9
FS700 OVERVIEW
Introduction
The FS700 LORAN-C Frequency Standard produces a highly stable
and accurate 10 MHz output by locking an internal crystal oscillator to
the Cesium clock controlled LORAN-C radio transmission. The FS700
system consists of a receiver, containing amplifiers, filters, and data
acquisition circuitry, and a remote antenna, with an internal preamplifier.
Data acquisition circuitry allows the FS700 to frequency lock its internal
oscillator to the third positive zero crossing of the LORAN-C
transmission. In addition to providing an ultra-stable 10 MHz output, the
FS700 also provides a user selectable TTL compatible frequency
output in the range of 0.01 Hz to 10 MHz in a 1, 2.5, 5 sequence. An
internal phasemeter circuit allows precise frequency calibrations of
other oscillators in the range of 100 kHz to 10 MHz.
Antenna
The FS700 should be used with the supplied antenna. Do not use
another antenna because the antenna box contains filter and amplifier
circuits that are necessary to the FS700. Mount the antenna outside,
vertically, and preferably on the roof of a building. Connect the antenna
to the FS700 with shielded cable up to 1000 feet long. Use either 50 or
75 ohm cable, since the impedance of the cable is not critical. A 100
foot, 50 ohm cable is supplied with the FS700.
Signal Acquisition
After the user chooses and enters the desired GRI, the FS700 will
acquire the LORAN-C signal (the SEARCH LED will turn on). First,
automatic gain control (AGC) software adjusts the receiver gain so that
the signal at all points in the GRI is at full scale (about 6 V pk-pk at the
LORAN OUT connector). Next, the entire GRI is searched for the
presence of LORAN pulses. After the pulses are found, and the
stations identified, the desired station is chosen. This station may be
selected automatically to be the largest signal detected, or the user may
enter a specific station of choice. After the station is chosen the
receiver identifies and matches the phase coding of the selected
station.
The FS700 then begins to frequency lock to the entire pulse envelope
of the selected station. This initial frequency locking removes any initial
gross frequency offset (up to 2 x 10
-7
) and allows more time for the
determination of the third zero crossing position. When the frequency
offset has been reduced to better than 1 x 10
-9
, the phase offset
between the internal oscillator and the LORAN signal is set to zero and
the frequency lock is terminated. The FS700 then identifies the location
of the third zero crossing of the LORAN pulse. After the third zero
crossing has been located, the frequency lock is restarted to lock to the
third zero crossing position. At this point the FS700 enters lock mode
(the LOCK LED will turn on) and begins tracking the third zero crossing.
