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SRS Labs SIM940 Operation And Service Manual

Rubidium frequency standard
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Operation and Service Manual
Rubidium Frequency Standard
SIM940
Stanford Research Systems
Revision 1.3
May 8, 2007

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  • Page 1 Operation and Service Manual Rubidium Frequency Standard SIM940 Stanford Research Systems Revision 1.3 May 8, 2007...
  • Page 2 Copyright c Stanford Research Systems, Inc., 2003, 2007. All rights reserved. Stanford Research Systems, Inc. 1290–D Reamwood Avenue Sunnyvale, CA 94089 USA Phone: (408) 744-9040 Fax: (408) 744-9049 www.thinkSRS.com e-mail: info@thinkSRS.com Printed in U.S.A. Document number 9-01568-903 SIM940 Rubidium Frequency Standard...

  • Page 3: Table Of Contents

    Testing connection ....3 – 3 Connecting to the SIM940 ....3 – 3 Terminal program .
  • Page 4 5.2 Component parts list ....5 – 5 5.3 Schematic Diagrams ....5 – 5 Addendum: PRS10 Manual SIM940 Rubidium Frequency Standard...

  • Page 5: General Information

    Do not install substitute parts or perform any unauthorized modifi- cations to this instrument. The SIM940 is a double-wide module designed to be used inside the SIM900 Mainframe. Do not turn on the power until the module is completely inserted into the mainframe and locked in place.

  • Page 6: Symbols

    Symbols you may Find on SRS Products Symbol Description Alternating current Caution - risk of electric shock Frame or chassis terminal Caution - refer to accompanying documents Earth (ground) terminal Battery Fuse On (supply) Off (supply) SIM940 Rubidium Frequency Standard...
  • Page 7 General Information Notation The following notation will be used throughout this manual: Front-panel indicators are set as Overload. Remote command names are set as ID?. Literal text other than command names is set as OFF. SIM940 Rubidium Frequency Standard...

  • Page 8: Specifications

    72 h Stratum 1 level Frequency retrace (72 h o , then 72 h on) Control Settability Trim range (0 to 5 VDC) 0 5 ppm (remote interface) Warm-up time 6 minutes (time to lock) 7 minutes (time to 1 SIM940 Rubidium Frequency Standard...
  • Page 9 95 % (non-condensing) General Characteristics Interface Serial (RS-232) through SIM interface Power 24 VDC Supply current 2.2 A at start-up, 0.6 A after warm-up period Weight 5 lbs Dimensions 1 5 W 3 6 H 7 0 D SIM940 Rubidium Frequency Standard...
  • Page 10 General Information SIM940 Rubidium Frequency Standard...

  • Page 11: Overview

    1 Overview This chapter gives the necessary information to get started quickly with the SIM940 Rubidium Frequency Standard. In This Chapter 1.1 Introduction to the Instrument ... . . 1 – 2 Quick Start Guide .

  • Page 12: Introduction To The Instrument

    This interface is used for frequency calibration, 1 pps timing and status determination. A Windows application program is provided. The SIM940 will time-tag the rising edge of a user supplied 1 pps input, which usually comes from a GPS receiver. The module can phase-lock to this input to cancel long-term aging.

  • Page 13: Quick Start Guide

    1.2 Quick Start Guide 1. Turn o the SIM900 mainframe. 2. Install the SIM940 in the SIM900 mainframe. (Use slots #7 & #8 if available for best thermal environment and shortest connection for 10 MHz to SIM900 timebase input.) 3.
  • Page 14 1 – 4 Overview Figure 1.1: The SIM940 front and rear panels. SIM940 Rubidium Frequency Standard...

  • Page 15: Sim Interface

    SIM940 is mated to a SIM900 Mainframe via this connection, either through one of the internal mainframe slots, or the remote cable interface. It is also possible to operate the SIM940 directly, without using the SIM900 Mainframe. This section provides details on the interface. 1.3.1 SIM interface connector The DB–15 SIM interface connector carries all the power and commu-...
  • Page 16 RS-232 serial port of a personal computer. Connect RXD from the SIM940 directly to RD on the PC, TXD directly to TDS. In other words, a null-modem style cable is not needed. To interface directly to the DB–9 male (DTE) RS-232 port typically found on contemporary personal computers, a cable must be made with a female DB–15 socket to mate with the SIM940, and a female...

  • Page 17: Operation

    DG535 Digital Delay Generator ..2 – 4 Direct Frequency Control of the SIM940 ..2 – 4 Using an External 1 pps Reference ... 2 – 5...

  • Page 18: Power

    Within seven minutes, the module will be locked to within 0.001 ppm of 10 MHz. 2.3 Connection to other instruments All of the outputs on the rear of the SIM940 have a source impedance of 50 and are intended to drive 50 loads.

  • Page 19: Connection To Srs Instruments

    For convenience, it is best to install the SIM940 in the slot 7 position of the SIM900 (filling slots 7 & 8), so that a short length of RG58 coax cable can tie one of the 10 MHz outputs of the SIM940 to the SIM900 TIMEBASE IN.

  • Page 20: Direct Frequency Control Of The Sim940

    Some revisions of the DG535 required a 10 MHz square wave input, and may not operate properly with the 10 MHz sine wave output from the SIM940. The user should verify proper operation of the DG535 when locked to an external time base with an SR620 Time Interval Counter.

  • Page 21: Using An External 1 Pps Reference

    However, when a 1 pps signal is seen, the phase-lock algorithm “hijacks” the SF parameter to phase-lock the rubidium standard to the 1 pps input. The 1 pps from another SIM940 will work fine, providing it is not terminated into 50 ohm. SIM940 Rubidium Frequency Standard...
  • Page 22 , or 36 ns. The SIM940 indicates that it is synchronized to the 1 pps input by a front panel LED. The unit will indicate that is synchronized when the 1 pps input is present and if it occurs within 1 s of the 1 pps output.
  • Page 23 It is quite common for a unit to indicate that it has acquired, and lost, synchronization depending on the initial freqneyc o set between the SIM940 and the 1 pps reference. Suppose that a SIM940 is out of cali- bration by 1 10 and a 1 pps input is applied while using the default PLL time constant of PT8 or 2.25 hours.
  • Page 24 2 – 8 Operation SIM940 Rubidium Frequency Standard...

  • Page 25: Monitor And Control

    Testing connection ....3 – 3 Connecting to the SIM940 ....3 – 3 Terminal program .

  • Page 26: Setting Up Communications

    Stanford Research Systems. All commands sent to the SIM940 are passed directly to the PRS10. All commands received from the SIM940 come directly from the PRS10. No bu ering or filtering of commands is performed by the SIM940. The PRS10 oper- ation and service manual, which is an addendum to this document, serves as a programming reference for the SIM940.

  • Page 27: Testing Connection

    3.4 Connecting to the SIM940 Initially, the RbMon program will only be communicating with the SIM900 mainframe. To control a SIM940 installed in a mainframe slot, we must inform the SIM900 which slot number to communicate with. This is done with the CONN command. The format of this...

  • Page 28: Terminal Program

    Following the CONN command, issue a ID? to verify that communi- cations are now established with the SIM940. For example, if SIM940 is installed in slot 7 of the SIM900 mainframe (occupying the rightmost two slots), the screen should appear as below.

  • Page 29: Rubidium Stats

    PRS10 settings. The settings will be continuously updated until you disconnect from the communications port by clicking on the ”Connected” check box a second time to uncheck it. A sample screen shot, taken while the PRS10 was warming up, is shown below. SIM940 Rubidium Frequency Standard...

  • Page 30: Factory Settings

    In the screen shot, the value of LO is zero, indicating that the unit has not yet locked to the atomic tran- sition. It is highlighted with a blue box to indicate that this value is lower than normal. SIM940 Rubidium Frequency Standard...

  • Page 31: Analog Output Values

    . The interpretation of the boxes is as follows: (Red) Value is higher than normal. (Blue) Value is lower than normal. (Black)Value is di erent from the value stored in EEPROM. SIM940 Rubidium Frequency Standard...
  • Page 32 3.7.2 Help menu The Help menu has two options to choose from. 3.7.2.1 Contents Choose this menu item to display help information. 3.7.2.2 About Choose this menu item to display the program name and version information. SIM940 Rubidium Frequency Standard...

  • Page 33: Calibration

    4 Calibration This chapter describes the calibration procedure for the SIM940. In This Chapter 4.1 With a frequency counter ....4 – 2 With a 1 pps reference ....4 – 4 Calibration of the time-tag o set parameter .
  • Page 34 1. Be certain that the SIM940 has been operated continuously for at least 24 hours. 2. If the SIM940 has been o for more than one week, it should be operated continuously for at least 72 hours before the adjust- ment is made.

  • Page 35: With A Frequency Counter

    5. Measure and record the dc voltage on the center pin of the FREQ CTL BNC. 6. Power cycle the SIM940 by switching o the SIM900 mainframe for at least three seconds. This will make certain that the unit will track the calibration potentiometer (which can be ignored if the unit was phase-locked to a 1 pps input or the frequency had been previously set via the SF command.) The unit should...
  • Page 36 To use this procedure you will need a source of a precise 1 pps signal, such as a GPS timing receiver. You will also need a way to commu- nicate with the SIM940 via the serial interface, such as a PC or laptop computer with a serial port operating at 9600 baud. Any Windows computer together with the SIM940 application program, RbMon, will work well.

  • Page 37: With A 1 Pps Reference

    PLL to about 2.25 hours, which reduces the noise of the 1 pps reference by averaging. (PT8 is the default value.) 9. If the SIM940 has been o for more than one week, it should be operated continuously for at least 72 hours before the adjust- ment is made.
  • Page 38 1 pps reference. 4.3 Calibration of the time-tag offset parameter The SIM940 can time-tag the leading edge of a rear-panel 1 pps input. Results of the time-tag are returned over the serial interface, in re- sponse to the TT? command, in integer nanoseconds with respect to the 1 pps output.

  • Page 39: Calibration Of The Time-Tag O Set Parameter

    9. Iterate as necessary, adjusting TO, until the TT? returns 5 ns. 10. Save the TO parameter to EEPROM by sending the command 11. Verify that the TO was saved to EEPROM by reading it back from EEPROM with the TO!? command. SIM940 Rubidium Frequency Standard...
  • Page 40 4 – 8 Calibration SIM940 Rubidium Frequency Standard...
  • Page 41 5 SIM940 Circuit Description This chapter presents a brief description of the SIM940 circuit design. A complete parts list and circuit schematics are included. In This Chapter 5.1 Circuit Discussion ....5 – 2 5.1.1...

  • Page 42: Circuitry

    5 – 2 Circuitry 5.1 Circuit Discussion The SIM940 provides an operating environment for the PRS10 Rubid- ium Frequency Standard. The PRS10 operation and service manual details operation, circuits, and software interface for the rubidium frequency standard as an addendum to this manual.

  • Page 43: Lock And 1 Pps Output

    SIM900 Mainframe and installed SIM modules is 9600 baud. 5.1.5 Power Supplies The SIM940 is powered from the 24 VDC from the SIM900 main- frame. An L C filter (L1 & C1) is used to reduce ripple from the mainframe’s power supply.

  • Page 44: 10 Mhz Outputs

    The output has a source impedance of 50 and amplitude of 1.41 Vpp with very low phase noise. The SIM940’s 10 MHz output drivers were designed to maintain the low phase noise of the PRS10’s output and to reduce the harmonic distortion with high output-to-output isolation.

  • Page 45: Component Parts List

    0-00208 R19,R24,R29,R34 4-01088 SIM RUBBER FEET 0-00188 R20,R25,R30 4-01447 FRONT PANEL LEXAN 7-00654 5.3 Schematic Diagrams Schematics for the SIM940 follow this page. For schematics of the PRS10, see the attached PRS10 Operation and Service Manual. SIM940 Rubidium Frequency Standard...

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