Automatic gain control for a line locator

An automatic gain control (AGC) system for use in line locators that detect concealed conductors is presented. The AGC system includes determining an AGC signal from a reference sensor and processing signals from one or more observed sensors utilizing the AGC signal. The reference sensor is located farther from the concealed conductor than the observed sensors. The AGC system can be implemented in analog form or can include a digital AGC determination in a microprocessor.

BACKGROUND 
1. Field of the Invention 
This invention relates to AGC systems and, in particular, to an AGC system 
for signal processing in concealed conductor locator devices. 
2. Background 
It is often necessary to locate buried conduits, which are employed by 
numerous utility companies, in order to repair or replace them. In 
addition, it is important to locate conduit lines in order not to disturb 
them when excavating for other purposes (such as, for example, addition of 
new conduits). Examples of buried conduits include pipelines for water, 
gas or sewage and cables for telephone, power or television. Many of the 
conduits are conductors, such as metallic pipelines or cables. In other 
applications, it is often useful to locate concealed elongated conductors, 
such as power lines or copper water lines, concealed in the walls of 
buildings. It is well known to locate concealed elongated conductors 
("lines") by detecting electromagnetic emissions from them. 
A conducting conduit (a line) may be induced to radiate electromagnetically 
by being directly or inductively coupled to an external transmitter. In 
some instances, such as with power lines, the line may radiate without an 
external transmitter. 
A line locator detects the electromagnetic radiation emanating from the 
line. Early line locators included a sensor that detects a time-varying 
magnetic field. The line locator detects a peak amplitude signal or a 
minimum amplitude signal from the sensor, depending on the orientation of 
the sensor, when the line locator is passed over the line. Later line 
locators have included multiple sensors to provide further information to 
the line locator regarding the line (such as, for example, proximity 
information). 
FIG. 1 shows a line 4, beneath surface 7, that is radiating a magnetic 
field 5. Magnetic field 5 is generally directed in a circular fashion 
around line 4. Line locator 1 is held over line 4 by operator 6. Line 
locator 1 includes sensor 3 that detects magnetic field 5 and displays a 
signal on a display 2 that is indicative of the magnetic signal strength 
measured by sensor 3. Depending on the orientation of sensor 3 (i.e., 
whether it is sensitive to horizontal or vertical components of magnetic 
fields), display 2 will indicate a maximum signal or a minimum signal when 
line locator 1 is held directly above line 4 (where the magnetic field 5 
is directed horizontally). 
FIG. 2A shows a line locator 200 having a top sensor 202 and a bottom 
sensor 201. Top sensor 202 and bottom sensor 201 are both horizontally 
oriented to detect magnetic fields oriented horizontally with respect to 
surface 7. Each of top sensor 202 and bottom sensor 201 has an output 
signal indicative of its position relative to the source of the time 
varying magnetic field 5, line 4. 
FIG. 2B shows a block diagram of a circuit 217 for receiving and processing 
output signals from bottom sensor 201 and top sensor 202. The output 
signal from top sensor 202 is input to amplifier 203. The gain of 
amplifier 203 is set in response to a control signal G. The output signal 
from bottom sensor 204 is received by amplifier 204. The gain of amplifier 
204 is set by control signal G. Switch 210 selects between the output 
signals from amplifier 203 or the output signal from amplifier 204. In 
operation, switch 210 is typically a momentary contact switch that 
normally selects the output signal from amplifier 204 (i.e., bottom sensor 
201) and selects the output signal from amplifier 203 (i.e., top sensor 
202) only when temporarily engaged. When switch 210 is engaged to receive 
output signals from top sensor 202, the control signal G is not allowed to 
vary. The output signal from switch 210 is filtered in filter 205. Filter 
205 is typically a band-pass filter that passes signals of a particular 
frequency. In active operation, a current of a particular frequency is 
induced in line 4 by a transmitter (not shown) coupled to line 4. Typical 
frequencies of operation include 9.8 kHz and 82 kHz, but any convenient 
frequency can be used. In some instances, such as in a power line, line 4 
may radiate electromagnetic radiation passively, i.e. without a 
transmitter. 
The output signal from filter 205 is amplified in amplifier 206. The gain 
of amplifier 206 is set in response to control signal G. The output signal 
from amplifier 206 is converted to a DC signal in AC/DC converter 207 and 
amplified by DC amplifier 208. 
The resulting output signal from circuit 217 is displayed to an operator 
(i.e., audibly with a speaker or visually through a meter) as an 
indication of the position of the line locator with respect to line 4. 
Engaging switch 210 so that the output signal from amplifier 203 is 
processed, in comparison with the processed output signal from bottom 
sensor 201, indicates the depth of line 4 below surface 7 or can be 
utilized for environmental noise compensation. 
FIG. 2C shows the top signal and the bottom signal as a function of lateral 
distance from line 4 with control signal G held at a constant voltage. 
Often the signals from sensors 202 and 201 are weak and their absolute 
magnitude may vary over a wide range in amplitude (in some cases about 100 
dB) in response to environmental parameters such as current strength, 
depth and soil type. In such a case, the control signal G, which is often 
manually adjusted by the operator, must continuously be adjusted to 
compensate for the widely varying magnitude of the signals. 
In some cases, an automatic gain control (AGC) circuit 209 is used to 
produce a control signal G. In that case, an AGC circuit 209 is coupled to 
the output terminal of DC amplifier 208. In FIG. 2B, circuit 217 is shown 
to include AGC circuit 209, which controls the gain of amplifiers 203, 204 
and 206 by varying control signal G so that the output signal from DC amp 
208 is at a constant amplitude regardless of the output signal from sensor 
201 or sensor 202. The useful information about magnetic field strength, 
then, is embedded in the control signal G. 
However, the control signal G, determined by AGC circuit 209, is typically 
very nonlinear with the magnitude of the input signal, which often varies 
drastically with parameters. For example, the first 20 dB of input signal 
change takes two thirds of the available AGC signal and the next 80 dB of 
input signal change takes about one third of the available control signal 
range. The available control signal, outputted by AGC 209, is the total 
range of voltages that AGC 209 is capable of producing and to which 
amplifiers 203, 204 and 206 are capable of responding. 
Processing useful information from the AGC determined control signal 
results in the use of highly complicated circuitry. Therefore, there 
exists a need for a line locator system with an AGC system where useful 
signals are more easily obtained. 
SUMMARY 
In accordance with the present invention, an AGC correction voltage is 
derived from a reference channel which includes a reference channel 
sensor. The AGC signal can then be used in the AGC amplifiers of one or 
more observed channels, which include observed channel sensors. The 
reference channel sensor is arranged to be farther from the concealed 
elongated conductor than any of the observed channel sensors. In that 
case, the reference channel provides the AGC signal that is used in all of 
the channels and the observed channels provide output signals for display 
or further processing. 
In one embodiment, the line locator includes a bottom sensor and a top 
sensor. The reference channel includes the top sensor and the observed 
channel includes the bottom sensor. The amplitude of the observed channel 
output signal, therefore, provides information about the lateral 
positioning of the line locator with respect to the underground conductor. 
The AGC signal, along with the bottom sensor output signal, can be used to 
indicate depth. 
In another embodiment, the line locator includes more than two sensors. For 
example, a left/right line locator can include a bottom sensor, a top 
sensor, a left sensor and a right sensor. The AGC signal is determined in 
a reference channel that includes one of the sensors and the other 
channels are observed channels that provide output signals to a display. 
In the left/right line locator, an AGC signal may be derived from the top 
sensor output signal and used in other channels of the detection circuit 
for processing the output signals from the bottom sensor and the left and 
right sensors. In another example, the line locator may have one sensor 
for an observed channel, a second sensor in a channel to indicate the 
depth of the concealed conductor, and a third sensor for a channel that 
generates an AGC signal that controls the other channels.

DESCRIPTION OF THE INVENTION 
FIG. 3A shows a block diagram of a signal processing circuit 300 according 
to the present invention. Circuit 300 includes two channels, a reference 
channel 318 and an observed channel 319. In general, a line locator 
according to the present invention includes any number of observed 
channels. 
Reference channel 318 includes sensor 316, preamp 301, AGC amp 302, filter 
303, AGC amp 304, rectifier 305, DC amp 306 and AGC generator 307. The 
observed channel includes sensor 317, preamp 309, AGC amp 310, filter 311, 
AGC amp 312, rectifier 313, DC amp 314 and display 315. 
Sensors 316 and 317 are mounted within a line locator and can correspond to 
the top sensor 202 and bottom sensor 201 of FIG. 2A, respectively. In 
general, a line locator can include any number of sensors. Sensors 316 and 
317 can be any pair of sensors included in a line locator where sensor 316 
is located farther from line 4 than any of the sensors in observed 
channels. 
Sensors 316 and 317 represent any device capable of detecting a magnetic 
field, including sensor coils, Hall effect probes, flux-gate sensors, or 
any other device capable of detecting a magnetic field. In most 
embodiments, sensors 316 and 317 comprise coils of wire wound about a 
bobbin. The center of the bobbin may contain a magnetic core. Sensor coils 
with no magnetic coils, i.e. air coils, are useful for high frequency 
applications, but sensor coils having magnetic cores will provide higher 
sensitivity at lower frequencies. 
In general, the magnetic field sensors are sensitive to the orientation of 
the magnetic field and detect components of the field that are directed 
along an orientation axis of the magnetic field sensor. A horizontally 
oriented sensor, such as bottom sensor 201 in FIG. 2A for example, detects 
fields that are horizontally directed with respect to surface 7. In FIG. 
2A, surface 7 may be the earth's surface or may be any other surface that 
conceals an elongated conductor such as, for example, a wall. 
In FIG. 3A, the output signal from sensor 316 is amplified in preamp 301 
and amplified again in AGC amplifier 302. The gain of an AGC amplifier is 
determined by the value of an inputted AGC signal. The output signal from 
an AGC amp is tailored to provide an input to the circuit element that 
follows the AGC amp. In FIG. 3A, the output signal from AGC amp 302 is 
input to filter 303. 
In general, circuits for processing signals from the sensors will include 
multiple AGC amplifiers because including all of the gain required in the 
circuit in one amplifier is likely to overload components of the circuit 
that follow the amplifier. Including several amplifiers allows signals to 
be amplified as needed during processing through the circuit, without the 
risk of overloading particular components of the circuit. Each of the AGC 
amplifiers inputs a common AGC signal and each of the AGC amplifiers has a 
gain that is dependent on the AGC signal. The response of one AGC 
amplifier to the AGC signal may differ from other AGC amplifiers in the 
circuit. 
Filter 303 of FIG. 3A is typically a band-pass filter tuned to pass signals 
of a particular frequency. In some embodiments, Filter 303 includes a 
single band-pass filter. Filter 303 is arranged to pass a frequency that 
is determined by an external transmitter or by the passive emission from 
the concealed conductor. For example, one embodiment includes a band-pass 
filter that passes a frequency of about 35 kHz. Other embodiments can pass 
single frequencies of 9.8 kHz, 88 kHz, 982 Hz, RF frequencies (about 10 
kHz to about 20 kHz), 50/60 Hz signals, or any other frequency range that 
is required. Some embodiments of filter 303 can include the ability to 
select from one or more discrete frequencies or may include a tuner that 
is capable of selecting frequencies from a continuous spectrum of 
frequencies. FIG. 3B shows a filter 303 having the ability to pass signals 
selected from the frequencies 9.8 kHz, 88 kHz, 35 kHz, 50 Hz, 60 Hz or 
harmonics of 50 or 60 Hz, and a custom frequency. 
FIG. 3B shows an embodiment of filter 303 that includes a filter array 
having a separate signal path for each of a 9.8 kHz filter 320, an 82 kHz 
filter 321, an active filter 322 for passing harmonics of 50/60 Hz, a 35 
kHz filter 323, a custom filter 326, an RF filter 325, and a 50/60 Hz 
filter 324. In general, a separate path for any number of individual 
filters can be included in filter 303. Also, the filters can be of any 
type (e.g., active filters, passive filters, digital filters and 
pass-through filters). 
In FIG. 3B, filter 303 receives input signals into each of the filter 
paths. In general, filter 303 can include any number of separate filter 
paths. In some embodiments, as shown in FIG. 3B, each filter path includes 
a separate AGC amplifier (i.e., AGC amplifiers 328-334) whose output 
signals are separately arranged to provide appropriate input signals for 
each filter (i.e., 9.8 kHz filter 320, 82 kHz filter 321, active filter 
322, 35 kHz filter 323, custom filter 324, RF filter 325 and 50/60 Hz 
filter 326, respectively). In embodiments where an AGC amplifier is 
included in each filter path, AGC amplifier 302 is generally excluded. 
In FIG. 3B, filter 320 is a band-pass filter arranged to pass signals 
having a frequency of about 9.8 kHz. Filter 321 is a band-pass filter 
arranged to pass signals having a frequency of about 82 kHz. Active filter 
322 is a notch and band-pass (comb) filter that passes 50 Hz, 60 Hz or 
harmonics of 50 or 60 Hz. Filter 323 is a band-pass filter arranged to 
pass frequencies of about 35 kHz. Filter 324 is a filter designed to pass 
frequencies that are determined by the particular application of the line 
locator. Filter 325 passes RF frequencies (about 10 kHz to about 20 kHz). 
Filter 326 passes 50 or 60 Hz signals for power line locators. In addition 
to these frequencies, embodiments of this invention may include filters 
appropriate for specific applications such as, for example, detection of 
power lines. In addition, filters appropriate for particular transmitters 
210, which may not use the more conventional 9.8 kHz or 82 kHz 
frequencies, can be included in filter 303. 
The output signals from each of the filters is received by frequency select 
327. Frequency select 327, in response to a FREQSEL signal, has an output 
signal corresponding to the output signal of one of the filters in the 
filter array. The FREQSEL signal can originate from an operator controlled 
switch or controller. In some embodiments, frequency select 327 separately 
outputs signals from active operation (for example, 9.8 kHz or 82 kHz 
signals) and from passive operation (for example 50 or 60 Hz signals from 
power lines) Additionally, Filter 320 can be a digital filter such as that 
described in U.S. Pat. No. 5,065,098, issued to Salsman et al., herein 
incorporated by reference in its entirety. Digital signal processing is 
especially useful if a microprocessor controls the line locator and 
processes signals received from the sensors. 
In FIG. 3A, the output signal from filter 303 is amplified in AGC amplifier 
304. The gain of AGC amplifier 304 is determined by the AGC signal. 
Rectifier 305 rectifies the output signal from AGC amplifier 304. 
Rectifier 305 can be any device for converting an AC signal to a DC 
signal. DC amplifier 306 amplifies the DC output signal from rectifier 
305. 
AGC generator 307 compares the output signal from DC amplifier 306 with a 
reference signal from set point 308 and outputs the AGC signal in response 
to this comparison. AGC generator 307 and the various AGC amplifiers 
(i.e., AGC amplifiers 302 and 304), through the AGC signal, create a 
feedback circuit. The AGC signal is varied such that the output signal 
from DC amplifier 306 is a constant voltage. 
FIG. 3C shows a diagram of an AGC generator 307 that can be used in the 
present invention. AGC generator 307 includes analog comparator 335. 
Analog comparator 335 compares the input signal with a reference signal 
and outputs an AGC signal in response to the difference between the input 
signal and the reference signal. 
Additionally, if channel 318 is being processed digitally by a 
microprocessor, then the AGC signal can be digitally generated in the 
microprocessor. Digital AGC generation is described in copending 
application "Line Locator Having Left/Right Detection," Application Ser. 
No. 09/136,767, by Gopal Parakulam and Stevan Polak, assigned to the same 
entity and filed on the same date as the present invention, herein 
incorporated by reference in its entirety. In addition, digital AGC 
generation is described in copending application "Line Locator Having 
Left/Right Detection," Application Ser. No. 09/136,794, by Gopal Parakulam 
and Stevan Polak, assigned to the same entity and filed on the same date 
as the present invention, herein incorporated by reference in its entirety 
In most embodiments, observed channel 319 and reference channel 318 are 
substantially identical except that the output signal from DC amplifier 
314 in observed channel 319 is inputted to a display 315 and the output 
signal from DC amplifier 306 in reference channel 318 is inputted to AGC 
generator 307. In some embodiments, however, reference channel 318 and 
observed channel 319 may differ. One difference, for example, is that DC 
amplifier 306 may have a different gain than DC amplifier 314. In 
addition, observed 319 channel may include additional filtering either 
included in filter 311 or in separate filters (not shown) than reference 
channel 318. 
In FIG. 3A, the output signal from sensor 317 is amplified in preamplifier 
309 and amplified in AGC amplifier 310. The amplitude of the output signal 
from AGC amplifier 310 is determined by the AGC signal from AGC generator 
307. Filter 311 inputs the output signal from AGC amplifier 310. Filter 
311 typically is a band-pass filter that passes signals having a 
particular frequency. Filter 311 can be substantially the same as that 
described for filter 303 of reference channel 318. 
The output signal from filter 311 is amplified in AGC amplifier 312. The 
output signal of AGC amplifier 312 has an amplitude determined by the AGC 
signal. The output from AGC amplifier 312 is rectified in rectifier 313, 
which is substantially the same as rectifier 305 of reference channel 318. 
The DC output from rectifier 313 is amplified in DC amplifier 314 before 
being inputted into display 315. Display 315 may include a speaker having 
a volume or a tone (i.e., pitch) that is indicative of the displacement of 
the line locator from a line or a visual indicator (such as a meter or a 
bar graph) that is indicative of the displacement of the line locator from 
a line. Additionally, display 315 may be a microprocessor based 
controller. 
FIG. 4 shows the output signal from reference channel 318 and the output 
signal from observed channel 319 as a function of lateral displacement of 
the line locator from the line. The output signal from reference channel 
318 is the output signal from DC amplifier 306. The output signal from 
observed channel 319 is the output signal from DC amplifier 314. As a 
result of AGC generator 307 and its control of circuit 300, the output 
signal from DC amp 306 in FIG. 3A is a constant voltage (arbitrarily 
designated as 1.0 in FIG. 4). The output signal from observed channel 319 
has the advantage over the corresponding signals shown in FIG. 2C in that 
it does not vary in amplitude with outside parameters. The observed 
channel output, therefore, is simpler to utilize and requires much less 
circuitry than that required to obtain the corresponding information from 
the AGC signal itself. Additionally, because the sensors of the reference 
channel 318 are farther from the concealed conductor than the sensors of 
the observed channel, the output signal from observed channel 319 has a 
peak at a lateral displacement of zero. 
Line locators according to this invention may have other observed channels 
as well. For example, in copending application "Line Locator Having 
Left/Right Detection," which has previously been incorporated by reference 
into this disclosure, a left/right line locator having an AGC circuit 
according to the present invention is described. FIG. 5A shows a line 
locator 500 having a bottom sensor 503, a top sensor 504, a left sensor 
501 and a right sensor 502. In most embodiments, top sensor 504 and bottom 
sensor 503 are horizontally oriented to detect components of the magnetic 
field parallel to surface 7. Left sensor 501 and right sensor 502 may have 
any orientation but are shown in FIG. 5 to be vertically oriented. For 
left/right determination, left sensor 501 and right sensor 502 are 
symmetrically placed around bottom sensor 503. Bottom sensor 503 serves as 
an observed sensor. Top sensor 504 can be included in a reference channel 
for generating an AGC signal and may be placed anywhere in line locator 
500. If top sensor 504 is being further used to determine the depth of 
line 4, top sensor 504 should be placed such that left sensor 501 and 
right sensor 502 are symmetrically located with respect to top sensor 504 
as well. 
FIG. 5B shows a block diagram of a detection circuit 543 for a left/right 
detecting line locator 500 according to the present invention. Detection 
circuit 543 includes a left/right channel 540, a reference channel 541, 
and an observed channel 542. The AGC signal is generated in reference 
channel 541. 
The output signal from top sensor 504, the reference sensor, is amplified 
by a preamplifier 517 and filtered in filter 518. Filter 518 passes 
signals of a particular frequency in response to a FREQ. SEL. signal. The 
FREQ. SEL. signal is usually set by the operator. As was previously 
described (see FIG. 3B), filter 518 may have several different filter 
paths. In filter 518, each filter path has an AGC amplifier and a filter 
for a particular operating frequency. In some embodiments, a single AGC 
amplifier is supplied for all of the filter paths. 
In some embodiments, filter 518 splits active frequencies (those requiring 
the use of an external transmitter 511) and passive signals (such as from 
a 50 or 60 Hz power line or broadband RF signals). Further filtering is 
easily possible on the active frequencies, which are generally narrow band 
and at higher frequencies defined by the transmitter (not shown). In 
contrast, passive signals are either at low frequencies (50 or 60 Hz) or 
are broadband RF signals (about 10 kHz to 20 kHz) ant are not further 
filtered. In addition, filter 518 may be a digital filter, as was 
previously discussed. 
In the embodiment shown in FIG. 5B, the active output signals from filter 
518 are mixed with a LO FREQ signal in mixer 519. In most embodiments, the 
LO FREQ signal is the frequency chosen by the FREQ. SEL. signal (i.e., the 
active operating frequency) plus a center frequency (about 560 Hz in some 
embodiments, although other embodiments may use a different center 
frequency). Mixer 519 receives the LO FREQ signal and the active output 
signal from filter 518 and outputs an output signal that has a beat 
frequency equal to the center frequency. Typically, mixer 519 subtracts 
the LO FREQ signal from the active output signal from filter 518. 
The output signal from mixer 519 is input to AGC amplifier 520. The gain of 
AGC amplifier 520 is determined by the AGC signal. IF filter 521 is a 
narrow bandwidth band-pass filter that passes signals with frequency equal 
to the center frequency. Switch 531 chooses between the active signal path 
and the passive signal path in accordance with an active/passive signal. 
The active/passive signal is usually operator selected and is generally 
determined by the operator selected FREQ. SEL. signal (i.e., if 60 Hz is 
selected then the active/passive signal is set to passive). 
The output signal from switch 531 is amplified by AGC amplifier 522. The 
output signal from AGC amplifier 522 is determined by the AGC signal. 
AC/DC converter 523 is a rectifier/amplifier that converts the output 
signal from AGC amplifier 522 into a DC signal. The DC signal from AC/DC 
converter 523 is filtered in low pass filter 524 and inputted into AGC 
generator 525. 
AGC generator 525 outputs the AGC signal in response to a comparison of the 
output signal from low pass filter 524 with an AGC reference signal. AGC 
generator 525 can be the same as that shown in FIG. 3C or, as was 
previously discussed, can be a digital AGC generator. In FIG. 5B, the AGC 
signal, as determined in reference channel 541, is used throughout the 
remainder of signal processing circuit 543. 
The output signal from left sensor 501 is amplified in preamp_506. The 
output signal from right sensor 502 is amplified in preamp 507. If left 
sensor 501 and right sensor 502 are vertically oriented, then the output 
signals from preamps 506 and 507 are summed in amplifier 508. When the 
output signals from preamps 506 and 507 are summed, the resulting output 
signal advantageously utilizes the polarity change in the vertical 
component of the magnetic field 5 (FIG. 5A) that is detected as the line 
locator is laterally passed over the concealed conductor. The polarity 
change in input signals in vertically oriented left/right sensors is 
further discussed in the copending application "Line Locator Having 
Left/Right Detection". Amplifier 508 may also convolute the output signals 
from preamps 507 and 508 using other convolution functions depending on 
the orientation of left sensor 501 and right sensor 502. For example, if 
left sensor 501 and right sensor 502 are horizontally oriented, amplifier 
508 amplifies the difference between the output signals from preamplifiers 
506 and 507. 
Filter 509 can be any of the filters that have been discussed. The active 
output signal from filter 509 is mixed with the LO FREQ signal in mixer 
510. The output signal from mixer 510 is inputted to AGC amplifier 511 and 
the output signal from AGC amplifier 511 is inputted to IF filter 512. 
Switch 513 selects the active frequency path or the passive frequency path 
in response to the active/passive signal. The output signal from switch 
513 is inputted to AGC amplifier 514. 
The output signal from AGC amplifier 514 is inputted to phase detector 515. 
Phase detector 515 receives a PHREF signal and outputs an output signal 
that is dependent on the amplitude and polarity of the output signal from 
preamp 508. Phase detector 515 compares the PHREF signal with the output 
signal from AGC amplifier 514 in order to determine the output signal. 
Phase detection is utilized in this context so that information regarding 
the polarity change in the magnetic field strength that is included in the 
output signal of amplifier 508 is not lost. 
As shown in FIG. 5B, the PHREF signal is determined in a bottom sensor 
channel 542. Alternatively, the PHREF signal can be received into the line 
locator directly from the external transmitter or may be determined from 
reference channel 541. Additionally, the PHREF signal may be determined 
from another channel not shown in FIG. 5B. The PHREF signal has the same 
frequency as the magnetic field strength that is detected by sensors 501 
through 504. 
The output signal from phase detector 515 is inputted to low pass filter 
516. Low pass filter 516 removes any lingering AC component of the output 
signal from phase detector 515. The left/right output signal from low pass 
filter 516, L/R, indicates the lateral position of the line locator with 
respect to the line, including whether the line locator is to the left or 
the right of the line. 
The bottom sensor channel 542 is similar to reference channel 541 and 
left/right channel 540. The output signal from bottom sensor 503 is 
amplified in preamp 526 and filtered in filter 527. The active frequency 
output signal from filter 527 is mixed with the LO FREQ signal in mixer 
528, amplified in AGC amp 529 and filtered in IF filter 530. Switch 532 
selects between the active frequency signal and the passive frequency 
signal in response to the active/passive signal. The output signal from 
switch 523 is amplified in AGC amplifier 533 and converted to a DC signal 
in AC/DC converter 534. The output signal from AC/DC converter 534 is 
filtered in low pass filter 538 to create a field strength (FS) signal. 
Other signals may also be generated using this channel. For example, the 
FS signal may be further filtered to obtain a more stable signal 
indicating field strength that can be used in further calculations. 
The PHREF signal that is utilized in left/right channel 540 can be derived 
from the bottom channel signal in zero crossing detector 535. 
Alternatively, reference channel 541 may include a zero crossing detector 
535 for generating the PHREF signal. Also, the PHREF signal can be 
inputted, either with a direct electrical connection or with a wireless 
transmitter and receiver, from an external transmitter that is used to 
induce current in the concealed conductor. Zero crossing detector 535 
outputs the PHREF signal, which has the same frequency as the operating 
frequency of the system (i.e., the same frequency as the magnetic field 
strengths measured at sensors 501 through 504), in response to the output 
signal from AGC amplifier 533. 
The AGC signal from AGC generator 525 is used in AGC amplifiers 514, 527 
and 533 as well as AGC amplifiers contained in filters 509, 518 and 527. 
Obtaining the AGC signal from top sensor 504 in a signal detection channel 
separate from the signal detection channels for other signals (i.e., 
left/right channel 540 and bottom channel 542) eliminates the previously 
discussed problems with using the AGC signal as data signals. The output 
signals from sensors 501, 502 and 503 can be used to display relevant data 
without the use of costly and complicated circuitry for obtaining that 
data from AGC signals associated with each signal processing channel. 
FIG. 5C shows an embodiment of a display/controller 538. Display/controller 
538 provides information to the operator and, usually in response to 
operator input, generates control signals (i.e., FREQ. SEL. and 
Active/Passive signals). Display/controller 538 receives the L/R signal 
and the FS signal and displays the relevant data to an operator. The data 
may be displayed visually (as with bar graphs and meters) or audibly (as 
with speaker tones or volumes). In FIG. 5C, meter 539 displays the 
information contained in the L/R signal; bar display 540 displays the 
information contained in the FS signal and speaker 541 operates with 
either the L/R signal or the FS signal to provide an audio display. 
Additionally, display/controller 538 generates control signals and the LO 
FREQ signal. The FREQ. SEL signal and the Active/Passive signal are 
determined in by operator controlled selector knob 542. Local generator 
544 generates the LO FREQ signal in response to an input signal from 
selector knob 542. The AGC reference signal is selected by adjustment 543 
that can be an internal setting or can be operator controlled, i.e. with a 
knob. In some embodiments, display/controller 538 can include a 
microprocessor that further processes input signals and sets control 
signals in response to operator input. 
The above examples are demonstrative only. One of ordinary skill in the art 
will recognize numerous variations from the example embodiments of this 
invention. It is intended that these variations be included within the 
scope of this invention. As such, the invention is limited only by the 
following claims.