Technical Topics



Datum 2000 rubidium GPS receiver

Most GPS disciplined oscillators are crystal controlled. This is true of the HP GPS receivers like the Z3801A and Z3816A. However, Datum offers a choice between either crystal or rubidium oscillators. The Datum System2000 is a second generation GPS based Time and Frequency System. The use of a rubidium oscillator allows for improved holdover performance should the GPS signal be lost. The System2000 is also designed to provide the user with a variety of plug-in option circuit boards for different applications.

This receiver is an excellent performer, especially so in its rubidium variant with the Efratom LPRO-101 oscillator.

Datum System2000 Basic Specifications

Type: 6-channel C/A-Code-Receiver, tracks up to 6 satellites continuously. An active amplified antenna is required.

8 x 10-13 Frequency Stability under Selective Availability (24 hours)
Rb-Oscillator Holdover Stability (Aging ca. 1.3 x 10-12 per day)
Rubidium Long Term Stability: 3.9 x 10-11 per month

Input / Output
1 pps Input: TTL into 50 Ohm for system check without GPS (for ext. Sync) or for free running operation with external synchronization

1 pps Output: TTL into 50 Ohm

10 MHz Output: sine / 0.5 Vrms into 50 Ohm

Relay Outputs A6 Warning Secondary-Alarm LED amber A7 Defect Primary-Alarm LED red Number: 2 x changeover contacts / potential free Contact Current Capability: 1 A

Power Supply: 38 to 74 Vdc (48 Vdc nominal) or 230 Vac Power: Typical 30 W, max. 120 W (warm-up)

Operation Temperature: 0° C to +45° C (Unit) -55° C to +85° C (Antenna)

Dimensions: 483 x 132. x 320 mm / w x h x d (about 19 x 5 x 12 inches)

RS232-Interface: 150 Baud to 9600 Baud (hardware handshake, cable detail below)

Front Panel Detail

The front panel keypad, LED indicators, and LCD status indicator are shown.

Datum 2000 front panel detail

Rear Panel Options

The rear panel is configured to provide for numerous user options. The System 2000 is a modular design. The basic system provides add-on capabilities to satisfy customer
needs for specific frequencies and varying numbers of outputs. Up to 24 BNC and four Sub D9 outputs can be installed. The picture below shows the a 48V version with additional 4 x 2.048kHz output option.

Rear panel Datum 2000

Modular Card Design

The Datum System2000 has plenty of room inside the cabinet for optional circuit boards.

Inside the Datum 2000

Efratom LPRO-101 Rubidium Oscillator

The GPS disciplined rubidium oscillator provides a 10 MHz output. The Efratom LPRO-101 is designed with a goal of ten years of maintenance-free operation. According to Datum, each LPRO has been designed to have enough rubidium fill in the lamp to last for the required period, sufficient pulling range for the voltage controlled oscillator, and adequate dynamic range of the rubidium control loop.

Datum indicates the EFC control sensitivity is roughly 6 parts in 1E13 per millivolt, so relatively fine resolution is available on the EFC software graph.

The Efratom oscillator is a single circuit board design, has extensive RFI shielding, and occupies one slot inside the receiver cabinet.

Efratom rubidium details

Control Software 

Datum supplies basic control software for their System2000. This is similar in appearance and function to the HP Satstat software. Every command works normally in Terminal mode. Received data is displayed in an output window. One phase measurement graph is also generated that displays the last 500 values of phase error from the unit's memory.

Datum 2000 standard software

GPSCon Software version D2k

This receiver has significant differences from those of the HP or Symmetricom families. Because of this, there is a special version of GPSCon called GPSConD2k which is specifically designed to work with this receiver. This software offers a high performance alternative to that supplied by Datum.

There are fundamental differences in the amount of information which can be obtained from the Datum compared to the HP receivers. Firstly, there is no information regarding satellites which are not being tracked, only those which are currently being used by the receiver. Secondly there is no predicted uncertainty for holdover (free run) mode. Thirdly, the values for TI and EFC are updated less frequently than a similar HP GPS receiver. This has a modest impact on the appearance of a long term graph of 48 hours or more. However, a short term graph is less detailed than one from an HP receiver.

GPSConD2k will produce a high quality graph of the electronic frequency control, and the time interval between the 1 PPS GPS clock and the 1 PPS locally generated clock. The number of satellites being tracked is recorded as for an HP. The satellite map has all the transit paths of the tracked satellites.

Note that GPSConD2k was written specifically for the Datum System2000, and does not support a variety of additional HP style receivers like the Z3801A. GPSCon Pro remains the software of choice for HP GPS receivers.

GPSConD2k Software

Since the Datum System2000 has its own illuminated LCD display and a keypad, it can be completely operated without a PC. However, GPSConD2k adds another dimension to ones appreciation of this fine receiver. A glance at the built-in long term receiver plots will alert you to important changes in receiver performance. Long term plots reveal subtle characteristics impossible to determine with basic software.

Optional Web-plots for the Datum System2000

This is a built in feature that sends current receiver status plots to an Internet server. This is useful for remote monitoring applications. The screen capture below is from an actual Internet Web site.

RS-232 Serial Interface

The Datum receiver supports hardware handshaking only. The NULL modem serial cable must have RTS and CTS, DTR and DSR, together with TXD and RXD, connected as crossovers. In addition, each end of the cable needs its DCD locally connected to DSR.

Com cable wiring diagram


When a receiver is moved, the stored position must be re-set. This can be carried out by issuing a reset command (which deletes all stored data) or by deleting the stored position only. The stored position can be deleted by selecting the position display screen, pressing "E" and selecting "yes". If the location is incorrect, the unit will not lock to GPS.

Antennas with low power consumption trigger the "antenna feed fault" error. This does not affect the running of the unit and the alarm relay and LED can be disabled by setting the mask on the error handling to ignore GPS errors. (One user noted that he avoided the "antenna fault" condition by adding a 470 ohm resistor threaded through a ferrite bead and then connected to an SMA T piece. It still detects if the real antenna is removed.)

Try to locate the System 2000 unit away from sources of heat or cold and in a location with minimal temperature variation.

Some early units were sold under the Efratom name.

(Thanks to Graham Leighton)