Stanford Research Systems FS700 - Frequency Time Standards

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.

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.