Pendulum GPS-12, User Manual, Page: 20 / 62

Functional
Description
The GPS-12X series of frequency standards is
continuously disciplined by control signals
from
satellites belonging to the GPS
[GPS-12(R)] or the GLONASS/GPS
[GPS-12RG] navigation satellite systems. The
signals have very low long-term uncertainty
(5*10
-13
per 24h), and are traceable to different
national standards for time and frequency at,
for instance, the National Institute of Standards
and Technology (NIST) via the US Naval Of-
fice (USNO). The GPS-12X contains a satellite
receiver module, generating a stable 1 pps sig-
nal, plus a local voltage-controlled rubidium or
crystal oscillator (OCXO) and a high resolution
measurement kernel that is continuously
phase-comparing the received satellite signal
and the local oscillator. This means that the lo-
cal oscillator is continuously monitored and ad-
justed.
The GPS-12X can operate in two different
modes. Either the local oscillator is free-run-
ning with a frequency offset that increases with
time, due to aging, or the results of the phase
comparisons are used for adjusting the local os-
cillator, thereby compensating for aging. These
two modes are called:
–
Hold-over mode (free-running local oscil-
lator)
–
Disciplined mode (monitored and adjusted
local oscillators)
Disciplined mode is the default mode.
Hold-over mode is automatically entered when
the disciplining fails for some reason (e.g. loss
of satellite contact). Hold-over mode can also
be forced via the User Interface (UI). When the
satellite contact is lost, the GPS-12X can not be
continuously controlled (intercompared with
the satellite reference clocks in real time) and
reverts temporarily to hold-over mode specifi-
cations.
User Interface
General Principles
The UI is straightforward. All interaction be-
tween operator and instrument takes place by
navigating a cursor (marked by text inversion)
on the LCD through a set of text-based menus.
The navigating tools are just four arrow keys,
UP (
p
), DOWN (
q
), LEFT (
t
),
RIGHT (
u
), and one ENTER key. The differ-
ent menus are described in the fol-
lowing paragraphs.
Default Menu
This is the menu entered at start-up
and shows the current instrument sta-
tus. Here
RB-osc is selected, and to
the right you can find one of the fol-
lowing short informative texts:
•
Warming up
•
Unlocked
•
Hold-over
3-4 Functional Description
Using the Controls
Satellite
Receiver
2.048 MHz Outputs
To PC (USB)
PLL-Controlled
Frequency Reference Oscillator
(Rubidium or OCXO)
1 PPS
1 PPS Output
Output Opt.
Other Frequencies
Output Opt.
User Interface (UI)
(Front Panel Keyboard & Display)
Microprocessor
&
Memory
I/O Option
1 PPS Input
1 PPS Output
10 MHz Output
Figure 3-1. The internal high-stability oscillator of the
GPS-12X is continuously compared with
and controlled by the satellite receiver.