1. Introduction

1.1. Cesium-controlled Frequency via Satellites

The GPS-controlled frequency standards GPS-12(R) and the GLONASS/GPS-controlled frequency standard GPS-12RG deliver a precision frequency and time reference everywhere in the world. They gain their long-term frequency stability from Cesium standards in the navigation satellite systems GPS and GLONASS. They are also designed to provide very high short-term stability and are cost-efficient and extremely accurate frequency standards.

These reference sources are very suitable as frequency standards in the telecommunication and electronics industry. They fit both as stationary frequency references – for instance in test systems and as local references in the design department – and as portable, highly accurate reference sources for field use.

These instruments are off-air frequency standards with an internal architecture according to Figure 1-1. They have one antenna input and a number of optional frequency outputs.

There is also an option for disciplining with an external 1 pps source, e.g. an in-house cesium standard.

Fig 1-1. Simplified block diagram of the GPS-12X.

1.2. Optional I/O Boards

The GPS-12X comes as standard with two 2.048/1.544 MHz square wave outputs and one 1 pps pulse output with approx. 10 µs duration. TheGPS-12RGhasalso3x10MHz and 1x5MHz outputs. See Option 70B below. There are six options to choose from, options 70B, 71B, 72B, 73B, 74B and 79/01. All but the 79/01 allow for four extra frequency outputs to be mounted. Any combination of none, one or two of these units is possible. Only one 79/01can be mounted.

Option 70B gives 3×10 MHz and 1×5 MHz sine wave outputs, 1 VRMS in 50 Ω. The GPS-12RG is as standard equipped with one such option.

Option 71B gives four sine wave outputs, 1VRMS in 50 Ω, of resp. 10 MHz, 5 MHz, 1 MHz and 0.1 MHz.

Option 72B gives 2 x 2.048 MHz clock outputs,±1.2 V square wave in 75 Ω, plus 2 x 2.048 Mbps data outputs (G.703:10), for telecommunication testing and clock synchro-nization.

Option 73B gives 4 x 13 MHz square wave out-puts with TTL levels in 50 Ω.

Option 74B gives 2 x 1.544 MHz plus 2 x 1.544 Mbps outputs for SONET applications.

Option 79/01 gives 2 x 10 MHz sine wave outputs, 1 VRMS in 50 Ω, plus 1 x 1 pps output, TTL levels in 50 Ω, plus 1 x 1 pps disciplining input, TTL levels.

1.3. Reference Oscillators

The GPS-12 is equipped with an oven-con-trolled crystal oscillator (OCXO), the GPS-12R has a standard rubidium oscillator, and the GPS-12R/HS has a high-stability, low phase noise rubidium oscillator. The GPS-12RG is also equipped with the latter oscillator.

1.4. Two Operating Modes

The disciplined mode is the default mode. It eliminates long-term frequency drift, also called aging. As long as there is a valid satellite signal, the local rubidium oscillator is continu-ously adjusted to minimize the deviation from the satellite-derived reference signal.

The hold-over mode is entered either automati-cally, if the satellite contact is lost, or manually. The automatic adjustment is replaced by the normal aging characteristics of the rubidium oscillator.

The manual hold-over mode is mainly intended for the hopefully rare occasions when the satel-lite contact is sporadic, causing unacceptable mode switching interference.

1.5. Portability

By meansofOption78orOption78/HS,anin-ternal rechargeable battery unit, it is possible to maintain stability during transportation, as the internal reference oscillator is continuously powered.

Field use without access to AC line power is also practicable.

Option 78/HS has the additional feature of ac-ceptinganexternal12VDC supply. Thus it is possible to realize a true uninterruptible power supply (UPS), as the DC source and the AC source can be applied simultaneously. The AC source takes precedence, as long as it is present, and so does the external DC supply over the internal battery, which is kept charged by one of the external sources.

In case of a power line failure you will benefit from double security, if you utilize an external DC source in addition to the AC supply.

1.6. Stationary Use

Option 77 makes it possible to apply -48 VDC as an alternative to the standard AC line volt-age, thus allowing permanent use as local fre-quency standard in telephone exchange sta-tions. AC line voltage and -48 VDC can be ap-plied at the same time in order to realize a true uninterruptible power supply (UPS).

1.7. Overall Accuracy

The cesium standards of the satellites are con-trolled by primary frequency standards like the US Naval Observatory, and ultimately to all na-tional standards (e.g. NIST, NPL, PTB, SP etc.).