RD8000 Operation Manual 39
12.2 CD reset
12.2.1 About CD reset
When you trace a signal on very long target lines, the
transmitter signal gradually bleeds into the ground
by capacitance. This means the phase angle of the
remaining signal gradually changes.
This is referred to as phase-shift and can occur whenever
an alternating current signal is flowing in a system
of conductors that have a significant capacitance or
inductance. The relative phase angle between the two
frequencies will alter, but only over extended distances.
Figure 12.2: CD reset
Figure 12.2 illustrates a buried line with significant
capacitance to ground. It illustrates the gradual phase-
shift that occurs along the line. The diagram shows a
reference point on each waveform. At first, the reference
point occurs right on the peak of a positive half cycle
(A). It gradually moves towards the zero crossing point
(D) and ends up on the peak of a negative half cycle (F).
The direction has shifted from forwards to backwards.
At the intermediate point (D) the current flow cannot be
determined. At this point the CD arrows on the RD8000
receiver will flash on and off. In the example shown in the
illustration, by returning to (C) which is the last point at
which a CD reading was achieved, and performing a CD
reset, the response at (C) will be equivalent to that of (A).
In most cases the phase angle shift does not occur until
many kilometers from the transmitter and in some cases
a CD rest may only be required every 20 kilometers (14
miles).
12.2.2 Using a CD clamp or stethoscope
When using either of these accessories it is important to
RESET the CD at a known point, near the transmitter,
before making any identification measurements. On both
the CD/CM clamp and the CD stethoscope there is an
arrow that indicates which way the clamp or stethoscope
is facing in relation to the transmitter. If the arrow is
pointing away from the transmitter when the CD reset is
performed the receiver display shows 000 and the bar
graph reads 100%. If the arrow is pointing towards the
transmitter, the receiver display shows 111 and there is
no bar graph reading.
Refer to Section 10 for more information on clamps and
stethoscopes.
12.2.3 Performing a CD reset
You must perform a CD every time you select a CD
frequency. CD reset can only be performed when in CD
mode.
To perform a CD reset:
Select any CD frequency using the key.
Press the key to enter the menu.
Scroll to the CDR menu using the or
arrows.
Press the key to enter the CDR menu.
Press the key to reset CD and exit to the
main menu.
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