GB2282501A - Detecting a narrow band signal in wide band noise - Google Patents

Detecting a narrow band signal in wide band noise Download PDF

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Publication number
GB2282501A
GB2282501A GB9320038A GB9320038A GB2282501A GB 2282501 A GB2282501 A GB 2282501A GB 9320038 A GB9320038 A GB 9320038A GB 9320038 A GB9320038 A GB 9320038A GB 2282501 A GB2282501 A GB 2282501A
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GB
United Kingdom
Prior art keywords
signal
differential amplifier
detector
paths
band
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
GB9320038A
Other versions
GB2282501B (en
GB9320038D0 (en
Inventor
Christopher Keith Richardson
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Roke Manor Research Ltd
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Roke Manor Research Ltd
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Filing date
Publication date
Application filed by Roke Manor Research Ltd filed Critical Roke Manor Research Ltd
Priority to GB9320038A priority Critical patent/GB2282501B/en
Publication of GB9320038D0 publication Critical patent/GB9320038D0/en
Publication of GB2282501A publication Critical patent/GB2282501A/en
Application granted granted Critical
Publication of GB2282501B publication Critical patent/GB2282501B/en
Anticipated expiration legal-status Critical
Expired - Fee Related legal-status Critical Current

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Classifications

    • GPHYSICS
    • G01MEASURING; TESTING
    • G01RMEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R23/00Arrangements for measuring frequencies; Arrangements for analysing frequency spectra
    • G01R23/16Spectrum analysis; Fourier analysis
    • G01R23/165Spectrum analysis; Fourier analysis using filters
    • GPHYSICS
    • G01MEASURING; TESTING
    • G01RMEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R19/00Arrangements for measuring currents or voltages or for indicating presence or sign thereof
    • G01R19/0007Frequency selective voltage or current level measuring
    • HELECTRICITY
    • H04ELECTRIC COMMUNICATION TECHNIQUE
    • H04BTRANSMISSION
    • H04B1/00Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
    • H04B1/06Receivers
    • H04B1/16Circuits

Abstract

A signal detector comprising a differential amplifier having two input terminals 2, 3 and an output terminal 4, a plurality of signal paths connected mutually in parallel, each of which signal paths includes an amplitude detector 12 - 15, 7 serially connected with a bandpass filter, wherein one of the said signal paths which is connected to one in put terminal of the differential amplifier includes a bandpass filter 6 having a relatively wide pass band 'f' and wherein the other of the said signal paths include bandpass filters 8 - 11 which cover mutually different relatively narrow frequency hands which fall within the band 'f' which said other signal paths are connected to the other input terminal of the differential amplifier, an output signal from the signal detector being derived via the output terminal of the differential amplifier. In operation all of diodes 12 - 15 except that carrying the desired signal are cut off by the higher voltage produced by the signal. The signal through the wide band amplifier sets up a threshold voltage on terminal 3 of the differential amplifier. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO SIGNAL DETECTORS This invention relates to signal detectors and more especially it relates to the detection of small signals in noise.
A particular problem arises when it is required to detect a small wanted signal in a narrow frequency band lying within a much wider received signal band, wherein noise aggregates so as to swamp the wanted signal.
A conventional solution to this problem would be to use a narrow band filter tuned to the wanted signal. This however, is not possible when the wanted signal tend to drift in frequency, or when its precise frequency is not known. Moreover, this conventional solution requires highly stable and consequently costly components.
One important object of the present invention therefore is to provide a relatively simple and inexpensive signal detector which will detect a weak narrow band signal below wide band noise.
According to the present invention a signal detector comprises a differential amplifier having two input terminals and an output terminal, a plurality of signal paths connected mutually in parallel, each of which signal paths includes an amplitude detector serially connected with a bandpass filter, wherein one of the said signal paths which is connected to one input terminal of the differential amplifier includes a bandpass filter having a relatively wide pass band 'f' and wherein the other of the said signal paths include bandpass filters which cover mutually different relatively narrow frequency bands which fall within the band 'f' which said other signal paths are connected to the other input terminal of the differential amplifier, an output signal from the signal detector being derived via the output terminal of the differential amplifier.
The signal detector may include an amplifier, the gain of which is adjustable and via which the said other signal paths are fed.
An output signal from the differential amplifier may be derived via a low pass filter.
The band pass filter in each signal path may be followed by a diode detector, which diodes constitute the amplitude detectors.
The diode detectors in the said other paths may be coupled together at ends thereof remote the bandpass filters and coupled to the said other input terminal of the differential amplifier, the diode detector in the said one path being connected, at an end thereof remote its associated bandpass filter, to the said one input terminal of the differential amplifier.
A smoothing capacitor may be connected to each input terminal of the differential amplifier.
One embodiment of the invention will now be described by way of example with reference to the accompanying drawing.
Referring now to the drawing, a signal detector which might be referred to as a signal to noise ratio detector, comprises a differential amplifier 1 having input terminals 2 and 3 and an output terminal 4. The input terminal 3 of the differential amplifier 1 is fed from an input signal line 5 via a band pass filter 6 which is connected in series with a diode detector 7. The input terminal 2 of the differential amplifier 1 is fed via a plurality of band pass filters 8, 9, 10 and 11 which are connected in series with diodes 12, 13, 14 and 15 respectively, thereby to define four signal paths 16, 17, 18 and 19 which are connected mutually in parallel and arranged to feed the input terminal 2.The band pass filters 8, 9, 10 and 11 are fed from the line 5 via a gain controlled amplifier 20 and the output terminal 4 of the differential amplifier 1 is arranged to feed an output line 25 via a low pass filter 26.
The bandwidth of the band pass filter 6 is arranged to be relatively wide, and the frequency bands covered by the band pass filters 8, 9, 10 and 11 are arranged to be contiguous and to fall within the bandwidth of a frequency band 'f' covered by the band pass filter 6.
Operation of the detector is as follows. An input signal on the line 5 consists of a wanted narrow band signal together with broad band noise at a relatively high level. The total signal on the line 5 is fed to two signal paths 21 and 22 which feed the amplifier 20 and the band pass filter 6 respectively. If the wanted narrow band signal lies within the pass band of one of the filters 8, 9, 10 or 11, the voltage on its associated diode 12, 13, 14 or 15, as the case may be, will rise and tend to cut off the other diodes. Thus, if for example, the wanted signal falls within the pass band of the band pass filter 9, the voltage at the diode 13 will rise and the other diodes connected thereto, i.e. the diodes 12, 14 and 15, will tend to be cut off. The circuit comprising the diodes 12, 13, 14 and 15 thus behaves very much like an OR gate.
It will be apparent therefore that the signal to noise ratio which obtains at the input terminal 2 of the differential amplifier 1, is appropriate to a receiver system operating with a bandwidth corresponding to a single one of the band pass filters 8, 9, 10 or 11 rather than with the overall bandwidth of the system. The total signal and noise is fed via the line 22 and the relatively wide bandwidth filter 6, to provide a signal on the line 3 which amounts to an adaptive threshold signal. The system is therefore not level dependent nor is it affected by high level wideband transient signals.
In order initially to set up the detector, with no wanted signal present, the gain of the amplifier 20 is adjusted so that the signal on the line 3 just exceeds the signal on the line 2. The signals on the lines 2 and 3 will generally be dc levels which are smoothed by capacitors 23 and 24 respectively. When a signal is received in one of the signal paths 16, 17, 18, 19, that signal will be applied to the differential amplifier input terminal 2 and exceed the threshold set on the input terminal 3 thereby to provide an output signal on the line 25.
It will be appreciated that various modifications may be made to the arrangements shown without departing from the scope of the invention, and for example, more narrow band filters may be added without loss of performance. Additionally, although single diode rectifiers have been used in the present example, as amplitude detectors, it is contemplated that in an alternative embodiment a full wave rectifier arrangement may be used.
A signal detector in accordance with the present invention has been found particularly useful for detecting small Morse signals and for this application, 'f', i.e. the bandwidth of the relatively wideband filter 6, is arranged to cover the audio spectrum and the other filters 8, 9, 10 and 11 are arranged to cover contiguous frequency bands within the band 'f'. Although four filters only shown in the drawing by way of example, it will be appreciated that any number of filters may be provided in accordance with the application in view. If the Morse signal drifts in frequency such that it moves between the signal paths 16, 17, 18 and 19 then operation will not be affected since the signal in the path in use will tend to cut off the diodes in the other paths thereby suppressing the noise contribution in those paths.
Although the detector is eminently suitable for audio signal applications as just before described, it is also contemplated that detectors for operation in the HF or other radio frequency bands may be used to good effect.

Claims (9)

1. A signal detector comprising a differential amplifier having two input terminals and an output terminal, a plurality of signal paths connected mutually in parallel, each of which signal paths includes an amplitude detector serially connected with a bandpass filter, wherein one of the said signal paths which is connected to one input terminal of the differential amplifier includes a bandpass filter having a relatively wide pass band 'f' and wherein the other of the said signal paths include bandpass filters which cover mutually different relatively narrow frequency bands which fall within the band 'f' which said other signal paths are connected to the other input terminal of the differential amplifier, an output signal from the signal detector being derived via the output terminal of the differential amplifier.
2. A signal detector as claimed in claimed 1, comprising an amplifier, the gain of which is adjustable and via which the said other signal paths are fed.
3 A signal detector as claimed in claim 1 or claim 2, wherein the output signal from the differential amplifier is derived via a low pass filter.
4. A signal detector as claimed in claim any preceding claim, wherein the band pass filter in each signal path is followed by a diode detector, which diodes constitute the amplitude detectors.
5. A signal detector as claimed in claim 4, wherein the diode detectors in the said other paths are coupled together at ends thereof remote the bandpass filters and coupled to the said other input terminal of the differential amplifier, the diode detector in the said one path being connected, at an end thereof remote its associated bandpass filter, to the said one input terminal of the differential amplifier.
6. A signal detector as claimed in any preceding claim, wherein smoothing capacitor is connected to each input terminal of the differential amplifier.
7. A signal detector as claimed in claim 1 and as hereinbefore described with reference to the accompanying drawing.
8. Radio receiving apparatus including a signal detector as claimed in any preceding claim.
9. Radio receiving apparatus as claimed in claim 8, wherein the signal detector is arranged to include bandpass filters effective for operation in the audio frequency range.
GB9320038A 1993-09-29 1993-09-29 Improvements in or relating to signal detectors Expired - Fee Related GB2282501B (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
GB9320038A GB2282501B (en) 1993-09-29 1993-09-29 Improvements in or relating to signal detectors

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB9320038A GB2282501B (en) 1993-09-29 1993-09-29 Improvements in or relating to signal detectors

Publications (3)

Publication Number Publication Date
GB9320038D0 GB9320038D0 (en) 1993-11-17
GB2282501A true GB2282501A (en) 1995-04-05
GB2282501B GB2282501B (en) 1997-10-01

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Family Applications (1)

Application Number Title Priority Date Filing Date
GB9320038A Expired - Fee Related GB2282501B (en) 1993-09-29 1993-09-29 Improvements in or relating to signal detectors

Country Status (1)

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GB (1) GB2282501B (en)

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2329290B (en) * 1996-05-31 2001-04-18 Tracker Network Method of transmitting and receiving data,system and receiver therefor
US8054061B2 (en) 2006-07-18 2011-11-08 University Of Sussex Electric potential sensor
GB2518010A (en) * 2013-09-09 2015-03-11 Crfs Ltd Frequency discriminator

Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2329290B (en) * 1996-05-31 2001-04-18 Tracker Network Method of transmitting and receiving data,system and receiver therefor
US8054061B2 (en) 2006-07-18 2011-11-08 University Of Sussex Electric potential sensor
GB2518010A (en) * 2013-09-09 2015-03-11 Crfs Ltd Frequency discriminator
WO2015033156A1 (en) * 2013-09-09 2015-03-12 Crfs Limited Frequency discriminator
US9729363B2 (en) 2013-09-09 2017-08-08 Crfs Limited Frequency discriminator

Also Published As

Publication number Publication date
GB2282501B (en) 1997-10-01
GB9320038D0 (en) 1993-11-17

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Legal Events

Date Code Title Description
PCNP Patent ceased through non-payment of renewal fee

Effective date: 20110929