GB2105130A

GB2105130A - Synchronous AM envelope detector

Info

Publication number: GB2105130A
Application number: GB08220067A
Authority: GB
Inventors: Leonard R Kahn
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-08-31
Filing date: 1982-07-09
Publication date: 1983-03-16
Also published as: CA1197904A; FR2512291B1; DE3230606A1; IN157383B; KR840001412A; AU8578682A; KR880000869B1; BR8205084A; ZA825531B; NL8203256A; FR2512291A1; AU550418B2; JPS5846704A; MX153217A; NZ201230A; GB2105130B

Abstract

An envelope detector for an amplitude and phase modulated carrier includes in-phase and quadrature-phase product demodulators 104, 106 whose outputs are passed through low pass filters 110, 112 separately. These separate filtered outputs are then squared 114, 116 and summed 118. The square root 120 of the resulting sum signal is taken to produce the desired output signal whose amplitude is proportional to the envelope of the composite modulated carrier. <IMAGE>

Description

SPECIFICATION Synchronous envelope detector Background of the invention This invention relates to the reception of amplitude modulated radio frequency carrier waves and, more particularly, to envelope detectors for such carrier waves.

In a typical receiver for monaural amplitude modulated (AM) radio frequency signals, the first stage is a superheterodyne circuit that converts the radio frequency signal to an intermediate frequency (I.F.) signal. The envelope of this signal, which represents the amplitude modulation, is then detected, i.e. a signal equivalent to the amplitude modulation is created from the l.F. signal. This envelope detection may be accomplished by the series combination of a diode, which half-wave rectifies the signal, and a low pass filter. Alternatively, envelope detection can be achieved by an in-phase product demodulator wherein the l.F. signal is multiplied by a signal at the frequency of, and in phase with, the carrier. Higher order harmonics and high frequency noise are then filtered out by use of a low pass filter.

One proposed system of AM stereo broadcasting amplitude modulates the carrier with a signal representing the sum of the left and right stereo audio signals (L + R) and phase modulates it with a signal representing different (L - R). A receiver for such a broadcast signal is disclosed in the inventor's prior U. S. Patent No. 4,108,994. One embodiment of this receiver utilizes an in-phase product demodulator as an envelope detector to recover the L + R signal.

Whether in a monaural or a stereo receiver, an envelope detector of the diode or in-phase product demodulator type is subject to fading and reduced selectivity problems. In particular, a diode detector cannot differentiate between the desired carrier and a stronger interfering carrier that falls in the l.F.

passband. An in-phase product demodulator is subject to problems resulting from incidental phase modulation, which is a common occurence with AM transmitters.

It is, therefore, an object of the present invention to improve the selectivity and resistance to fading of AM receivers by providing an envelope detector which utilizes both in-phase and quadrature product demodulation.

Summary of the invention In an illustrative embodiment of the invention a received AM signal that has been translated into the l.F. band is applied to two product demodulators.

The carrier of the I.F. signal is isolated, e.g. by a phase-locked loop, and a corresponding quadrature carrier, i.e. the isolated carrier phase-shifted by 90 degrees, is generated. The carrier is applied to one of the demodulators and the quadrature carrier is applied to the other. The outputs of the demodulators are separately squared and then added together. The square root of the resulting signal is taken in order to produce the final enveloperepresentative signal. By using product demodulators, i.e. synchronous detection, the rejection of strong interfering carriers is possible. Also, the use of both the in-phase and quadrature detected signal makes the envelope detection process insensitive to transmitter incidental phase modulation.

For a better understanding of the present invention, together with other and further objects, reference is made to the following description, taken in conjunction with the accompanying drawing, and its scope will be pointed out in the appended claims.

Brief description of the drawing Figure 1 of the drawing shows a block diagram of a synchronous envelope detector according to the present invention.

Description of the invention In a stereo system such as that disclosed in U. S.

Patent No.4,018,994, pertinent portions of which are incorporated herein by reference, the broadcast signal has an amplitude modulation component representative of the L + R stereo information and a phase modulation component representative of the L - R stereo information. If the in-phase signal component is represented by a vertical vector, the quadrature phase component will be represented by a horizontal vector.Thus the envelope modulation of the broadcast signal may be represented by a vector varying in angle between 0 and 90 degrees and having an amplitude that is equivalent to the square root of the sum of the squares of the in-phase component (I) and the quadrature component (Q), i.e. 12 + Q2 The circuit of Figure 1 accomplishes envelope detection by generating the in-phase component (I) in product demodulator 104 and the quadrature-phase component (Q) in product demodulator 108. In order produce the l.F. signal needed for product demodulators 104 and 108, a composite amplitude and phase modulated R.F. signal is received by an antenna and processed by a conventional superheterodyne circuit (not shown) wherein the corresponding l.F. signal is generated. This I.F.

signal is not only applied to the product demodulators 104 and 108, but is also applied to a carrier isolation circuit 102. As stated in the above-cited patent, carrier isolation can be accomplished by means of a phase-locked loop arrangement. The isolated carrier is then applied to a phase shift circuit 106 wherein in-phase and quadrature-phase carrier signals are derived. Such signals, however, are already present in an AM stereo receiver such as that shown in the Patent 4,018,994.

The output of product demodulator 104 is applied to low pass filter (LPF) 110 while the output of demodulator 108 is applied to low pass filter 112. It is necessary that the product demodulator 104 and low pass filter 110 pass the DC component which results from multiplying the l.F. signal by the carrier. It is also desirable for the DC component to be maintained in the quadrature path through product demodulator 108 and filter 112. However, there are some situations where this may be unnecesary.

Filters 110 and 112 can provide improved selectivity, assuming the product demodulators 104 and 108 have low distortion characteristics. For example, an active filter having five poles can be inexpensively constructed. Such a filter will provide 45db attenuation at 1 OKHz and 1 dub attenuation at 5KHz. At 5.4KHz the signal would be down 6.6db. Although not always neccesary, it is advantageous to include some selectively in the l.F. circuitry in order to avoid overload of the product demodulators. The selectivity achieved by low pass filters 110 and 112 is symmetrical, which is an advantage in most AM reception situations, but the main advantage is that it allows the use of the two active low pass audio filters, which can be constructed using integrated circuit techniques, in place of a more difficult to make l.F. filter.

From filters 110 and 112 the signals are coupled to squaring circuits 114 and 116, respectively, in order to generate the 12 and Q2 signals. These signals are then combined in summing circuit 118 and the sum signal is coupled to a square root circuit 120 which produces the desired envelope detected output signal.

Claims

1. An envelope detector for receiving an amplitude modulated carrier signal comprising: means for generating an in-phase and a quadrature-phase carrier signal from a received amplitude modulated carrier signal; first and second product demodulator means having said in-phase carrier signal and said quadrature-phase signal, respectively, coupled thereto as well as said received amplitude modulated signal, for producing first and second demodulated signals respectively; first and second squaring circuit means for squaring said first and second demodulated signals respectively; means for summing said first and second squared signals to form a sum signal; and means, responsive to said sum signal for develop ing an output signal proportional to the square root thereof and representative of the amplitude of the envelope of said received amplitude-modulated signal.

2. An envelope detector as claimed in claim 1, wherein said means for generating an in-phase and a quadrature-phase carrier signal comprises: a phase-lock loop response to said received modulated carrier, for producing an unmodulated carrier having the same frequency and phase as said modulated carrier; and a 90 degree phase shift circuit for producing a quadrature-phase signal from the output of said phase lock loop.

3. An envelope detector as claimed in claim 1 or 2, wherein a low pass filter is coupled in the output of each of said first and second product demodulator means.

4. A Synchronous Envelope Detector of the type specified and substantially as illustrated in the accompanying drawing and described in the specification with reference thereto.