Showing papers on "Demodulation published in 1985"

Adaptive FSK demodulator and threshold detector

Abstract: An FSK demodulator of the differential type is arranged to provide two versions of an input FSK signal that, relative each other, (1) have a linear phase shift versus frequency characteristic and (2) are phase shifted by 90 degrees at a center frequency halfway between the upper and lower FSK frequencies. The linear phase shift is provided by an N sample delay element, while a constant phase shift circuit which includes a K factor gain element provides the 90 degree phase shift. By virtue of this arrangement, the demodulator characteristics can be adaptively varied and the demodulator tuned simply by changing the value of K. A similar adaptive technique is applied to a threshold detector arranged to position a threshold midway between the extremes of an input signal.

MSK digital demodulator for burst communications

Abstract: A receiver for burst transmission of MSK-modulated binary signals includes in-phase (I) and quadrature (Q) demodulation under the control of reference sine and cosine signals. Initially, the reference signals are near the nominal carrier frequency of the received signal, but at arbitrary phase, so that noncoherent detection takes place. A control circuit is provided to slew the reference signals to the same phase as the MSK carrier for low-noise coherent detection. Because of the burst nature of the communications, data throughput is significantly reduced if reference phase acquisition time is large. In order to reduce the phase acquisition slew time, each burst of data is preceded by a predetermined preamble. In the receiver, the noncoherently detected preamble is applied to a set of four preamble-matched correlators which respond to the I and Q portions of the preamble, and to the cross-coupled Q and I portions contaminating the I and Q portions respectively. At the moment that the preamble completely fills the correlators, each correlator produces a correlation peak having a magnitude and polarity. The relative magnitudes and polarities of the four correlation outputs at the instant of receipt of the preamble uniquely determines the relative phase difference (θ) between the sine reference signal and the MSK carrier. The phase difference signal θ slews the reference signal phase to near the desired phase at the beginning of the data portion of the burst transmission. In order to keep the reference signals in-phase with the MSK carrier during the data portion of the transmission, the received data is used as a predetermined signal once the bit decisions have been made. The bit-decided data is applied to set of four correlators the taps of which are adaptively controlled in response to the bit decisions to continuously generate θ for control of the reference phase.

Demodulation of optical DPSK using in-phase and quadrature detection

Abstract: A 140 Mbit/s optical DPSK system experiment employing a 90 degree optical hybrid to achieve in-phase and quadrature detection is reported. The principle of the optical hybrid is outlined and the effect of polarisation misalignment on the optical performance is compared with that for standard homodyne/heterodyne detection.

Performance of Noncoherent Direct-Sequence Spread-Spectrum Multiple-Access Communications

Abstract: The performance of noncoherent reception in direct-sequence spread-spectrum multiple-access communications systems is investigated for additive white Gaussian noise channels. Analytical and numerical results on the probability of error are presented for binary and M -ary frequency-shift-keying data modulation with noncoherent demodulation and differential-phase-shift-keying data modulation with differentiallycoherent demodulation. Both synchronous and asynchronous systems are analyzed. Systems which employ deterministic as well as random signature sequences are considered. The multiple access capability of noncoherent DS/SS systems is evaluated and compared to that of coherent DS/SS systems with the same parameters. The comparison shows that the loss in the performance of DS/SSMA systems due to noncoherent reception can be considerably larger than the loss incurred to noncoberent single-user systems operating in additive Gaussian noise.

QPSK modulator or demodulator using subharmonic pump carrier signals

Abstract: The present invention relates to a QPSK modulator or demodulator for modulating two different input bit streams with a subharmonic pump carrier signal to produce an appropriately encoded QPSK output signal. The subharmonic pump carrier signal used is a submultiple of a predetermined microwave or millimeter-wave carrier frequency. The QPSK modulator or demodulator is capable of being fabricated on a planar substrate using appropriate stripline filters and a mixer diode in each of an in-phase and quadrature signal path. A fin line arrangement, also capable of fabrication on the substrate, can be used to introduce an appropriately phased subharmonic pump carrier signal into each of the in-phase and quadrature signal paths before each mixer diode. Each mixer diode mixes the associated data bit stream and one of the two appropriately phased pump carrier signals to produce separate output signals which modulate along separate orthogonal axes, which output signals are combined to produce the QPSK modulator output signal.

Coherent Hybrid DS-SFH Spread-Spectrum Multiple-Access Communications

Abstract: The performance of synchronous and asynchronous hybrid direct-sequence/slow-frequency-hopped spread-spectrum multiple-access communications over additive white Gaussian noise channels is examined. Systems employing binary or quaternary phase-shift-keying modulation with coherent demodulation are investigated. Both deterministic and random signature sequences and frequency-hopping patterns are considered and several possible assignments for them are discussed. It is shown that the multiple-access capability of hybrid spread-spectrum is superior to that of pure frequency-hopped spread-spectrum, and inferior to that of pure direct-sequence spread-spectrum for systems with identical bandwidth expansion which employ the same data modulation and demodulation scheme and random hopping patterns and signature sequences.

Super high frequency receiver

Abstract: Apparatus for receiving super high frequency signals having an outdoor unit which includes a converter for converting super high frequency signals received by an antenna to ultra high frequency signals, a local oscillator for generating a local signal supplied to the converter a switch for switching the frequency of the local signal, and a detecting circuit for detecting an operating voltage supplied to the outdoor unit and the value of which is changed according to a band range of the super high frequency signals, the output of the detecting circuit controlling the switch, an indoor unit which includes a demodulator demodulating the ultra high frequency signals, and a circuit generating the operating voltage; and a coaxial cable supplying the ultra high frequency signals from the outdoor unit to the indoor unit and also supplying the operating voltage from the indoor unit to the outdoor unit.

Antimultipath communication by injecting tone into null in signal spectrum

Abstract: A transmitter for digital radio communication creates a null by balanced encoding of data modulated on an RF carrier, and inserts a calibration tone within the null. By having the calibration tone coincide in phase and frequency with the transmitted radio frequency output for coherent demodulation of data at the receiver where the tone calibration signal is extracted and used for multipath fading compensation.

Data modem system

Abstract: A data modem system incorporates, digital filters (11, 21, 59, 60) in transmitter and receiver units having intermediate frequency stages which operate at a frequency which is an integral multiple of the modulation symbol rate. This enables filtering, modulation and demodulation to be carried out in a relatively simple manner using digital hardware, with conversion between different forms of modulation such as QPSK, BPSK and offset BPSK being possible without substantial hardware modifications. In one embodiment of the invention, the receiver employs a phase estimating circuit (124) to compensate for phase and frequency errors arising when a free-running local oscillator is used as an alternative to one controlled via a phase-locked loop.

Clock rate spread spectrum

Abstract: A transmitter and receiver for direct sequence spread-spectrum (SS) communications is described. The audio or other analog information to be communicated controls the clock rate of a pseudorandom sequence (PRS) to produce clock rate encoded direct sequence spread spectrum signals. The signals are decoded by a feedback loop including a voltage controlled oscillator which clocks a local PRS generator. A phase detector responds to the clock rate encoded received PRS signal and to the local PRS signal to produce a control signal. The control signal is filtered and applied to the VCO to control the local PRS clock rate. The decoder loop forces the VCO rate to track the clock rate of the encoded signal, and the decoder loop VCO control signal reproduces the analog information. A demodulator for a clock rate encoded SS signal upconverted by a carrier includes a pair of mixers, a first of which receives the signal to be demodulated and the local PRS sequence from the decoder. When the local and received sequence are phase coincident, the first mixer produces the carrier. The second mixer receives the signal to be demodulated and the carrier from the first mixer and generates therefrom demodulated baseband clock rate encoded SS signals for application to the decoder.

Fiber Optic Video Transmission Employing Square Wave Frequency Modulation

Abstract: Investigations are carried out on the performance of video transmission systems employing square wave frequency modulation (SWFM). A simple formula for the signal-to-noise ratio (SNR) in the SWFM video transmission systems is derived. The SNR performance is compared to that of frequency modulation (FM) and pulse frequency modulation (PFM). SWFM is shown to display the greatest receiver sensitivity when the fiber 6 dB bandwidth is less than 100 MHz. Video transmission experiments demonstrate the advantages of the SWFM, including 1) degree of simplicity in modulation/demodulation circuits as well as in optical transmitter and receiver circuits, 2) tolerance to nonlinearities in the system, and 3) insensitivity to the fiber baseband frequency-phase characteristics. Both the theoretical and experimental results have made clear the superiority of SWFM optical video transmission systems in application to CATV distribution systems and subscriber loop systems.

Polar loop transmitter

Abstract: A polar loop transmitter having particular, but not exclusive, application in VHF single-sideband radio systems. In order to avoid having to match the amplitude detectors used in known polar loop transmitters, it is proposed as an alternative to provide a subtracting circuit which produces a difference signal from between the low level input signal and the attenuated output signal. The difference signal is amplitude demodulated in a mixer and the demodulated signal is used as a control signal for the amplitude modulator. First and second limiting amplifiers are used to derive the carrier signals from the input and output signals. The outputs of the limiting amplifiers are applied to a phase sensitive detector formed by another mixer whose output is used as a correction signal for a voltage controlled oscillator coupled to the amplitude modulator. The output of the first limiting amplifier is also used as the local oscillator signal for the amplitude demodulator. Means are proposed for compensating for the phase shifts introduced by the limiting amplifiers.

Upstream signal control apparatus in bidirectional CATV system

Abstract: A CPU in a subscriber terminal generates upstream data and a transmission request signal as an enable signal for sending the upstream data to a broadcast center. An oscillator generates a carrier wave while the enable signal is being held at a predetermined level. A multiplier multiplies the carrier wave with the upstream data and generates the upstream signal. A transmission level adjusting amplifier amplifies the upstream signal, and the amplified signal is sent onto a cable. A detecting means receives a signal associated with the enable signal and discriminates the presence of the carrier wave. A time constant circuit measures the period of time for which the carrier wave is present in response to output from the detecting means. A switching circuit stops supplying at least the carrier wave of a first frequency onto the cable in response to output from the time constant circuit which indicates that the carrier wave has been exceeded the predetermined period of time.

Astronomical polarimeter with 2-D detector arrays.

Abstract: It is shown how fast (50–100 kHz) piezoelastic modulation of the full Stokes vector can be used in combination with large CCD-type detector arrays with long integration times. The technique is to use an optical demodulation system (replacing the lockin amplifiers in corresponding single-channel detector systems). This allows the CCD detectors to be used with integration times and readout rates as in ordinary photometry. Including an optical phase switch in the system, the effect of the large pixel-to-pixel sensitivity variations can be removed from the recorded polarization images. The beam splitter that suppresses atmospheric noise can be located immediately before the detectors instead of being part of the polarization analyzer.

Modem for RF subscriber telephone system

Abstract: A modem including a modulator section for converting a bit stream, wherein each given number of successive bits defines a symbol, into a phase modulated intermediate frequency (IF) signal, the symbols in the modulator section being phase modulated and then digitally filtered to provide a filtered signal that, when converted into an analog signal, provides a modulation signal of a frequency that is centered about a predetermined frequency and deviates therefrom in accordance with the value of the phase modulated symbol, the analog signal then being filtered and passed to a mixer for up conversion to an IF signal; and a demodulator section for converting the IF signal into a bit stream, the demodulator section including a microprocessor which not only filters received digital signals but contains a memory for storing filter coefficients for use in the filtering of received digital signals; the modem being also operable in a training mode wherein a sequence of predetermined symbols is provided to the filter in the modulator section and the resultant phase modulated IF signal is receivable in the demodulator section, the microprocessor acting to adjust the filter coefficients stored in its memory until the reconstructed phase modulated symbols correspond to the sequence of predetermined symbols provided to the filter in the modulator section.

Compatible am broadcast/data transmission system

Abstract: A system for transmitting a composite signal comprising a data transmission signal component and an AM broadcast signal component. The broadcast signal component may be monophonic or stereophonic. The level of the data signal component is made a function of the modulation level so that the data signal is masked by the program modulation and, therefore, AM radio listeners will not be disturbed by the data signal. The rate of data transmission, in one embodiment, is reduced as the level of the data signal is reduced. The data signal is in quadrature with the AM carrier so as to minimize detection of the data signal by an envelope demodulator. Suitable data receivers are also disclosed.

Transceiver for multi-drop local area networks

Abstract: TRANSCEIVER FOR MULTI-DROP LOCAL AREA NETWORKS ABSTRACT A multi-drop local area network utilizes alternating current power lines as a transmission channel. Identical transceivers are used as a master and as up to 255 slaves. Each transceiver comprises modulator and demodulator units and a microprocessor control unit which may be connected to a host. The modulator and demodulator are connected to the alternating current power line through a high pass filter comprising a split bobbin, iron core transformer having an air gap which also provides impedance matching. The demodulator includes a clipper and analog circuitry providing a band pass filter and generating a square wave corresponding to the received carrier. A state machine acting as a digital filter produces a received carrier signal only when each half cycle of the carrier square wave is of the proper duration and the carrier signal has existed for slightly more than one half of the transmitted bit duration, which is a predetermined number of carrier cycles. The microprocessors are programmed to provide for block ahead acknowledgment, alternate transmissions between master and slaves; each block message comprises an acknowledgment non-acknowledgment bit, a message sequence bit, a polling sequence bit, and a longitudinal redundancy check. The carrier frequency utilized lies in the range of 20 to 40 KHz and preferably within the range between 27 and 33 KHz for a power line transmission channel. Error free 1200 Baud transmission rates are achieved in full duplex. Higher carrier frequencies and transmission rates are possible over less noisy transmission channels such as wire pairs.

Low error rate digital demodulator

Abstract: An on-off keyed carrier signal is demodulated by quadrature sampling of the incoming carrier signal at twice carrier frequency to provide short term digital patterns indicating detection of an incoming signal of carrier frequency. A number of phase counting channels are provided which count the number of times a particular pattern is detected during a longer interval, preferably a sixth of a bit interval at the baud rate of the received message. Since the phase counters are reset every sixth of a bit they can collectively accommodate a substantial drift in phase of the received carrier during reception of a complete 33 bit message while still operating asynchronously with respect to the received carrier.

Baseband demodulator for FM and/or AM signals

Abstract: A demodulator of the type employed in a "zero-IF" system uses a local oscillator for providing quadrature output signals at the center frequency of a received signal to be demodulated. The demodulator has first and second mixers for separately mixing the received signal with the quadrature signals to provide a first and second output signal each in quadrature at the outputs of said mixers. These signals are low pass filtered. The demodulator includes third and fourth mixers with each mixer respectively receiving at an input one of the first and second low pass signals. At another input the mixers receive third and fourth signals. The third and fourth signals are derived from mixing a variable controlled oscillator (VCO) signal with local oscillator quadrature signals the frequency of said is independent of the center frequency of the received signal. The outputs of the third and fourth mixers are applied to the inputs of a difference amplifier, the output of which controls the frequency of the VCO in an automatic frequency control mode (AFC) to cause the output of the difference amplifier to provide demodulation of an FM signal. Additional mixers are provided which mix the first and second low pass signals respectively with the fourth and third signals, with the signals appearing at the outputs of such additional mixers being summed to provide demonstration of an AM signal independently of the FM demodulation.

Bidirectional digital signal communication system

Abstract: A bidirectional digital signal communication system includes a plurality of terminal units in communication with a center unit. Each of the terminal units includes a signal generator for generating digital data spread with a PN (pseudo-noise) code, a modulator for modulating the digital data on a carrier, and a transmitter for transmitting the modulated digital signal to the center unit through a transmission line. The center unit includes a receiver for receiving the modulated digital signal, a PN code generator for locally generating a PN code which corresponds to the transmitted PN code, a multiplier supplied with outputs of the receiver and the PN code generator to produce a digital signal de-spread from the PN code, a demodulator connected to the multiplier to reproduce the digital data from the modulated digital signal, and a memory for storing the PN code when the signal level of the multiplier exceeds a predetermined value. The stored PN code is retrieved from the memory to enable rapid synchronization with the next transmission received from the terminal unit.

Method for remote control of electrical equipment and corresponding receiver

Abstract: 1. Process for the remote control of electrical equipments, wherein : A) in a transmission centre : - there is formed a digital signal corresponding to a function to be remote controlled, this signal is encoded by at least one error detecting and correcting code, the analog signal is modulated by the encoded digital signal and the modulated signal is transmitted to equipments to be remote controlled ; B) in receiving means associated with each of the equipments : - the transmitted signal is received, it is demodulated, the demodulated signal is decoded in order to retrieve the digital signal corresponding to the function to be remote controlled and a command for the execution of this function is generated, this process being characterized in that : A) in the transmission centre : - the digital signal is formed by an address word (A) and a function word (F), - this signal is successively processed by two error detecting and correcting codes, a first BCH code and a second REED-MULLER code, - a synchronization word (MS) is made to precede the doubly encoded signal, the synchronization word - address word - function word together forming a digital frame, - this frame is repeated cyclically, - each digital frame modulates a subcarrier (SP) with a phase shift modulation, - this subcarrier (SP) thus modulated is used in order to modulate the frequency of a carrier which is then radio transmitted by a frequency modulation radio transmitter ; B) in each receiver, - the transmitted signal is picked up by a tuned filter (10) coupled to an electric current distribution system (16) supplying the equipment to be remote controlled (50), - a frequency demodulation is carried out in order to retrieve the modulated subcarrier, - the subcarrier is demodulated by a phase shift demodulation in order to retrieve the digital frame, - the digital frame is processed firstly by recognition of the synchronization word (MS), then by a first REED-MULLER typ decoding, which enables the possible errors to be detected and corrected, and then by a second BCH type decoding, which enables the residual errors to be detected, and the digital signal is thus retrieved with its address word (A) and its function word (F), - the address word (A) is decoded, - the received address is comparied with the receiver's own address, - if the received address and the receiver's address are the same, the received function word (F) is processed by comparing it with a plurality of function words stored in the receiver, - the number of coincidences between the cyclically received function word and one of the stored function words is counted and, when this number reaches a predetermined value, a command for execution is initiated corresponding to this function in the associated equipment.

Modified absolute phase detector

Abstract: Disclosed is a modulation-demodulation system and method for transmitting a plurality of sequentially received information bit sequences, the system including a transmitter having a state machine for expanding by a coding process each information bit sequence into an expanded bit sequence with a coded bit group portion and, in some cases, an uncoded bit group portion. The transmitter further includes a modulator for modulating a carrier signal by one of a plurality of multilevel symbols in a two-dimensional complex plane in response to each sequentially applied expanded bit sequence, with the coded bit group portion being used to specify a subset of the multilevel symbols which when rotated in the complex plane maps upon another subset for each adverse angular rotation and the uncoded bit group portion being used to specify for a selected multiple symbol subset the transmitted multilevel symbol of the carrier signal. The system further includes a receiver having a demodulator and slicer for demodulating and detecting the modulated carrier signal to obtain the expanded bit sequences; a phase rotation detector, coupled to the slicer, for uniquely identifying each adverse angular rotation by analyzing a plurality of non-rotated and rotated sequentially received coded bit group portions; and a phase corrector, coupled to the phase rotation detector, for compensating for the adverse angular rotation.

Method and apparatus for reduction of sinusoidal phase jitter in a high speed data modem

Abstract: An apparatus for cancellation of sinusoidal varying phase jitter in a data modem is implemented in firmware using microprocessor technology. An estimate of the frequency and phase of the phase jitter is computed in a first stage. Substantially independently, an estimate of the amplitude of the phase jitter is computed in another stage. These estimates are combined to form a composite estimate of the phase jitter which is utilized to cancel out the sinusoidal phase jitter in the demodulator. Digital phase locked loop technology (DPLL) is utilized to lock onto the phase jitter components. The capture range of the phase locked loop is dynamically altered during a training sequence to allow for capturing a wide range of jitter frequencies. During the training sequence the damping factor of the loop is gradually altered thereby substantially reducing the capture range and response time of the loop once jitter acquisition has occurred. This results in enhanced noise performance while still retaining the capability of locking to a wide range of jitter frequencies. Quantization of the error signal is utilized to compute estimates of the frequencies and phase of the jitter signal so that continuous updating occurs virtualy without regard for the amplitude or change in amplitude of the jitter signal.

Current sense demodulator

Abstract: An apparatus for use with an amplifier (51-56) regulates the current through a load (35) according to a drive signal (93) having first and second states and comprises switchless sensing means (62, 64) and polarity correction means (66, 68). The switchless sensing means (62, 64) provides a sense signal continuously representative of the magnitude of the current through the load (35) without the use of switching circuitry. The polarity correction means is directly coupled to the sensing means (62, 64) and receives the sense signal and is responsive to the status of the drive signal (93) for providing an analog signal (102) continuously representative of both the instantaneous magnitude and polarity of the current through the load.

Cross-polarization interference canceller

Abstract: In order to reset a cross-polarization interference canceller for use in a digital radio communications receiver, two polarized-signal discriminators are provided in a manner to be coupled respectively to receive reproduced digital data from the corresponding demodulators. In the event that the demodulator produces an output not characteristic thereof, the corresponding polarized-signal discriminator produces a reset signal to reset the canceller.

Characterisation of digital radio signals

Abstract: A method of characterizing a modulated carrier signal by generating electrical signals which represent arithmetic ratios and which can be used to construct a histogram indicative of the quality of the modulated carrier signal. For a digitally modulated carrier signal, time spaced samples of in band amplitude dispersion of the signal are generated and first counts are made of the number of occurrences of particular values of in band dispersion, over a range of such values. The error ratio of the decoded modulated carrier signal is also monitored and seconds counts are accummulated, of the numbers of occurrences of respective values of in band amplitude dispersions which at least substantially coincide with the value of said error ratio crossing a predetermined level. The ratio of each second count to each corresponding first count is then taken and these ratios are then representative of the probability that, at the respective in band amplitude dispersion value, the value of the error ratio will cross the predetermined level. When practicing the method with analogue modulated carrier signals, values of the signal to noise ratio are employed instead of the values of error ratio. A method of the invention permits construction of a histogram which pictorially represents the quality of the modulated carrier signal.

A single-chip MSK coherent demodulator for mobile radio transmission

Abstract: Design and performance of a single-chip minimum shift keying (MSK) coherent demodulator fabricated by complementary metal oxide semiconductor-integrated circuit (CMOS-IC) technology is described. The demodulator consists of a phase detection circuit, carrier recovery circuit, data recovery circuit, and timing-clock recovery circuit. For the carrier recovery circuit, three types of Costas-loop are reviewed from the viewpoint that MSK modulation format has a close relationship to binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK). Among these loops, a loop of center-frequency locking scheme modified from a BPSK Costas-loop, termed MSK Costas-loop in this paper, is adopted for IC implementation. Digital IC design techniques are next described. Utilizing the sample-and-hold operation of the digital devices, a divided-frequency locking scheme of the quadrature coherent demodulation is proposed. Finally, IC demodulator performance is experimentally shown in the static and Rayleigh fading environments. The bit error rate performance and error-burst characteristic are measured. It is concluded that the single-chip coherent demodulator is suitable for digital mobile radio application.

General time, space and frequency multiplexed acousto-optic correlator

Abstract: Time, space and frequency multiplexed time integrating acousto-optic correlators and exemplary uses thereof. The correlators utilize a plurality of radio frequency (RF) modulators, each operating at the same or a different RF frequency to provide excitations to an acousto-optic cell representing the sum of the outputs of the modulators. A corresponding plurality of detectors are positioned so that light from the acousto-optic cells corresponding to the correlation output of various pairs of the RF modulators is incident to a respective one of the detectors. Uses for the correlators include demodulation and synchronization applications.

Target-identifying metal detector

Abstract: The target signal produced by the receive coil of an induction-balance metal detector is demodulated and filtered in matched V X and V R demodulator and filters (11, 13) driven by quadrature versions (TA/TB) of the signal that drives the transmit coil of the detector. The resultant phase quadrature signals--V X and V R --are applied to a ramp computer (15) that internally produces a ramp signal whose slope is a function of the magnitude of the sum of the V X and V R signals. The ramp computer (15) inludes a comparator that combines the ramp signal with a fixed signal whose magnitude is a function of the magnitude of the V X signal. The result is a pulse whose width is a function of the phase angle and, thus, the identity of the target that created the V X and V R signals. The pulse is integrated (17) and the results stored at a TI voltage whose magnitude is functionally related to the identity of the target. The TI voltage is used to control the state of a display (23) calibrated in terms of target identity. A controller (37) that controls the foregoing operations is subject to preventing the display from occurring when a discriminate circuit (25) determines that the TI voltage denotes a target (such as a nail) that is not of interest. The discrimination disablement of the display is subject to being overridden by a signal from a target-select circuit (27) when the TI voltage denotes a target of interest, (e.g., a nickel) falling within the phase angle range that is normally subject to discrimination. Further, the controller (37) only allows the detection sequence to operate if the target produces a signal having a magnitude above a minimal level. The controller is also subject to being overridden by the output of a deep-target circuit (35) designed to be operator-enabled when the detector is being used to locate deeply buried targets in ground having a low mineralization content.