AccuBeat NAC1 User manual

Document No. ZD24010
Rev. B2

Nano Atomic Clock
NAC1
User Manual
AccuBeat Ltd
5 Ha’Marpeh Str., Har Hotzvim
P.O. Box 45102, Jerusalem 91450
Israel

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Change Record
Rev.
Description
Date
ECO No
Approved
A
Initial Revision
10-09-15
-
A.S
A6
Specification updated
11-09-16
-
A.S
B1
Specification updated
19-12-16
-
A.S
B2
Specification updated
29-04-18
-
C.L

Tel: [phone]
Fax: [phone]
E-mail: [e-mail]
Web: http://www.accubeat.com

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Warnings
Do not operate the unit before reading this manual.
To avoid electrostatic discharge (ESD) damage, proper ESD handling procedures must be
observed in unpacking, assembling, and testing the NAC1.
Warranty
•The Rubidium Frequency Standard purchased under your order is warranted for one
year against defects in material or workmanship.
•This warranty does not cover cases of inappropriate operation.
•Do not attempt to open the unit!
Before Cleaning
•Disconnect the product from the power before cleaning or maintenance.
General Notes
1. To avoid electric shock or fire hazard:
•Apply voltage to the connectors as specified in the product Electrical ICD
specifications.
•Do not connect the unit to power supply with reverse polarity.
•Operate this unit under dry conditions.
•Operate this unit in un-explosive atmosphere.
2. Use output load impedance of 1MΩ.
3. Use frequency corrections instruction with caution.
Uncontrolled corrections can cause the frequency accuracy to get out of calibration or
even cause the clock to be unlocked!
4. Solder: Hand solder using 63/37 Tin/Lead solder with maximum soldering tip
temperature of 330ºC (626ºF).
The information and specifications included in this manual are subject to change
without prior notice.
The information contained within this manual is the proprietary of AccuBeat Ltd.

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Table of Contents
1. General description .............................................................................................................. 6
1.1. Introduction .......................................................................................................................... 6
1.2. Key features ......................................................................................................................... 7
1.2.1. The NAC1’s main features ............................................................................................... 7
1.2.2. Special features................................................................................................................ 7
2. NAC1 Overview ................................................................................................................... 8
2.1. Precautions .......................................................................................................................... 8
2.2. Packaging ............................................................................................................................ 8
2.3. Absolute Maximum Ratings ................................................................................................. 8
2.4. Mechanical Interface and Mounting ..................................................................................... 9
2.5. Recommended Operating Characteristics ......................................................................... 10
3.Evaluation Kit .................................................................................................................. 11
3.1. Installing the NAC1 on the evaluation board ..................................................................... 11
3.2. Installing the NAC1 GUI software ...................................................................................... 12
3.3. Cabling ............................................................................................................................... 14
3.4. Evaluation Board Overview ............................................................................................... 14
3.5. Initial Start Up .................................................................................................................... 16
3.5.1. Initial Power-On.............................................................................................................. 16
3.5.2. Establishing Communication with NAC1........................................................................ 16
3.5.3. Basic NAC1 GUI Features ............................................................................................. 17
3.5.4. Frequency Tuning........................................................................................................... 18
4. Functional Description ....................................................................................................... 19
4.1. Principle of Operation ........................................................................................................ 19
4.2. Built-In Test (BIT) ............................................................................................................... 20
4.2.1. S/W indication................................................................................................................. 20
4.2.2. H/W indication ................................................................................................................ 20
4.3. 10 MHz Output Characteristics .......................................................................................... 21
4.4. Frequency Adjustments ..................................................................................................... 22
4.5. 1PPS Output ...................................................................................................................... 22
4.6. Rubidium free run .............................................................................................................. 23
4.7. Rubidium disciplined to 1PPS ........................................................................................... 23
4.8. 1PPS Input ......................................................................................................................... 23
5. Disciplining Mode ............................................................................................................... 24
6. Appendices ........................................................................................................................ 26
Appendix A: Mechanical ICD...................................................................................................................... 26
Appendix B: Electrical ICD.......................................................................................................................... 27
Appendix C: Software ICD (CLI)................................................................................................................. 28
Appendix D: Specifications......................................................................................................................... 35

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List of Figures
Figure 1 : NAC1 Mechanical drawing and Pinout ..............................................................................................9
Figure 2 : Protective sheet for Evaluation Board .............................................................................................11
Figure 3 : NAC1 on Evaluation board.............................................................................................................12
Figure 4: LabVIEW85RuntimeEngineFull installation......................................................................................13
Figure 5 : visa441runtime installation .............................................................................................................13
Figure 6 : Evaluation Board Connections........................................................................................................14
Figure 7: NAC1 GUI Software.......................................................................................................................16
Figure 8 : NAC1 Block Diagram.....................................................................................................................20
Figure 9 : BIT LED indication.........................................................................................................................21
Figure 10 : NAC1 10MHz Output Driver .........................................................................................................21
Figure 11 : Evaluation Board 10MHz Output Driver.........................................................................................22
Figure 12 : Mechanical ICD...........................................................................................................................26
List of Tables
Table 1: Absolute Maximum Ratings................................................................................................................8
Table 2 : Recommended Operating Characteristics ........................................................................................10
Table 3: Electrical ICD .................................................................................................................................27
Abbreviations
1PPS –1 Pulse Per Second
BIT - Built In Test
CLI –Command Line Interface
DAC –Digital to Analog Converter
H/W - Hardware
TCXO –Temperature Compensated Crystal Oscillator
P/N –Part Number
S/N –Serial Number
S/W –Software
I/O –Inputs/Outputs
Rb –Rubidium

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1. General description
1.1. Introduction
AccuBeat's 1st generation Nano-Atomic-Clock (NAC1) is an ultra-small, ultra-low power Rubidium Atomic
Clock. NAC1 utilizes a modulated miniature diode laser to optically pump a miniature Rubidium glass cell.
The modulation frequency is derived from a local crystal oscillator which is then locked to the Rubidium
atomic line, thereby delivering the high stability of the atomic quantum transition to NAC1 outputs at
10MHz and 1PPS.
The small size and low power consumption of the NAC1, enables atomic timing accuracy in various
applications such as: GPS receivers, UAV’s, Autonomous sensors and Backpack secure communication
radios.
This manual provides technical guidance to facilitate mechanical, electrical, and functional integration of
the NAC1.
This manual also describes the NAC1 Evaluation Kit (AccuBeat P/N: AA50766), which includes an
evaluation board, cabling, and the NAC1 Graphic User Interface (GUI) software.
Installation and use of the NAC1 Evaluation Kit is presented in Section 3 of this User Manual. The
description of NAC1 functionality in Section 4 includes examples from the Evaluation Kit.

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1.2. Key features
1.2.1. The NAC1’s main features are as follows:
Input: 1PPS (for disciplining).
Outputs:
•Main frequency output (10MHz).
•1PPS
Medium term stability: 2E-11 @ 100s.
Phase noise: <-148 dBc/Hz @ 100 KHz (relative to the main 10MHz output).
Size: 32cc (41.1mm X 35.3mm X 22 mm)
Weight: ≤75 grams.
Communication: RS-232 (CMOS 3.3V, 1MΩ).
Power supply: 3.3VDC
1.2.2. Special features:
•Disciplined to 1PPS: The NAC1 is disciplined to a 1PPS signal, which improves the long-term-
stability as well as the accuracy.
.

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2. NAC1 Overview
2.1. Precautions
Do not operate the unit before reading this manual.
To avoid electrostatic discharge (ESD) damage, proper ESD handling procedures must be
observed in unpacking, assembling, and testing the NAC1.
2.2. Packaging
Please retain the original NAC1 ESD-safe packaging material in the event that the device needs to be
returned to AccuBeat for service.
2.3. Absolute Maximum Ratings
Table 1 indicates the absolute maximum ratings to which the NAC1 can be subjected without
permanent unrecoverable damage. Note that the NAC1 cannot be expected to operate normally when
operated outside of the Recommended Operating Conditions (Table 2) and no performance is
guaranteed under absolute maximum ratings.
All ratings apply at 25°C, unless otherwise noted.
Rating
Parameter
4.1 V
Supply Voltage (Vcc)
1PPS in, Tx, Rx, BIT: +/- 2 mA1PPS out, 10 MHz out: +/- 20 mA
Maximum current draw
-40°C to +90°C
Storage Temperature
Table 1:Absolute Maximum Ratings

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2.4. Mechanical Interface and Mounting
The physical dimensions of the NAC1 are 1.62” x 1.41” x 0.87” H.
Detailed dimensions and NAC1 Pinout are shown in Figure 1.
Pin # 3 is not present in the NAC1.
For initial testing and evaluation it is recommended that the pins should not be modified or soldered to a
PCB. The recommended socket for connecting the PCB is SAMTEC P/N: SC-2P7-TT or SC-2P7-GG.
When soldering the NAC1 to a PCB, please notice that the bottom surface of the NAC1 will be
parallel to the PCB in order to prevent electrical shortages.
Figure 1 : NAC1 Mechanical drawing and Pinout

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2.5. Recommended Operating Characteristics
The NAC1 pinout is shown above in Figure 1. The electrical function of each pin is defined in this section.
Refer to the Reference Section for a detailed functionality description of each pin.
PIN #
NAME
FUNCTION
VOLTAGE LEVELS
NOTES
REFERENCE
SECTION
1
N/C
Not Connected
2
GND
Ground
-
-
-
3
N/A
Not Applicable
4
BIT
Built-In Test
2.5 V < Logic H < Vcc0 V < Logic L < 0.5 V
1,5
4.2
5
Tx
Comm. Transmit
2.5 V < Logic H < Vcc0 V < Logic L < 0.5 V
5
Appendix C:
Software ICD
(CLI)
6
Rx
Comm. Receive
6
7
Vcc
Input Voltage
3.3 VDC +/- 0.1 VDC
4
8
GND
Ground
9
1PPS IN
1 Pulse Per Second
Input
2.5 V < Logic H < Vcc0 V < Logic L < 0.5 V
2,6
4.8
10
1PPS OUT
1 Pulse Per Second
Output
2.5 V < Logic H < 2.85 V0 V < Logic L < 0.5 V
2,3,5
4.5
11
Fact. Use
Factory Use
-
12
10MHz OUT
10MHz Output
2.5 V < Logic H < Vcc 0 V < Logic L < 0.5 V
5
4.3
Table 2 : Recommended Operating Characteristics
Notes:
1. Built-in Test (BIT) output:
0: Normal Operation
1: Unlocked
2. Timing reference is the rising edge of an input pulse.
3. Output 1PPS is 20μs in duration.
Timing reference is the rising-edge of Pin # 10. Rise time < 10 ns.
4. Regulated 3.3V DC.
Recommended regulator : MICREL P/N: MIC39100-3.3WS
5. Load Impedance = 1MΩ.
6. Input Impedance = 1MΩ.

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3. Evaluation Kit
The Evaluation Kit enables the developer top easily evaluate and test the NAC1.
It includes all of the necessary hardware, software, and cabling to facilitate validation of performance
and software interface development.
The Evaluation Kit (AccuBeat P/N: AA50766) contains the following items:
1. Evaluation Board (AccuBeat P/N: AA50666).
2. CD-ROM: Graphic User Interface- GUI (AccuBeat P/N: SW50051).
3. Optional - 1PPS Input reference (AccuBeat P/N: AR133A-00).
4. USB to Serial Cable (AccuBeat P/N: ZE00314).
5. Protective sheet (AccuBeat P/N: MT30160).
3.1. Installing the NAC1 on the evaluation board
In an ESD- safe environment carefully remove the NAC1 and the evaluation board from their ESD
protective bags. Note that the NAC1 pinout is “keyed” (see Figure 1) so the NAC1 can only be
inserted in the proper orientation.
Gently insert the NAC1 into the socket on the evaluation board as shown in Figure 3 below.
Note: In order to prevent a short circuit between NAC1 signals, please verify that the protective
sheet provided with the NAC1 Evaluation Kit is located between the NAC1 pins and the
Evaluation board sockets (see Figure 2).
Figure 2 : Protective sheet for Evaluation Board

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Figure 3 : NAC1 on Evaluation board
3.2. Installing the NAC1 GUI software
The AccuBeat NAC1 GUI software (P/N: SW50051) provides a convenient graphical user interface
(GUI) for monitoring and controlling the NAC1.
After quick installation of the drivers, it will run on any PC running Microsoft Windows®XP or Windows®
7 and having at least one available RS232 (COM) or USB port. Note that multiple NACs can be
monitored from a single PC, provided additional COM ports are available.
To install the NAC1 GUI software, insert the provided CD-ROM into the CD drive of the PC. The
installation will not start automatically.
Please browse to the CD-ROM drive in Windows Explorer. In the GUI Drivers directory you may find
two .exe files.
For both files please perform the following procedure:
- Double-click on visa441runtime.exe / LabVIEW85RuntimeEngineFull.exe.
- The following message will open:
- Click on 'OK'.
- The following Unzip message will open:

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- Click on 'Unzip'.
- The directories will be unzipped and the following installation messages will open:
Figure 4: LabVIEW85RuntimeEngineFull installation
Figure 5 : visa441runtime installation
- Finish both installations by clicking three times the "Next >>" button.
- Restart your PC.

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- Browse to the CD-ROM drive in Windows Explorer. Open the SW50051- Graphical User Interface
(GUI) for NAC1 customer directory and double-click on NAC1 Customer GUI.exe to open the
NAC1 GUI.
3.3. Cabling
Connect the provided RS-232 cable between the evaluation board and the COM port on the PC. On
laptops without an available COM port, a USB-to-RS232 adapter might work.
We recommend using the USB to Serial adapter (Support RS232 Serial communication) that was
supplied with the NAC1 Evaluation Kit.
Connect banana to banana cables (Red & Black) between J36 and J52 power input connectors and
a 6V DC power supply VCC and Ground respectively.
NAC1 signal outputs and inputs are available from the evaluation board on connectors J97 (10 MHz
Output), J38 (1PPS Output) and J37 (1PPS Input). Connect either (or both) of these to your test
equipment (1PPS Input reference, frequency counter, spectrum analyzer, etc.).
3.4. Evaluation Board Overview
Detailed schematics of the evaluation board are provided in Figure 6 below that shows the connections
to the evaluation board.
Figure 6 : Evaluation Board Connections

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10 MHz Output (SMA) –The NAC1 output is a 10MHz, CMOS 0-3.3VDC waveform.
6V DC Power Input –Input power to the evaluation board is provided from an external power supply
through twisted pair banana cables. The use of a current limiter to 2A is recommended.
RS232 Connection (D9) –The evaluation board provides a level shifter (U1), which converts the NAC1
0-3.3 VDC serial interface to the RS232 standard +/- 12 V for direct interface with a PC COM port.
Connect the test fixture (J22) to a PC with a standard (non-Null) DB9F-DB9F RS232 cable. To avoid
problems, please use the proper cable which is provided by AccuBeat with the Evaluation Kit.
BIT Lock Indicator LED –Indicates normal operation following initial acquisition of the clock signal.
Note that this is the logical complement of the BIT output (NAC1 Pin # 4).
Power LED –Indicates the state of the external power supply.
1PPS Input (BNC) –The 1PPS input connection to the evaluation board accepts a 1PPS reference of
arbitrary amplitude (logic high: 2.5V < Vin < 5V) and passes it directly to the NAC1.
1PPS Output (BNC) –The 1PPS output can be buffered by a CMOS 0-3.3 V logic gate on the
evaluation board. For this option please transfer J99 Jumper to J98 Header.
Note: Do not change the configuration of the other JUMPERs connectors.

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3.5. Initial Start Up
3.5.1. Initial Power-On
Make sure NAC1 is placed on its socket on the Evaluation board with the protective sheet.
Connect the Evaluation board voltage input to the Power Supply.
Connect power and RS232 to the Evaluation Board as described in Section 3.3.
Make sure the simulated COM port number is known to you.
Run the NAC1 GUI software:
Figure 7: NAC1 GUI Software
.
The GUI enables simple communication to the NAC1. The GUI provides visual presentation of the
NAC1 parameters such as BIT, serial number and other internal parameters.
3.5.2. Establishing Communication with NAC1
Choose the relevant COM port number from the ‘COM. Selection’ and click the ‘Open Terminal’ button.
You can use the ‘Close Terminal’ button to close communication with the device and ‘Exit’ button to
close the application.

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3.5.3. Basic NAC1 GUI Features
The ‘Read Parameters’ button triggers the read of the NAC1 status and setup. The ‘Rx Reading’
indicator will be lit during the process of data acquiring.
The ‘Received Data’ window is basically used for the visual confirmation of the last issued command.
When the reading of parameters is completed, the updated data will be displayed on the GUI screen.
Note: After power up, start-up scripts will operate for another 5 minutes from the moment the BIT is
valid (warm-up time). You cannot communicate with NAC1 until the start-up scripts will end.
‘BIT’ (Built-In Test) indicates the overall status of the NAC1 control loops and readiness to function.
See also ‘UMR’ command description (Appendix C: Software ICD (CLI)) and Section 4.2 for more
information.
The ‘SW Reset’ button performs NAC1 Soft Reset, which is equivalent to a power cycle.
See also the ‘RST’ command description (Appendix C: Software ICD (CLI)).
The ‘Disciplining’ dialog allows controlling the disciplining process. The default setting is ‘Enable’.
Selecting ‘Disable’ will disable the disciplining, effectively switching NAC1 to Holdover state, regardless
to the 1PPS input status.
See the ‘SDM’ command description (Appendix C: Software ICD (CLI)) and Section 5 for more
information.
The ‘Output Delay’ dialog allows deferring the ‘1PPS Output’ phase by the specified number of nano-
seconds. This is a pure administrative function and doesn’t affect the disciplining algorithm.
See also the ‘SPD’ command description (Appendix C: Software ICD (CLI)) for more information.
The ‘Ext. 1PPS Input Delay’ dialog allows deferring the ‘1PPS Input’ phase by the specified number of
nano-seconds.
See also the ‘SED’ command description (Appendix C: Software ICD (CLI)) for more information.
The VER CLI instruction can be used to remotely identify the unit.
The instruction returns the unit part number (for example: NAC1), S/W and F/W versions, and the
unit’s serial number.
See also the ‘VER’ command description (Appendix C: Software ICD (CLI)) for more information.

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3.5.4. Frequency Tuning
The ‘Frequency Tuning’ dialog allows tuning the NAC1 microwave synthesizer by applying corrections
or setting an explicit frequency value. Corrections are sent in steps of 7.6E-13, 1.96E-10 or 4.98E-8 and
can be positive or negative with an arbitrary number of steps. Moreover, corrections can be applied for
the current session only (until power down) or forever. In the latter case the correction value affects the
base frequency, stored in a non-volatile memory. Every new correction is added to the base frequency.
See also the ‘RFC’ command description (Appendix C: Software ICD (CLI)) and Section 4.4 for
more information.

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4. Functional Description
This chapter provides an overview of the operation principles of a Rubidium Frequency Standard and
describes the main features in the NAC1 unit.
4.1. Principles of Operation
Atomic Clocks, the most accurate man-built machines, have been commercially available for over 40
years. Recent developments in atomic clocks technology have enabled a substantial reduction of size
and power consumption of these clocks, using the phenomenon of Coherent Population Trapping
(CPT), with alkali atoms such as Rubidium and Cesium. The CPT utilizes the transparency of an
atomic vapor cell, which is created when illuminated by two laser beams that differ in their frequency
(wavelength) by an amount that equals the hyperfine transition frequency of a Rubidium (@ ~6.8GHz
) or Cesium (at@ ~9.2GHz ). Instead of using 2 lasers one uses a single laser, which is modulated by
half the hyperfine frequency synthesized from a local crystal oscillator, thereby creating two
sidebands. When the separation between the sidebands exactly equals the hyperfine frequency, the
vapor becomes transparent. One uses this transparency to lock the local crystal oscillator (that
provides the clock output) to the atomic line. With this technique one gets rid of the conventional
lamp and resonance microwave cavity which are being used in the traditional Rubidium Frequency
Standard technology, thereby saving substantial size and power.
AccuBeat’s NAC1 utilizes a novel and unique scheme where numerous servo-loops are operating
simultaneously to stabilize all the key parameters of the clock, and where the sensors of the loops
origin from the atomic vapor. This results in an improved stability of the clock’s output and low
sensitivity to disturbances. In addition, the vapor cell in NAC1 is based on a proven traditional glass
technology which has been used in Rubidium Clocks for dozens of years. This assures very high
reliability and confidence in the design. For the same reason AccuBeat selected to use Rubidium
rather than Cesium due to the long Rubidium heritage in vapor cell atomic clocks (Cesium is normally
used in atomic beam clocks which deploy a totally different approach).
A detailed block diagram of the NAC1 is provided in Figure 8 below:

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Figure 8 : NAC1 Block Diagram
The NAC1 provides outputs of 10MHz and 1PPS.
The NAC1 is comprised of a unique DFLL (Digital Frequency Locked Loop), where a TCXO is locked to
the Rubidium atomic line using an embedded processor. The algorithm improves temperature stability
and enables very fine digital frequency control.
4.2. Built-In Test (BIT)
Successful BIT result indicates that the internal TCXO is locked on the Physics Package.
You can expect the ‘BIT’ to turn ‘OK’ (Green) in 3 minutes after the NAC1 power-up at a room
temperature (25 ºC) or up to 10 minutes at -20 ºC.
The NAC1 has two options for BIT status indication: S/W indication and H/W indication.
4.2.1. S/W indication:
The CLI of the NAC1 contains the BIT indication. The unit BIT can be remotely monitored by using the
CLI. See also the ‘UMR’ command description (Appendix C: Software ICD (CLI)) for more
information
4.2.2. H/W indication:
The H/W BIT indication uses a high-impedance CMOS Compatible logic output in Pin 4 (BIT) of the
NAC1.
Frequency lock is indicated both by FLL status = 0 in the FLL status field of 'UMR' CLI command and
by the low (logic 0) electrical state of the BIT output pin, which is high (logic 1) upon initial power-on and
whenever FLL status = 1.
A visual indication of the BIT status can be created by connecting a LED in series to the power supply
and an appropriate resistor. Please see the following Figure 9.