Adjusting Oscillator Frequency; Disciplining To External 1 Pps; Pps Disciplining - Symmetricom 8040C User Manual

Adjusting Oscillator Frequency

There are two reasons to adjust the unit's frequency output:
 To adjust for the effects of aging on the Rubidium oscillator's frequency
 To syntonize (or tune) the Rubidium oscillator's frequency to that of a more
accurate primary frequency source.
The Symmetricom 8040C/8040CLN is a secondary frequency standard (i.e., much more
accurate than a quartz frequency standard, but not as accurate as a Cesium primary
frequency standard). By comparing the Symmetricom 8040C/8040CLN to an external
Cesium standard or GPS receiver, it can be readjusted periodically to match the primary
standard's greater accuracy.
The 8040C/8040CLN output frequency is adjusted by using the RS-232 interface.
The "f" command allows the user to adjust the output frequency in parts <1E-11.
NOTE: A typical external counter does not have a resolution or accuracy high enough to
set this device, so do not reset the frequency unless established metrology methods are
used for frequency measurement resolution of <1 E-1 1.
Example:
1. At the r > prompt, enter f
a. For a 5E-11 change, enter "f5<CR>"
b. For a -5E-11 change, enter "f-5<CR>"
2. Press ENTER
3. At the r > prompt, enter "t"
4. Under the "t", enter 5987717 (this is the pass code save command)
5. Press ENTER (unless changes are saved they will not be recorded)
6. Response "Tuning Data Save"

Disciplining to External 1 PPS

1 PPS Disciplining

The advent of low cost GPS technology has brought about its widespread use
throughout the telecommunications industry. The GPS system provides 1 PPS with
extremely good long-term stability (e.g., <1E-12 averaged over 24 hours). However the
short-term stability of this signal is not suitable due to inherent noise perturbations in
GPS related to background noise, atmospheric conditions, cross talk, multipath, and
instabilities in the oscillators of GPS satellites and GPS receivers.
In order to provide the required stability for telecommunications, system designers must
combine the benefits of short-term stability (such as from a Rubidium or low noise
OCXO) with long-term stability (such as from GPS, Loran-C, GLONASS, or Cesium). In
the past, external disciplining circuitry was required to combine short-term and long-term
stability. The traditional approach involved adding an external circuit to the oscillator that
had a phase lock loop detector to handle disciplining algorithms.
Symmetricom is a leader in time and frequency design and has pioneered the use of
Rubidium oscillators in telecommunications. The model SA.22C Rubidium oscillator in the
Symmetricom 8040C/8040CLN provides an important new feature – built-in disciplining to
a 1 PPS input from an external reference. This new feature eliminates the need for
additional external disciplining circuitry. The 8040C/8040CLN is inherently capable of
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Rev. C, 07/12/2011