-current i(t) appears also (after the transient) at 2 discrete values i1 and i2. The difference between i1 and i2 produces the error signal used to adjust the crystal oscillator center frequency until the mean value of f1 and f2 is exactly equal to the rubidium hyperfine frequency.

Atomic resonance signal capture
Figure 4: Atomic resonance: error signal detection

Temperature controllers

Since the temperature of the VCSEL and the Cell must be adjusted independently, two separated heaters are necessary to control the temperature of the PP main parts. The temperatures are controlled by compensating thermal losses using heating elements (no cooling). Temperature regulation servo-loops are based on a temperature sensor (NTC within a Wheatstone bridge), Proportional-Integral (PI) regulator and a heating element.

C-field

A pair of C-field coils inside the PP generate a magnetic field which separates the rubidium spectral lines. This magnetic field allows fine-tuning of the output frequency by shifting the rubidium resonance frequency by the second-order Zeeman effect.

A stabilized current drives the coils. The current is adjustable for fine- tuning of the output frequency and can be set by a software interface or by an analog voltage applied to the mRO pin #1.

Telemetries

The user can access these data from a software GUI shown in Figure 5. Telemetry parameters shown include:

Error signal of the atomic lock loop
DC signal on photodiode
VCSEL temperature
Control signal of the VCSEL temperature controller
Cell temperature
Control signal of the Cell temperature controller
EP temperature
VCSEL driving current
Voltage across the VCSEL
Control voltage of the 10 MHz crystal

Figure 5: Control and monitoring software for the mRO-50

More details are given in 4.8.3. More specific information on the mRO-50 Ruggedized – Evaluation Kit document

Frequency adjustment

The C-field coil within the PP provides fine frequency adjustment capabilities. The value of the output frequency is settable by applying an analog voltage to pin #1 (see 4.8.4) or by software.

The mRO-50 Ruggedized SPECIFICATIONS

The specifications of this product are available on our website.

mRO-50 Ruggedized installation and operation

Introduction

This chapter of the manual contains information regarding the installation and operation of the mRO-RUG. It is recommended to read this chapter carefully prior to operate the unit.

Use proper ESD precautions

Ensure that all cables are properly connected

The equipment contains small quantities of rubidium metal hermetically sealed inside the glass lamp and cell assemblies, hence, any dangers arising from ionizing radiation are caused for human health (exemption set in article 3 to Council directive 96/29/Euratom).
For further information, ask for the «rubidium product data sheet».
Handling the product in a reasonably foreseeable conditions do not cause any risk for human health, exposure to the SVHC (substances of very high concern) would require grinding the component up.

Environmental Responsibility

The equipment contains materials, which can be either re-used or recycled.
Do not deposit the equipment as unsorted municipal waste. Leave it at an authorized local WEEE collection point or return to Safran to ensure proper disposal.
To return the appliance:
Submit a support ticket at [email protected] and request an RMA.
We will contact you for more information and/or with shipment process details.

Shipping and receiving information

The mRO-50 RUG is packaged and shipped in a foam-lined box. The unit is inspected mechanically and
electrically prior to shipment. Upon receipt of the unit, a thorough inspection should be made to ensure that
no damage has occurred during shipping. If any damage is discovered, please contact

SAFRAN TIMING TECHNOLOGIES SA

PHONE: +41 32 732 16 66

CH-2000 NEUCHATEL / SWITZERLAND

Should it be necessary to ship the unit back, the original case and packing should be used. If the original case is not available, a suitable container with foam-packing is recommended.

CAUTION
Care must be taken for the transportation of the m RO-RUG to ensure that the maximum acceleration due to a shocks 50g/11ms is not exceeded. m RO-RUG contains glass bulbs, crystal resonators.

When m RO-RUG integrated into an instrument, such instrument shall be packed in a suitable container, similar to containers generally use for the transportation of instruments like scope, video display or computer.

Mounting

CAUTION
Care must be taken to ensure that the maximum operating temperature is not exceeded, (+80°C) as measured at the unit’s base plate)

This maximum temperature can be reached when operating the unit into forced air flow at +80°C.

The mRO-50 RUG is a well shielded unit, using several magnetic shield. Nevertheless, some consideration must be given to the operating location of the unit, regardless of its application. To minimize frequency offsets and/ or non-harmonic distortion, the unit should not be installed near equipment generating strong magnetic fields such as generators, transformers, etc…

The general information for the mechanical interface of the mRO-RUG unit is provided in Figure 6 (all dimensions in mm [inch]).

Dimension

Pin Layout:

HUNCTION

Frequency Adjust (Analog +1.5V ±1V)
GND
10MHz square output (0-3V)
GND
Power 5V or 3.3V depending on model
/LOCK (Bit)
TxD
R xD

Figure 6: Mechanical interface and dimensions of the mRO-50 RUG

Dimension

Normal operation

When 5 or 3.3 Vdc, depending on the option, is applied to pin #5 (+), the unit will immediately generate a 10 MHz signal from the crystal oscillator. Within approximately 120 seconds after application of input power, the unit will “lock”, i.e. the crystal is now synchronized by the atomic resonant frequency.

The unit is able to provide a single signal called ‘/lock ’ (pin #6) which toggles from high to low level when the internal crystal oscillator is locked to the Rb atomic resonance (see 4.9). The center frequency is tunable by applying an ultra-stable analog voltage to pin #1. It is also possible to adjust the frequency by software (see 4.8.4).

Serial interface operation

Introduction

4.8.1 Introduction The mRO-50 RUG integrates a micro-controller with A/D and D/A embedded converters. The micro-controller is used to set the parameters of the clock but also to lock the TCXO on the Rb absorption ‘dip’. The built-in serial interface allows an automatic parameter adjustment during the manufacturing process as well as a fine and coarse adjustment of the center frequency. All the working parameters are stored in a built-in EEPROM memory and are accessible through the serial interface for monitoring.

The user must send commands with the following pattern (TxD):

command | carriage return to validate command

Example: monitor1 or PI L_ cfield 0960 Remark: the system removes the line feeds and spaces and is case insensitive.

The returned response is decomposed as following (R xD):

response (if there is) + carriage return + line feed

Example: 0123

or error response (if there is) + “ ?” + 8 bits number corresponding to the error type + carriage return + line feed

Example: 0123 ?08

Serial interface connection

The data transfer from the mRO-50 RUG is made by direct connection to a PC or standard terminal.
The data transfer parameters are the following:

bit rate: 9600 bits/s
parity: none
start bit: 1
data bits: 8
stop bit: 1

Figure 7: Communication polarity of he mRO-50 RUG
Serial interface connection

mRO-50 Ruggedized parameters monitoring

The parameters monitoring is readable through the serial interface and with the use of the single command ‘MONITOR1‘ followed by a carriage return character. The mRO-50 RUG will respond to this command with an ASCII/HEX coded string as shown below.

AAAABBBBCCCCDDDDEEEEFFFFGGGGHHHHIIIIJJJJKKKKLLLLMMMMNNNNOOOO

(example: 08F90BCE10CC0F8C09600BFC07E207E507C00B5F0D970D1B09D709554D05)

This string is composed of the 15 parameters. The DEC(X) function is the conversion of the hexadecimal to decimal value. Each returned byte is an ASCII coded hexadecimal value. The value are coded full range (4800: 0x0000 to 0x12C0 and 4095: 0x0000 to 0x0FFF) but are truncated into the software of the mRO. •

(AAAA) Rubidium cell temperature setpoint:

during the optimization of the clock, the temperature of the glass cell is adjusted to maximize is define as setpoint.
mRO-50 Ruggedized parameters monitoring

The highest hexadecimal value is 0x0B86 which corresponds to 101°C.

(BBBB) LASER temperature setpoint: the temperature of the VCSEL is set to match the laser wavelength to the Rubidium resonance for a fixed driving current. The temperature determined during

The highest hexadecimal value is 0x0E42 which corresponds to 100°C.

(CCCC) LASER current setpoint during start-up: after switch-on, the mRO-50 runs the process of locking the light source to the Rubidium resonance which lasts 70 seconds. During this process, the current driving the VCSEL is fixed to a high value. The light control loop is closed and the current will converge to the value which enables the lock.

The maximum driving current is 1.8 mAmp (0x1130).

(DDDD) C-field current setpoint: a small current crosses the pair of C-field coils which generate a small axial static magnetic field to resolve the Zeeman sub-transitions of the hyperfine line and select the clock transition.

The maximum current of the C-field is 5882 µAmp (0x0000).

(EEEE) Differential integrator DYNAMIC setting: after switch-on, the mRO-50 runs the process of locking the light source to the Rubidium maximum absorption which lasts 70 seconds. During this period, a voltage bias is added at the input of the light lock corrector to ensure that the controller will converge to the absorption.

The voltage range is from 0 to 3 V (0x0000 to 0x12C0).

(FFFF) TCXO control voltage: the control voltage of the local oscillator (TCXO) is corrected in real time to lock its frequency to the Rubidium atomic transition at 3.036GHz.