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pulse in the middle of the screen. Set the scope to average about 1000
triggers. After the signal has averaged for a while, the LORAN pulse
should be easily visible. By expanding the vertical scale it is easy to
see the start of the pulse. It is simple to observe the tracking point by
counting zero crossings and remembering that the LORAN pulse starts
with a positive half cycle that doesn't count as a zero crossing.
Notch Filters
The FS700 has six adjustable notch filters that may be used to eliminate
strong interfering signals. Three of the filters can tune from 40 to 90
kHz while the other three tune from 110 to 220 kHz. The filters are
tuned by using the six rear panel screw adjustments. Because these
filters severely affect the passband shape of the FS700's rf circuitry,
they should only be used in cases of extreme interference, where the
interference is easily visible at the LORAN output (large random noise
does not count).
Tuning the Notch Filters
There are several ways to set the notch filters.
Best Method:
1) While the FS700 is locked or searching, use a rf spectrum analyzer
and look at the signal from the LORAN OUTPUT BNC. Because
the FS700's LORAN OUTPUT can only drive a high impedance
load, and most spectrum analyzers have a 50 ohm input, use a 1
kohm resistor in series with the LORAN OUTPUT and the probe.
Set a slow sweep speed because the LORAN emission is pulsed.
2) The spectrum of the LORAN pulse is a broad relatively uniform
hump from 80 to 120 kHz. Any severe interfering signals should be
visible as a large, discrete, repeatable spike on top of this signal.
3) Tune the appropriate notch filter to minimize this spike.
Method Two:
1) This method works if the frequency of the interference is known.
2) Stop the FS700's lock and set the SEARCH MODE to MANUAL so
that it doesn't restart. Attach a signal generator to the FS700's
antenna input using the input terminator shown in the
TROUBLESHOOTING section of the manual.
3) In CAL MENU 1 (press BSP and TIME together), set the
attenuators for 64 dB attenuation.
4) Set the signal generator to the interfering frequency and adjust the
amplitude so that it can be seen at the LORAN OUTPUT.
5) Adjust the appropriate notch filter to minimize the offending signal.
6) Reattach the antenna and restart the lock.
