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FS700 specificationsMany complex electronic systems require a stable, highly accurate timebase. Communication, automatic test and measurement, and precision time-measurement systems all require accurate frequency standards. Traditionally, users have looked to atomic clocks (cesium or rubidium) for high stability and accuracy. With the FS700 LORAN-C Frequency Standard, cesium-clock stability is now available at a fraction of the cost of atomic standards. The FS700 serves as a NIST traceable frequency reference in the US, Europe and Asia.
Cesium Stability
More than 50 LORAN transmitters are maintained throughout the Northern Hemisphere by the US Coast Guard. The timing of their transmissions is controlled by cesium clocks located at each transmitter site. The FS700 extracts the timing information from the transmitted signal and uses it to frequency-lock its own highly stable oscillator. The result is a 10 MHz output signal with the same stability as the cesium clock used to generate the LORAN transmissions.
Four 10 MHz outputs are available at the rear panel of the instrument, in addition to an adjustable frequency front-panel source. These outputs may used as timebase inputs to other laboratory instruments, such as frequency counters, synthesizers, spectrum analyzers, or pulse generators. The FS710 distribution amplifier can help you distribute the 10 MHz reference signal throughout your facility.
Phase Detector
A built-in phase detector measures the phase shift between an external timebase and the internal frequency source. The FS700 can easily calibrate precision oscillators between 100 kHz and 10 MHz, providing a visual display as well as a voltage output proportional to phase difference. You can use the analog output to drive recorders for long-term frequency-stability testing or to phase-lock other sources to the FS700.
NIST Traceable
The timing information in the LORAN signal is constantly monitored by NIST. Detailed reports of the accuracy of each station and projected station downtime are available. With the FS700, you not only get the accuracy of a single atomic standard, but an entire network of redundant, continuously monitored standards, all at a price far below that of even a single atomic clock.
LORAN-C Operation
LORAN-C stations are divided into groups called chains, each of which broadcast LORAN-C pulses at a common repetition rate, known as the GRI (Group Repetition Interval). Once every GRI, each station in the chain transmits a group of eight LORAN-C pulses each having the pulse shape shown below:
It is the third zero-crossing of these pulses which are accurately controlled by a 10 MHz cesium standard at the LORAN-C transmitter station. The zero crossings of the 10 MHz standard are precisely synchronous with the zero-crossings of the received 100 kHz LORAN-C carrier signal in the ratio of 100:1. Thus, by locking to the third zero-crossing of the 100 kHz pulses, the FS700's internal oscillator maintains the long term stability of the transmitter's standard.
To operate the FS700 you need only enter the GRI for the chain nearest your location. The FS700 automatically adjusts its receiver gain and begins searching for LORAN-C pulses. You can set the FS700 to search for the station with the largest detected signal in the chain, or you can select a particular station. Once LORAN-C pulses have been identified, the FS700 begins an initial frequency lock to the selected station to remove any initial gross frequency offsets. When this is accomplished, the FS700 searches for the position of the third zero-crossing, and locks to it. The entire search process takes between 15 and 40 minutes depending on the signal-to-noise ratio of the selected station. Front panel readout of the signal-to-noise ratio for any station is available.
Continuously Monitored Output
Once locked, the FS700 minimizes the frequency difference between its internal oscillator and the LORAN-C transmission by use of a firmware frequency-locked-loop (FLL). Frequency-locked-loops have much better phase-noise and short term stability than phase locked loops, which can introduce large instantaneous frequency offsets while attempting to maintain zero phase difference. While locked, the FS700 continuously monitors the received signal for error conditions. If errors are found, the unit suspends frequency updates for 20 minutes. If the error condition is cleared by that time, the FLL is resumed, otherwise the FS700 can either wait for the selected station to come back on-line, or search for a new station.
Frequency Outputs and Phase Meter
In addition to the four 10 MHz rear-panel sine outputs, the FS700 has a front panel adjustable frequency, TTL output frequencies between 0.01 Hz and 10 MHz, in a 1, 2.5, 5 sequence. The frequency source has the same accuracy as the 10 MHz outputs. A built-in phasemeter allows comparison of the frequency source with an external input. You can either display a bar-graph of the phase difference between the two signals, or the FS700 will compute the frequency difference between them by looking at the accumulated phase difference over a specified time interval. A front panel connector supplies a voltage output proportional to the measured phase, which can be used to drive recorders, or to phase-lock other sources to the FS700.[end]
Antennas and Filters
The FS700 comes complete with an 8-foot long remote active whip antenna, which is easily mounted on the roof of a building. The antenna is capable of driving up to 1000 feet of coaxial cable to the receiver. The weatherproof antenna base contains an FET preamplifier, and a 100 kHz bandpass filter. An optional lightning protection module can be inserted between the antenna and the receiver to provide protection in adverse weather conditions. Six adjustable notch filters in the receiver allow the user to reject interfering signals for optimum reception.[end]
FS710 Distribution Amplifier
Installations requiring remote outputs will benefit from the optional FS710 AGC distribution amplifier. This unit provides seven 10 MHz outputs from a single input at distances of up to 1 mile from the FS700. For more information read the "Some Questions About LORAN-C" tech note on the FS700.
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