Note: Descriptions are shown in the official language in which they were submitted.
7~
1 BACKGROUND OF THE INVENTION
2 This invention relates to the reception of
3 amplitude modulated radio frequency carrier waves and,
4 more particularly, to envelope detectors for such
carrier waves.
6 In a typical receiver for monaural amplitude
7 modulated ~AM) radio frequency signals, the first
8 stage is a superheterodyne circuit that converts the
9 radio fre~uency signal to an intermediate freauency
(I.F.) signal. The envelope of this signal 9 which
11 represents the amplitude modulation, is then detected,
12 i.e. a signal equivalent to the amplitude modulation
13 is created from the I.F. signal. This envelope
14 detection may be accomplished by the series
combination oF a diode, which half-wave rectifies the
16 signal, and a low pass filter. Alternatively,
17 envelope detection can be achieved by an in-phase
18 product demodulator wherein the I.F. signal is
19 multiplied by a signal at the frequency of, and in
phase withS the carrier. Higher order harmonics and
21 high frequency noise are then filtered out by use of a
22 low pass filter.
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1 One proposed system of AM stereo broadcasting
2 amplitude modulates the carrier with a signal
3 representing the sum of the left and right stereo
4 audio signals (L+R) and phase modulates it with a
signal representing difference (L-R). A receiver for
6 such a broadcast signal is disclosed in the inventor's
7 prior U. S. Patent No. 4~108,994. One embodiment of
8 this receiver utilizes an in-phase product demodulator
9 as an envelope detector to recover the L+R signal.
Whether in a monaural or a stereo receiver 9
11 an envelope detector of the diode or in-phase product
12 demodulator type is subject to fading and reduced
13 selectivity problems. In particular, a diode detector
14 cannot differentiate between the desired carrier and a
stronger interfering carrier that falls in the I.F.
16 passband. An in-phase product demodulator is subject
17 to problems resulting from incidental phase
18 modul~tion, which is a common occurrence with AM
19 transmitters.
It is, therefore, an nbject of the present
21 invention to improve the selectivity and resistance to
22 fading of AM receivers by providing an envelope
23 detector which utilizes both in-phase and quadrature
24 product demodulation.
O~
1 SUMMARY OF THE INVENTION
2 In an illustrative embodiment of the
3 invention a received AM signal that has been
4 translated into the I.F. band is applied to two
product demodulators. The carrier of the I.F. signal
6 is isolated, e.g. by a phase-locked loop, and a
7 corresponding quadrature carrier, i.e. the isolated
8 carrier phase-shifted by 90 degrees, is generated.
9 The carrier is applied to one of the demodulators and
the quadrature carrier is applied to the other. The
11 outputs of the demodulators are separately squared and
12 then added together. The square root of the resulting
13 signal is taken in order to produce the final
14 envelope-representative signal. By using product
demodulators, i.e. synchronous detection, the
16 rejection of strong interfering carriers is possible.
17 Also, the use of both the in-phase and uuadrature
18 detected signal makes the envelope detection process
19 insensitive to transmitter incidental phase modulation.
For a better understanding of the present
21 inventionS together with other and further objects,
22 reference is made to the following description, taken
23 in conjunction with the accompanying drawing, and its
24 scope will be pointed out in the appended claims.
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BRIEF DESCRIPTION OF THE DRAWINGS
-
The accompanying 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, 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 vec-tor. Thus the
envelope modulat;on of the broadcast signal may be represented
by a vector varying in angle between O 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. I2 + Q2. The circuit of
Fig. I 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 I.F.
17~
1 signal needed for product demodulators 104 and 108, a
2 composite amplitude and phase modulated R.F. signal is
3 received by an antenna and processed by a conventional
4 superheterodyne circuit (not shown) wherein the a
corresponding I.F. signal is generated. This I.F.
6 signal is not only applied to the product demodulators
7 104 and 108, but is also applied to a carrier
8 isolation circuit 102. As stated in the above-cited
9 patent, carrier isolation can be accomplished by means
of a phase-locked loop arrangement. The isolated
11 carrier is then applied to a phase shift circuit 106
12 wherein in-phase and quadrature-phase carrier signals
13 are derived. Such signals, however, are already
14 present in an AM stereo receiver such as that shown in
the Patent 4,018,994.
16 The output of product demodulator 104 is
17 applied to low pass filter (LPF) 110 while the output
18 of demodulator 108 is applied to low pass Filter 112.
19 It is necessary that the product demodulator 104 and
low pass filter 110 pass the DC component which
21 results from multiplying the I.F. signal by the
22 carrier. It is also desirable for the DC component to
23 be maintained in the ~uadrature path through product
24 demodulator 108 and filter 112. However, there are
some situations where this may be unnecessary.
26 Filters 110 and 112 can provide improved selectivity 7
27 assuming the product demodulators 104 and 108 have
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1 low distortion characterist;cs. For example, an
2 active filter having five poles can be inexpensively
3 constructed. Such a filter will provide 45db
4 attenuation at lOKHz and ldb attenuation at 5KHz. At
5.4KHz the signal would be down 6.6db. Although not
6 always necessary, it is advantageous to include some
7 selectivity in the I.F. circuitry in order to avoid
8 overload of the product demodulators. The selectivity
9 achieved by low pass filters 110 and 112 is
symmetrical, which is an advantage in most AM
11 reception situations, but the main advantage is that
12 it allows the use of the two active low pass audio
13 ~ilters, which can be constructed using integrated
14 circuit techniques, in place of a more difficult to
make I.F. filter.
16 From filters 110 and 112 the signals are
17 coupled to squaring circuits 114 and 116~
18 respectively, in order to generate the I2 and Q2
19 signals. These signals are then combined in summing
circuit 118 and the sum signal is coupled to a snuare
21 root circuit 120 which produces the desired envelope
22 detected output signal.
