Stanford Research Systems FS700, FS710 Service manual

MODEL FS700

LORAN-C

FREQUENCY STANDARD

MODEL FS710

Distribution Amplifier

1290-D Reamwood Avenue

Sunnyvale, California 94089

Phone: [phone] • Fax: [phone]

email: [e-mail] • www.thinkSRS.com

Copyright © 1997 by SRS, Inc.

All Rights Reserved.

Revision 2.5 (10/2001)

TABLE OF CONTENTS

FS700 LORAN-C Frequency Standard

Condensed Informati on

Safety and Preparation for Use iii
Symbols v
Specifications vi
Abridged Command List viii
Quick Start x

Operation

Introduction to LORAN-C 1

Signal Characteristics 1
Phase Coding 3
Signal Propagation 3
Blink Transmissions 3
Additional Information 4

LORAN-C Station List 5
FS700 Overview 9

Introduction 9
Antenna 9
Signal Acquisition 9
Locking to LORAN-C 10
Signal Errors 10

Front/Rear Panel Features 11

Front Panel Features 11
Rear Panel Features 12

Front Panel Operation 14

Introduction 14
LORAN Tracking

Overview 14

Tracking Menu 15

GRI Selection 15
Station Selection 15

Search Mode 16
Time Constant 16
Keyboard Lock 16

Status Menu 17

Gain, Signal to Noise 17
Phase 17
Stations Fo und 18
Status Timing 18
Signal Quality 18

14

Tracking P o int M o nito ring 18
Notch Filters 19
Tuning the Notch Filters 19
Search Problems 20

Tracking Problems 21
Frequency Generation
and Calibration 23

Frequency Menu 23

Phase Menu 23

Time Menu 25

Setup Menu 25

Programming

Programming the FS700 27

Communications 27

GPIB Communication 27

Front Panel LED's 27

Data Window 27
Command Syntax 27

Programming Errors 28
Detailed Command List 28

Tracking Co ntrol Co mmands 28

Tracking S tatus Co mmands 30

Phase Meter Commands 31

Status Reporting Commands 33

Hardware Test Commands 34

Status Byte Definitions 35

Status Reporting 35

Serial Poll Status Byte 35

Standard Event Status Byte 35

LORAN Status Byte 36

Programming Examples 37

Microsoft C - Example 1 37

IBM Basic - Example 2 39

Test and Calibr ation

Troubleshooting 41

Self Test Errors 41
GPIB Interface Problems 42

Hardware Troubleshooting 43

16-Bit DAC Test 43
RF Circuitry Test 43
Integrator Test 44

i

Peak Detector Test 44

Performance Tests 45

Introduction 45
Necessary Equipment 45
Tests 45

Start Tests 45
Front Panel Test 45
RF Bandwidth Test 46
Internal Self-Tests 46
Notch-Filter Check 47
10 MHz Oscillator Check 47
Phasemeter Check 48

FS700 Performance Test
Record - Scorecard 49

Calibration 50

Necessary Equipment 50
Coarse Oscillator Adjustment 50
Bandpass Filter Alignment 51
Notch Filter Alignment 53

FS700 Circuitry

Circuit Description 55

Front End/ Notch Filters 55
LORAN Front End 55
Microprocessor System 56
I/O Ports and Interrupt Driver 57
Gated Integrators 57
Gated Integrator Pattern RAM 57
A/D, D/A Converters 58
Clocks and Clock Outputs 58
Phase Comparators/Frequency
Output 59
Front Panel LED's/Switches 59
Unregulated Power Supplies 59
Power Supply Regulators 60
Active Antenna 60

Typical Specifications 75
Operation 76
Trouble Shooting 76
Calibration 76
Circuit Description 76
Line Voltage Selection 76
Line Fuse 76

FS710 Component Parts List 77
PC Layout 80

Schematic 81
Component Placement on PCB
Schematic Circuit Diagrams

Front End/Notch Filters 1/14
LORAN Front End 2/14
Microprocessor System 3/14
I/O Ports and Interrupt Driver 4/14
Gated Integrators 5/14
Gated Integrator Pattern RAM 6/14
A/D, D/A Converters 7/14
Clocks and Clock Outputs 8/14
Phase Comparator/Frequency
Output 9/14
Front Panel LED's/Switches 10/14
Unregulated Power Supplies 11/14
Power Supply Regulators 12/14
Spare IC's 13/14
Active Antenna 14/14

FS700 Component Parts List 61

FS710 Am plifier

FS710 – 10 MHz AGC Distribution
Amplifier

Introduction 75

Front Panel - Figure 1 75

ii

SAFETY AND PREPARATI ON FOR USE

WARNING: Dangerous voltages, capable of causing death, are present in this instrument. Use
extreme caution whenever the instrument covers are removed.

WARNING: Watch for overhead power lines when installing the FS700's antenna. Contact with power
lines can be fatal.

This instrument may be damaged if
operated with the LINE VOLTAGE
SELECTOR set for the wrong AC line
voltage or if the wrong fuse is installed.

Line Voltage Selection

The FS700 operates from a 100 V, 120 V,
220 V, or 240 V nominal AC power source
having a line frequency of 50 or 60 Hz.
Before connecting the power cord to a power
source, verify that the LINE VOLTAGE
SELECTOR card, located in the rear panel
fuse holder, is set so that the correct AC
input voltage value is visible.

Conversion to other AC input voltages
requires a change in the fuse holder voltage
card position and fuse value. Disconnect the
power cord, open the fuse holder cover door
and rotate the fuse-pull lever to remove the
fuse. Remove the small printed circuit board
and select the operating voltage by orienting
the printed circuit board to position the
desired voltage to be visible when pushed
firmly into its slot. Rotate the fuse-pull lever
back into its normal position and insert the
correct fuse into the fuse holder.

Line Cord

The FS700 has a detachable, three-wire power
cord for connection to the power source and to a
protective ground. The exposed metal parts of
the instrument are connected to the outlet ground
to protect against electrical shock. Always use an
outlet which has a properly connected protective
ground.

Antenna

The FS700's vertical antenna should be installed
on the roof of a building. Watch for power lines
when installing the antenna.

Furnished Accessories

- Power Cord

- Operating Manual

Environmental Conditi ons

OPERATING

Temperature: +10° C to +40° C
(Specifications apply over +18° C to +28° C)
Relative Humidity: <90% Non-condensing

Line Fuse

Verify that the correct line fuse is installed
before connecting the line cord. For 100
V/120 V, use an 1 Amp fuse and for 220
V/240 V, use a 1/2 Amp fuse.

NON-OPERATING

Temperature:-25° C to +65° C
Humidity: <95% Non-condensing

iii

FS700 Antenna Installation Instru ctions:

New FS700 Antenna Installation Instructions:

The new style PVC base LORAN-C antenna used with the FS700 LORAN Receiver comes with a 6"
galvanized pipe nipple and 2 U-bolt clamps for mounting. After securing galvanized pipe with U-bolts,
the antenna base can be screwed onto the pipe nipple and the BNC connector can be connected to
the lead-in cable. The piece of supplied special heat shrink tubing should be applied over the cable
connection and heated with a heat gun or other heat source to weatherproof the connection.

The new antenna does not have an internal attenuator as with the old style antennas. If excessive
signal strength is encountered, consult the factory.

!!NOTE ON GROUNDI NG AND L IGHTNING PROTECTION!!

All metal antenna attachments should always be connected to a good earth ground. In areas where
lightning is encountered, a lightning arrester(s) should be used in the antenna lead-in cable.

iv

v

SPECIFICAT IONS

Receiver Specifications

Sensitivity Will lock with signal-to-atmospheric noise level of -10 dB or better.
LORAN Output Filtered and gain controlled antenna signal, typically 6 V peak-to-peak.
Station Search All available stations pre-programmed. Auto-Seek finds and tracks

strongest station.

Notch Filters 6 adjustable 30 dB notch filters, 3 at 40 - 90 kHz, 3 at 110 - 220 kHz.
Antenna 8 Foot Active Whip with 30 dB switchable attenuator, bandpass filter,

and FET preamp in weatherproof housing.

Freq uency Specif ications

Frequency Stability

Long Term 10
Short Term 10

10 MHz Outputs 4 outputs, 1 Volt peak-to-peak sine wave into 50 W.
Internal Oscillator

Standard Option/01

Frequency 10.000 MHz 10.000 MHz
Type AT Cut Ovenized SC Cut Ovenized

Aging 5 x 10
Allan Variance (1 s) 5 x 10

Stability 0-50° C 0.005 ppm 0.005 ppm
Phase Noise (dBc) -120 dBc, 10 Hz offset from carrier

-12

, the same as LORAN-C transmitter Cesium clock.

-10

, standard oscillator.

-11

10

, low phase noise option.

-10

per day 5 x 10

-11

-10

per day

-12

5 x 10

-155 dBc, 100 Hz offset from carrier

-165 dBc, 1 kHz offset from carrier

Phasemet er S pecificati ons

Frequency Output 0.01 Hz to 10 MHz in 1, 2.5, 5 sequence, TTL level.

Can be 50 Ω terminated.

Oscillator Input 1 kΩ, 0.5 V peak-to-peak minimum level. 50 Volts max.

vi

Phase Output 0.01 V/degree, 0 to ±360°. Output proportional to phase difference

between OSC IN and FREQUENCY OUTPUT for frequencies between
100 kHz and 10 MHz.

Phase Resolution 3°

Interface

GPIB IEEE - 488 compatible interface. All instrument functions may be

controlled.

General

Operating 0 to 50° C.
Power 100, 120, 220 or 240 VAC +5% - 10%, 50/60 Hz, 50 Watts.
Dimensions 17" x 17" x 3.5". Rack mounting hardware included.
Weight 14 lbs.

vii

ABRI DGED COMMAND LIST

Commands which may be queried have a ? in parentheses (?) after the mnemonic. The ( ) are not

sent. Commands that may only be queried have a '?' after the mnemonic. Commands which may

not be queried have no '?'. Optional parameters are enclosed by {}. i and j are integers.

Tracki ng Co ntrol Commands

AUTO(?) {i} Sets the search mode to AUTO (i = 1) or Manual (i = 0).
FLLT(?) {i} Sets the receiver time constant.
GRIP(?) {i} Sets the GRI to i micro seconds.
STOP Stops LORAN station tracking.
STRT Starts station acquisition.
STTN(?) {i} Sets the station to be tracked.

Tracki ng St at us Commands

GAIN? Returns the current receiver gain.
INFO? i Returns station search information.
LFOS? Returns instantaneous frequency correction.
LPHA? Returns current phase of internal clock relative to LORAN-C signal.
LSTA? Returns the index of the station being tracked.
NSTA? Returns the number of stations found during search.
STON? Returns the noise margin of the station being tracked.
TIME(?) {h,m,s} Sets/reads the time of day.
TLCK? Returns the amount of time the FS700 has been locked.
TULK? Returns the length of the last unlock period.

Phasemet er Command s

DLTF? Returns the frequency offset between the FREQUENCY OUTPUT and the

OSC IN BNC's.

FREQ(?) {i} Sets the frequency of the FREQUENCY OUTPUT.
PHSE? Returns the phase difference between the FREQUENCY OUTPUT and the

OSC IN BNC.

Status Reporting Commands

*CLS Clears all status registers.
*ESE(?) j Sets/reads the standard status byte enable register.
*ESR? {j} Reads the standard status register, or just bit j of register.
*IDN? Returns the device identification .
*PSC(?) j Sets the power on status clear bit. This allows SRQ's on power up if desired.
*RST Clears instrument to default settings.
*SRE(?) j Sets/reads the serial poll enable register.
*STB? {j} Reads the serial poll register, or just bit n of register.
SENA (?) j Sets/reads the LORAN status enable register.

viii

STAT? {j} Reads the LORAN status register, or just bit n of register.

Hardw are Test Commands

(NOTE: These commands are not needed during normal operation.)
*TST? Starts self-test and returns status when done.

$ASC i Sets antenna input source. i = 0 = antenna, i = 1 = cal.
$DAT? Reads In phase and Quadrature gate data.
$GAT i Sets test gate patterns.
$INT? i Reads integrator data.
$POS i Sets gate position.
ATTN i Sets the attenuators to value i.
OSCF(?) i Sets the oscillator DAC to i.

ix

QUICK ST ART INSTURCTIONS

When Installing the FS700 for the first time, it i s recommended that th e followin g procedure be
carried out. If a problem is encountered, please read the detailed discussion on instrument
operati o n (pages 1-46).

1) Make sure that the correct line voltage has been selected on the rear panel power entry module.

2) Install the antenna according to the instructions on pages iv and 9 of this manual.

3) While holding the BSP key down, turn the front panel power switch of the FS700 to the ON
position. After a second or two, release the BSP key. This procedure will initialize all data in the
FS700’s RAM. Notice that the OVEN LED in the STATUS section is on. It will take 20 minutes
for the internal 10 MHz oscillator to stabilize. At that point, the OVEN LED will turn off. (It is not
necessary to wait for the OVEN LED to turn off before proceeding.)

4) Press the TRACKING menu button on the front panel. Using the FIELD button, position the

cursor the GRI field and type in the appropriate GRI for your area. (Note that the GRI can also

be selected by scanning the LOCATION field. This is done by using the ARROW keys.)

5) Now you must wait until the OVEN LED goes off (approximately 20 minutes). When the OVEN
LED goes off, the SEARCH LED will immediately turn on and the FS700 will start acquiring the

LORAN signal. The search process will take between 15 and 40 minutes. After the FS700 has

successfully locked to the LORAN signal, the LOCK LED will turn on. Now the FS700 is ready

for operation.

x

INT RODUCTION TO LORAN-C

LORAN-C is an accurate navigation system that is maintained by the
U.S. Coast Guard (LORAN stands for Long Range Navigation). A
receiver that measures the arrival times of the signals from three
LORAN stations can determine its position with an accuracy of about
1000 feet at a range of over 1000 miles. Because of the desire for
good long range position accuracy, the frequency and transmission
time of each LORAN transmitter is controlled by a set of Cesium clocks
or Hydrogen masers whose frequency accuracy is maintained by the
U.S. Naval Observatory. Because the timing characteristics of the
LORAN transmission are so tightly controlled, a receiver measuring the
signal from a single LORAN station can produce a very accurate
frequency output that is traceable to the U.S. Naval Observatory and
NIST. LORAN-C transmissions are also highly reliable. The stations
are functional more than 99% of the time and signal errors usually last
only a f ew minutes.

Signal Characterist ics

Signal LORAN-C stations transmit a pulsed signal at a carrier
frequency of 100 kHz. This frequency was chosen for stable
propagation characteristics and low ground wave attenuation. The
transmissions of the various stations are differentiated by the timing of
their pulses. The LORAN transmitters in a specific geographical region
are arranged in groups of at least three (the minimum number needed to
establish position) to at most 6 stations called chains. The chains are
differentiated by the repetition rate of the pulses transmitted by the
stations in the chain. This rate is called the Group Repetition Interval, or

GRI. For example, the U.S. West Coast chain has a GRI of 99400 µs

and each station in that chain (4) will transmit its signal once every

99400 µs. If the receiver synchronizes its timing with the desired GRI,

only stations in that GRI will produce a stable signal. Each chain has a
master station (labelled "M") and up to four secondary stations
(labelled "V", "W", "X", "Y", and "Z"). Once every GRI, each station will
transmit a group of eight pulses at the 100 kHz carrier frequency. Each

pulse is about 250 µs long, and the pulses are separated by exactly 1

ms. The master station has a ninth pulse that is transmitted 2 ms after
the eighth pulse. This ninth pulse identifies the station as the master.
The master station always transmits first in the GRI followed by each

Figure 1

Transmission of LORAN
Signals During a GRI

1

Graph 1

Shape of the Transmitted
LORAN-C Pulse

secondary station in a prescribed order. The signals are transmitted so
that the pulse groups will never overlap within the reception range of the
stations. This is shown in Figure 1. The receiver determines its
position from the time differences between the transmissions from each
station, and the known positions of the transmitters. If the master
station cannot be received, there is no way to identify the stations and
also no way to determine the receiver's position.

The Coast Guard controls the accuracy of the LORAN-C system by
precisely controlling the transmitter carrier frequency and the pulse
emission time. The pulse emission time is controlled by accurately
setting the emission time of the third positive zero crossing of the
LORAN rf pulse. This point is called the PULSE TIME REFERENCE
(PTR), and was chosen as a compromise between adequate signal-to­noise ratio and freedom from skywave interference (discussed below).
The shape of the LORAN pulse is shown in Graph 1. By carefully
tracking the PTR over long time intervals it is possible to extract the
inherent frequency accuracy of the LORAN transmitter's Cesium clock.
This is the function of the FS700 LORAN-C Frequency Standard. The
FS700 will produce a frequency output with the same long-term
accuracy as a USNO controlled Cesium clock as long as a single
station in any GRI is receivable.

2

Phase Codi ng

LORAN transmissions are Phase Coded to minimize the effects of
random noise, CW signal interference, and skywave interference.
Some of the pulses in the eight pulse group are transmitted with the
carrier signal inverted with respect to the rest of the pulses. That is,
some pulses are transmitted with the first cycle of the pulse starting by
going negative instead of positive. The phase code repeats in a two
GRI period called a Frame. A receiver, such as the FS700, that takes
phase coding into account will cancel any interference that occurs on an
interval longer than a frame. Also, the phase coding is arranged so that
any long delayed skywave interference will also be cancelled. The two
GRI pulse groups within a frame are called Group A and Group B and
have different phase codings. Additionally, master station frames have
a different phase coding than secondary station frames. The phase
codings for the various frames are shown in Figure 2.

Figure 2

Phase Codings of Master
And Secondary Stations

Signal Propagation

Group Master Secondary

A ++--+-+- + +++++--+
B +--+++++ - +-+-++--

The transmitted signal from a LORAN transmitter is split into two parts:
the groundwave which travels parallel to the surface of the earth, and
the skywave, which travels upward through the atmosphere, is reflected
by the ionosphere, and returns to earth. Because the height of the
ionosphere depends on the time of day, the season, and solar activity,
the skywave propagation path and the propagation delay are very
unstable. This makes the skywave transmission less than ideal for
accurate timing. The groundwave transmission does not suffer these
problems and is a very stable source of timing information. However,
the groundwave signal is rapidly attenuated by the atmosphere and
suffers contamination from the skywave signal which arrives slightly
later in time. These considerations limit the groundwave reception
range of the LORAN signal to about 1500 miles. At this range the
skywave interference will never disturb the position of the PTR.

Bli nk Tran smissions

Additional Information

Occasionally a LORAN transmitter will malfunction so that its
transmitted signal is no longer accurate. When this occurs the master
station and the affected secondary station begin blink transmissions.
The master station will blink its ninth pulse in a coded pattern indicating
which secondary is bad, while the secondary station will blink its first
two pulses in 0.25 seconds on, 3.75 seconds off pattern. The other
pulses in the transmission are unaffected. By checking for blink the
FS700 is able to ensure that it is locked to a healthy transmitting station.

More detailed information may be obtained from:

3

1) United States Naval Observatory, Time Services Division
34 Massachusetts Ave.
Washington, DC 20390
[phone]

Ask to be placed on the LORAN-C chain information mailing list.
LORAN-C station propagation delays may also be obtained from:

2) United States Coast Guard Headquarters
Washington, D.C. 20593
[phone]

The LORAN-C specifications may be obtained.

Publication numbers:

LORAN C User Handbook COMDTINST M16562.3
Specification of the LORAN C Transmitted Signal COMDTINST
M16562.4

4

X Fox Harbor, Labrador, Canada

Chain GRI Station Transmitter Location

Newfoundland East 72700 µs M Comfort Cove, Canada

Coast W Cape Race, Canada

W Jan Mayen, Norway

X Berlevag, Norway

W Jan Mayen, Norway

Bo 70010 µs M Bo, Norway

Ejde 90070 µs M Ejde, Faeroe Island, Denmark

X Bo, Norway

Y Vaerlandet, Norway

Z Loop Head, Ireland

X Soustons, France

Lessay 67310 µs M Lessay, France

Y Loop Head, Ireland

Z Sylt, Germany

X Lessay, France

Y Vaerlandet, Norway

Sylt 74990 µs M Sylt, Germany

French SNR 89400 µs M Lessay, France

X Soustons, France

X Lampedusa, Italy

Z Estartit, Spain

Mediterranean Sea 79900 µs M Sellia Marina, Italy

V Salwa, SA

Saudi Arabia North 70300 µs M Afif, SA

W Al Khamasin, SA

X Ash Shaykh Humayd, SA

Z Al Muwassam, SA

W Salwa, SA

X Afif, SA

Saudi Arabia South 88300 µs M Al Khamasin, SA

Y AshShaykh Humayd, SA

Z Al Muwassam, SA

Chain GRI Station Transmitter Location

West Coast USA 99400 µs M Fallon, Nevada, USA

W George, Washington, USA

X Middletown, California, USA

Y Searchlight, Nevada, USA

X Shoal Cove, Alaska, USA

Y George, Washington, USA

Z Port Hardy, BC, Canada

Canadian West Coast 59900 µs M Williams Lake, BC, Canada

W Baudette, Minnesota, USA

X Gillette, Wyoming, USA

North Central USA 82900 ms M Havre, Montana, USA

Y Williams Lake, BC, Canada

V Gillette, Wyoming, USA

W Searchlight, Nevada, USA

X Las Cruces, New Mexico, USA

Y Raymondville, Texas, USA

South Central USA 96100 ms M Boise City, Oklahoma, USA

Z Grangeville, Louisiana, USA

W Malone, Florida, USA

X Seneca, New York, USA

Y Baudette, Minnesota, USA

Z Boise City, Oklahoma, USA

W Grangeville, Louisiana, USA

Great Lakes 89700 µs M Dana, Indiana, USA

Southeast USA 79800 µs M Malone, Florida, USA

5

X Raymondville, Texas, USA

Y Jupiter, Florida, USA

Z Carolina Beach, NC, USA

W Caribou, Maine, USA

X Nantucket, Massachussetts, USA

Northeast USA 99600 µs M Seneca, New York, USA

Y Carolina Beach, NC, USA

Z Dana, Indiana, USA

Y Cape Race, Canada

Z Fox Harbor, Labrador, Canada

X Nantucket, Mass., USA

Canadian East Coast 59300 µs M Caribou, Maine, USA

X Raoping, PRC

6

Y Rongcheng, PRC

67300 µs X Raoping, PRC

Y Chongzuo, PRC

W Veraval, India

X Billamora, India

W Diamond Harbour, India

4/1/9

X Patpur, India

Chain GRI Station Transmitter Location

China East Sea 83900 µs M Xuancheng, PRC

W Petrozvodsk, Russia

China South Sea** 67800 µs M Hexian, PRC

X Solnim, Russia

Y Simferopol, Ukraine

Z Syzran, Russia

Bombay 60420 µs M Dhrangadhr, India

Calcutta 55430 µs M Balasore, India

W Petropavlo, Russia

X Ussuriisk, Russia

Y Kurilsk, Russia\

Z Ohotosk, Russia

** Broadcasting on two diferent GRI

W Kwang-Ju, Korea

X Gesashi, Okinawa

Y Niijima, Japan

Z Ussuriisk, Russia

W Gesashi, Okinawa, Japan

X Marcus Island, Japan

Y Tokatibutto, Hokkaido, Japan

Z Pohang, Korea

X Attu, Alaska, USA

Y Aleksandrovsk, Russia

USA

X Attu Alaska, USA

Y Point Clarence, Alaska, USA

Z Narrow Cape, Alaska, USA

X Narrow Cape, Alaska, USA

Y Shoal Cove, Alaska, USA

Z Port Clarence, Alaska, USA

X Xuancheng, PRC

Y Helong, PRC

Chain GRI Station Transmitter Location

Western Russia 80000 µs M Bryansk, Russia

Eastern Russia 79500 µs M Aleksandrovsk, Russia

East Asian 99300 µs M Pohang, Korea

Northwest Pacific 89300 µs M Niijima, Japan

6

Russian –American 59800 µs M Petropavlo, Russia

North Pacific 99900 µs M Saint Paul, Pribilof Is., Alaska,

Gulf of Alaska 79600 µs M Tok, Alaska, USA

China North Sea 74300 µs M Rongcheng, PRC

789

FS700 OVERVIEW

Introduction

Antenna

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.

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
determination of the third zero crossing position. When the frequency
offset has been reduced to better than 1 x 10
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.

-7

) and allows more time for the

-9

, the phase offset

The entire search process takes between 15 and 40 minutes depending
on the signal-to-noise ratio of the station selected.

It is important to understand that at large distances (>1000 miles) from
the LORAN transmitter, the LORAN skywave signal can be many times
larger than the groundwave signal. The FS700 can detect and correctly
handle this condition until the skywave is abo ut 5 times the gro undwave
amplitude. At this point, the FS700 will most likely lock to the skywave.
This results in poor long term stability because of the diurnal shifts in
the skywave timing. If this occurs, a different station should be chosen.

Lo cking t o LORAN-C

Signal E rrors

Once in LOCK the FS700 keeps the frequency difference between the
internal oscillator and the LORAN transmission at a minimum. This is
accomplished by using a software frequency-locked loop (FLL). The
frequency-locked loop adjusts the internal oscillator's frequency so that
the frequency difference at all times is zero. This is different than a
phase-locked loop (PLL). A PLL attempts to always keep the phase
difference at zero, and may introduce a large instantaneous frequency
offset to change the phase. Thus, a FLL will have better phase noise
and short-term stability than a PLL. The time constant of the FS700's
FLL is set according to the signal-to-noise ratio of the signal, and is
generally about 2000- 5000 GRI. The FS700 also has an auxiliary PLL
that keeps the phase difference between the FS700 and the LORAN
signal small. The time constant for the PLL is much longer than the FLL
time constant, and has no effect on short-term stability (the frequency
offset due to the PLL averages to zero and is rarely instantaneously
larger than 1-2 x 10

During locking, the FS700 monitors the received signal and checks for
error conditions. If an error is detected, the FS700 will halt its FLL (to
prevent erroneous frequency adjustments) and wait. If the error
condition goes away within 20 minutes, the FLL will restart and will
continue as before. If the error does not go away, one of two things
may happen. If the search mode is set to manual, the FS700 will
terminate its lock. If the search mode is set to auto, the FS700 will
attempt to reacquire a station. If the station selection is set to a
specific station (not auto station selection), the FS700 will continue to
try to acquire the station selected until the station returns to health.

-11

).

10

FRONT / REAR PANEL FEATURES

Front Panel Features

1) Power Switch Pressing the power switch turns the FS700 on and off. In STBY

position only, the ovenized oscillator is on. This minimizes warm-up
time.

2) Numeric Keypad The numeric keypad allows entry and modification of parameters in the

FS700's menus. Data can be entered in two modes depending on the
menu item displayed. In numeric mode (NUM LED on), data is entered
as a number with the keypad. The entry is terminated using the execute
(EXC) key. Typographical errors may be corrected using the
backspace (BSP) key. Pressing BSP with no number displayed will
cause the FS700 to return to the previous value. In cursor mode (<>
LED on) a menu item is modified pressing the up and down arrow keys
(8 and 2). If neither the NUM or <> LED's are lit, then the selected item
may not be modified.

3) Field Key In many menus, several selections are displayed. A flashing cursor

denotes the modifiable item. Pressing the FIELD key causes the cursor
to cycle through the available choices. If the FS700 is in GPIB remote
mode, pressing the FIELD key returns the unit to local mode.

4) Menu Keys The menu keys co ntrol which menu is displayed. Several menus have

more than one screen of data. The various screens of data may be
displayed by repeatedly pressing the associated menu key.

5) Status LED's The 5 status LED’s indicate the current status of the FS700.
LED Meaning

LOCK The FS700 locks to and tracks a station.

SEARCH The FS700 searches for a station.

ERROR An error has been detected. A relevant error message

will also be displayed. These errors can be signal
errors, command programming errors, etc.

OVEN The oven LED is on at power-up which indicates the

internal oven may be cold. The LED stays on for 20
minutes, and AUTO station search does not begin until
this warm-up period is over. This LED is lit when the
oscillato r uses up mo re than 75% o f its tuning range,
(about ±3 Hz).

ANT This LED indicates the antenna is faulty.

6) LCD Display The 32 character by 2 line LCD display shows menu items and

informational messages. If the displayed message is not a menu item,

11

it is either an error or an informational message. To retrieve the menu
display, just press any key.

7) LORAN Output This BNC outputs the amplified and filtered antenna signal. This may

be used in conjunction with the rear panel GRI SYNC and GATE
outputs to view the receiver's tracking point. The output is about 6 V
pk-pk and will drive a high impedance load.

8) Frequency Output This connector can be set to output a TTL level signal (2.5 V into 50

ohms) at a frequency between 0.01 Hz and 10 MHz in a 1, 2.5, 5
sequence. This output may be terminated into 50 ohms.

9) Oscillator Input This is the input to the FS700's internal phasemeter. The input signal to

the phasemeter should have the same frequency as the FREQUENCY
OUTPUT. The input has a 1 kohm input impedance and requires a
signal level of about 300 mV pk-pk. This input is protected to 100 VDC
and 40 VAC.

10) Phase Output This is the phasemeter analog output. The output voltage is

proportional to the phase difference between the OSCILLATOR INPUT
and the FREQUENCY OUTPUT with a coefficient of proportionality of

0.01 V/degree. This output has a range of ±360 degrees (±3.6 V) and
expects a high impedance load.

Rear Panel Features

1) Power Entry Module The power entry module contains the FS700's fuse and the line voltage

selection card. Be sure that the fuse rating matches that listed on the
rear panel and that the line voltage is set to the local value. To set the
line voltage: remove the power cord, remove the fuse, remove the
voltage selector card from the power entry module, insert the voltage
selector card so that the desired line voltage is visible through the clear
plastic window, replace the fuse with a correct rated fuse for the line
voltage selected, and reconnect the power cord.

2) IEEE-488 Connector This connector can be used with any standard IEEE-488 (GPIB) cable

to allow computer control of the FS700.

3) GRI SYNCH Output These two outputs may be used in conjunction with the front panel
Gate Output LORAN OUTPUT to view the FS700's tracking point. Both outputs

provide TTL level outputs and may have a 50 ohm termination. The
GRI SYNC output provides a negative going pulse at the start of each
GRI. When the receiver is in LOCK (LOCK LED on) this pulse occurs
500 ±5 ms before the receiver tracking point, and the GATE output
brackets the third zero crossing point with a negative going pulse.
These outputs may be used with an averaging digital oscilloscope (such
as HP 54501A) to view the Loran pulse tracking point. This procedure
is detailed in the tracking section of this manual.

12

4) 10 MHz Outputs These four outputs provide a 1 V pk-pk 10 MHz output when terminated

into 50 ohms. These outputs are individually buffered and are short­circuit protected.

5) Lock Output This is a TTL compatible output that is high when the receiver is locked

(LOCK LED on). This output is a transistor with a 10 kohm pull-up
resistor. This output may be wire-or'd with other similar outputs as long
as the current sink capability is greater than 0.5 mA.

6) Antenna Input This is the connector for the remote antenna. The antenna cable may

be up to 1000 ft long. The FS700 must be used with the antenna that is
supplied.

7) RS232 Connector This connector is used for RS232 communication.

(Optional)

13

FRONT PANEL OPERATION

Introduction

LORAN Tracking

The parameters controlling the operation of the FS700 are displayed
and adjusted in a set of six menus. Each menu may have one or more
screens (two line displays) of data. Each screen may contain one or
more field (adjustable parameter). Pressing a menu button displays that
menu. The screens that belong to a menu may be displayed by
repeatedly pressing the menu button. The field that is adjustable is
denoted by a flashing cursor bar. The various fields may be selected by
pressing the FIELD key.

The data entry mode for each field is indicated by the mode LEDs
above the field key. If the NUM LED is lit the data is entered in numeric
format using the keypad. Pressing the EXC key terminates the entry.
Errors are corrected using the backspace (BSP) key. If the <> LED is
lit the field is changed with the cursor keys (up and down arrow keys). If
neither LED is lit the display may not be changed.

Occasionally, an error or informational messages will be displayed on
the LCD display. The display may be returned to the normal menu
display by pressing any key.

The TRACKING and STATUS menus control the FS700's tracking of a
LORAN-C station. Adjustable tracking parameters are set in the
TRACKING menu, while tracking status is displayed in the STATUS
menu.

Overview Most of the FS700's operation is completely automatic. First, choose a

GRI and enter it into the FS700. Refer to the LORAN chain map to
choose a GRI that has stations near the receiver. Then, LORAN-C
station selection, acquisition, and tracking can begin. Station selection
and search mode may be either manually or automatically controlled.

Station selection is the process of choosing a specific station in the
GRI for tracking. In AUTO station selection, the FS700 will choose and
track the station with the largest signal strength. AUTO selection is the
FS700's default mode, and is recommended for general use. The
FS700 may be set to track a specific station in the GRI. If the master
station is not detected there is no way to identify the stations and
manual station selection will f ail. AUTO selection will pick the largest
signal even in the absence of the master.

The FS700's search mode controls the starting and stopping of
searches, and recovery from signal errors. In AUTO mode the FS700
will begin a search anytime that it is unlocked from a station. While in
MANUAL, the searches must be started by pressing the EXC key. The
FS700's default is AUTO. After power-on, an AUTO search will begin
after the oven oscillator warm-up period expires (the OVEN LED goes
off). A MANUAL search may be started at any time. However, if the

14

oscillator is not fully warmed up the search may fail. When in AUTO, if
a signal error forces the FS700 to terminate locking, the FS700 will
automatically start a new search. While in MANUAL, a new search
must be started by pressing the EXC key.

GRI: 99400 µs Station: X

Location: Middletown, CA USA

Tracki ng Men u

GRI Selection The first field is the GRI to which the FS700 is to lock. Set the GRI to a

Station Selection Station selection is set with the second field. Adjust this setting with the

The first screen has three adjustable parameters and is used to set the
station that the FS700 is to track. If the FS700 is tracking a station,
changing any of these parameters will cause the FS700 to unlock. To
prevent accidental unlocks, a warning message requiring confirmation
of the change will be displayed.

value appropriate to the location of the receiver by referring to the
LORAN chain map to select the GRI for the nearest stations. The GRI
must be set to a value between 40000 and 99990 ms in steps of 10 ms.

up and down arrow keys. The default setting is "AUTO", and the
FS700 will automatically choose the station with the largest signal. The
FS700 may be be set to a specific station - "M" (master), or one of
secondaries. If a chain existed at the chosen GRI when the FS700 was
shipped, the actual station names (a subset of V, W,X,Y, and Z) will be
displayed; otherwise the secondary may be set to an index of A,B,C, D,
or E - the order of transmission of the secondaries. The identification
of the secondaries is determined by the timing of the secondaries
relative to the master station. For a chain not in the FS700's station list
a default set of times will be used- an average of the timings of existing
chains. If the unknown chain's timing differs greatly from typical it may
not be possible to identify and select a particular secondary. However,
the master may always be selected, and AUTO will always select the
largest station- regardless of the identification of the secondaries. This
allows the FS700 to be set to any possible LORAN station, even if it
didn't exist at the time of the FS700's production. Once a station has
been chosen, during AUTO station selection the FS700 will display the
station identification in parentheses after the word AUTO. For
example: AUTO (Y) if Y has been chosen. If the FS700 is set to lock to
a specific station and the master station is not found, the selected
station will not be identified and the search will fail.

The third field is the station location field. This field displays the location
(station name) of the selected station. By scrolling through this list
using the up and down arrow keys a station near the receiver may be
chosen. All stations that existed at the time of the FS700's production
are in this list. If no station exists at the current GRI or station choice

15

the location message will display that fact. In AUTO station selection
the FS700 will display the station location once a station is chosen.

Search Mode: Auto
Start Station Acquisition (EXC)

Search Mode The second screen has two parameters and controls the starting and

stopping of the locking process. The first field controls the SEARCH
MODE. In AUTO search mode the FS700 will automatically begin a
station search any time it is unlocked. In MANUAL mode pressing EXC
will start the search. If a signal error occurs and the FS700 unlocks, the
FS700 will automatically reacquire a station in AUTO mode, while in
MANUAL the lock must be manually restarted. The second field
controls starting and stopping of locking and searches. If the FS700 is
not currently searching or locked, pushing the EXC button will start a
search. If the FS700 is searching or locking, pressing the EXC button
will stop the lock. In AUTO search mode, a new search will
automatically be started once the EXC button is pressed to stop the
search. If manual control is desired, the mode should be set to manual.

Receiver Time Constant: 4096 GRI

Time Constant The FS700's tracking time constant is set in the third screen in units of

GRI. The time constant is adjusted using the cursor keys, and can be
set between 128 and 16384 GRI. The time constant sets the amount of
signal averaging and the time between corrections of the FS700's
oscillator frequency. The setting is a trade-off between averaging
enough to reduce signal noise, and correcting frequently enough to
correct oscillator frequency changes with room temperature, etc.
Recommended settings are 2048 GRI with the standard oscillator, and
4096 GRI with option 1. If the signal-to-noise ratio of the station being
tracked is poor (< -3 dB), the time constants should be increased by a
factor of two or four. In general, there is no reason to reduce the time
co nstants below their nominal value.

Tracking Keys Lockout: On
Enter Password to Lock Keys:XXXXX

Keyboard Lock The menu items that control the FS700's station tracking may be locked

to prevent unauthorized modification. To lock all items in the
TRACKING and CAL menus, enter a password up to 4 digits long.
Entering the password again will unlock the menu items. On power-up
the keyboard lock is automatically cleared. Other features of the
FS700, such as the FREQUENCY OUTPUT and PHASE meter will
operate normally while the TRACKING menus are locked.

16

Receiver Gain: 75dB
Noise Margin: 33dB

Status Menu

Gain, Signal to Noise The first screen displays the FS700's gain and signal-to-noise ratio.

The gain is the gain necessary to bring the selected station to full scale
signal level. The maximum value is 120 dB. Noise Margin indicates the
current LORAN signal quality. A noise margin of 0.0 db is the poorest
quality signal useable by the FS700. The maximum value is +55 dB.
The gain number is not displayed until after station selection, while the
noise margin is not calculated until after the third zero crossing is
located.

LORAN Frequency Offset: 2.0 E-11
Phase: 0.2°

Phase The second screen displays the instantaneous frequency offset of the

FS700's oscillator from the LORAN signal. That is, it is the value of the
last correction that the FS700 made to its oscillator. The time average
of this value is zero. The oscillator has a sensitivity of about 1 x 10

11

minimum step size. Also displayed is the instantaneous phase of the
FS700's sampling gate relative to the LORAN pulse (in degrees). The
long-term fractional frequency difference over any time interval may be
calculated from the f o llo wing f o rmula:

-

For example: Suppose at the start of a 24 hour period, the phase is 0.1
degree. While at the end of the time interval, the phase is 0.2 degree.
Then the average frequency difference is :

This means that over a 24 hour period the FS700's oscillator is every
bit as good as the source in the LORAN transmitter. The transmitter's
Cesium clock is accurate to about 1-2 x 10
period.

-12

over the same time

Stations Found (Ident: Amp in dB)
M57 V38 W45 X70* Y44

17

Stations Found The third screen displays the identification and amplitude of all stations

found during the search phase of station acquisition. The amplitudes
are displayed in relative dB, and the station that the FS700 is locked to
has a "*" next to it. The station identifications will be displayed if the
FS700 knows them. If the master station is not found, the
identifications will be listed as "?". If there were no stations assigned at
the time of the FS700's manufacture the stations will be listed as "M",
"A","B", etc., since the station labels are unknown. This display is
useful in determining the number and strength of the stations in the
receiver's location.

Time Since Lock:12:34:56
Length Last Unlock: 0:00:00

Status Timing The fourth screen displays the amount of time that the FS700 has been

locked to its station. This timer can go up to 32767 hours (3.75
years)before it rolls over back to zero. Also displayed is the duration of
the most recent time at which the FS700 was unlocked. If the FS700
has never been unlocked this number will be 0:0:0. However, if the
FS700 ever became unlocked and had to reacquire a station, the length
of time until it became relocked will be displayed. These two times may
be used to determine when the FS700 became unlocked.

Signal Status: A r b n o
Press EXC to clear status

Signal Quality The last screen displays information on the quality of the received

signal and any transient conditions that may have occurred. The letters
a, r, b, n, and o indicate the status of antenna, rf, blink, noise margin,
and oscillator warnings. If the letter is displayed in lower case, the
condition has not occurred. If the letter is in UPPER case, the
condition has been detected in the past. When a warning condition is
detected, the associated status indicator is set. The indicators may be
reset by pressing EXC. Relocking the receiver will also reset the
indicators. The various warnings are described below in the
TRACKING PROBLEMS section.

Track Point Monitoring It is useful (or confidence building) to monitor the FS700's tracking point

(the third zero crossing). This is easily done using a digital oscilloscope
that is capable of averaging many signal traces (such as the HP
54501A). The averaging is necessary to reduce the noise on the signal.
While the FS700 is locked, the GRI SYNC output on the FS700's rear

panel provides a negative going pulse every GRI 500 ±5 µs before the

third zero crossing. The GATE output provides a negative going pulse
that brackets the third zero crossing. To monitor the tracking point,
trigger the scope with the GRI SYNC pulse. After the FS700 has
locked, display the LORAN OUTPUT and GATE output on the scope

using a 10 µs/div scale with 500 µs of trigger delay. This puts the gate

18