JPH0463574B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an FM receiver, and more particularly to an improvement in a receiver using a direct conversion method. The direct conversion method uses orthogonal detection to detect the received modulated wave using a signal obtained from the output of a local oscillator with a frequency approximately equal to the center frequency of the received modulated wave as a local oscillation signal. Compared to the heterodyne method used, it is possible to use a low-pass filter without requiring a band-pass filter, there is no image response problem, and channels can be easily selected over a wide band by simply changing the local oscillation frequency. It has the following advantages. For example, a frequency detector using the direct conversion method is described in British patent 1530602 "Demodulator for
A method such as that shown in "FM signals" is known. Although this method has the above-mentioned advantages, it requires a multiplier and an automatic gain control circuit with a wide dynamic range, making it difficult to implement.

An object of the present invention is to provide a direct conversion type FM receiver whose circuit is easy to realize.

According to the present invention, there is provided a quadrature detector whose local oscillation signal is the output of a first local oscillator having a frequency substantially equal to the center frequency of a frequency modulated wave; and a second low-pass filter, a second local oscillator, a phase difference separation circuit that branches the output of the second local oscillator into two with a 90° phase difference, and the phase difference separation circuit. As a local oscillation signal, one output of
A first frequency converter that converts the frequency of the output of the first low-pass filter, and the output of the second low-pass filter using the other branched output of the phase difference separation circuit as a local oscillation signal. a second frequency converter that performs frequency conversion; a circuit that adds or subtracts the outputs of the first and second frequency converters; an amplitude limiting circuit that receives the output of the addition or subtraction circuit; and the amplitude limiting circuit. The above object can be achieved by having a frequency detector which receives the output of the circuit as an input, and by using a signal obtained from the output of the frequency modulating detector as a received signal.

A detailed explanation will be given below using the drawings. An embodiment of the invention is shown in FIG. The FM modulated wave input to reception input terminal 1 is split into two, and each is sent to mixer 2.
1 and 22. This FM modulated wave can generally be expressed as follows.

Vr(t)=Acos[ _ωct +P(t) (1) Here, A is the amplitude, _ωc is the central angular frequency of the modulated wave, and P(t) is the phase term, which is expressed as follows.

P(t)=ω _n ∫t −∝S(t)dt (2) Here, ω _n is a constant indicating the degree of modulation, and S(t) is a transmission signal. The local oscillator 31 has its oscillation frequency selected to be approximately equal to the center frequency of the received FM modulated wave. This output is branched into two, and by inputting one to a 90° phase shifter 41, two local oscillation signals having phases different by 90° are obtained. The received signals are sent to mixers 21 and 2 using these signals as local oscillation signals.
After frequency conversion by 2, the signal is input to low-pass filters 57 and 52. This low-pass filter is for removing noise and interference waves of different frequencies. 51, 52 of low pass filter
The two output baseband signals are respectively expressed as follows.

V ₁ (t) = Bcos {P(t) + ΔQ(t)} (3) V ₂ (t) = Bsin {P(t) + ΔQ(t)} (4) Here, B is a constant proportional to A , ΔQ(t) is a phase term that changes slowly due to the slight difference between the center frequency of the modulated wave and the local oscillation frequency.

The two baseband signals V ₁ (t) and V ₂ (t) are
The output of the local oscillator 32 with angular frequency ω _i is divided into two,
A signal obtained by passing one of the signals through a 90° phase shifter is used as a local oscillation signal, and the frequency is converted by mixers 23 and 24. The obtained signals V ₃ (t) and V ₄ (t) are respectively expressed as follows.

V ₃ (t)=Bcos {P(t)+ΔQ(t)}cos(ω _i t
+θ _i )=1/2Bcos {ω _i t+P(t)+ΔQ(t)+
θ _i } +1/2Bcos {ω _i t−P(t)−ΔQ(t)+θ
_i }(5) V ₄ (t)=Bsin{P(t)+ΔQ(t)}sin(ω _i t
+θ _i )=-1/2Bcos {ω _i t+P(t)+ΔQ(t)
+θ _i } +1/2Bcos {ω _i t-P(t)-ΔQ(t)+θ
_i }(6) Here, θ _i is a phase constant.

By inputting these signals to the subtraction circuit 6, its output signal V ₅ (t) becomes as follows.

V ₅ (t)=V ₃ (t)−V ₄ (t)=Bcos {ω _i t+P(
t)+ΔQ(t)+θ _i }(7) Since this signal is a frequency modulated wave with a center angular frequency ω _i , it is passed through the amplitude limiter 7 and then input to the frequency discriminator 8. Then, the received signal Vd(t)=Aod/dt{P(t)+ΔQ(t)+θ _i }=
Ao {S(t)+d/dtQ(t)}(8) is obtained. Here, Ao is a constant, and d/dtQ(t) is the difference between the center frequency of the received modulated wave and the oscillation frequency of the local oscillator 31, which is selected to be small and can be ignored. Therefore, the received output becomes the transmitted signal S(t).

The present invention eliminates the need for a differentiator and a multiplier that were necessary in the conventional direct conversion method. This effect is significant because multipliers are generally difficult to implement. Also, instead of the conventionally required automatic gain control circuit, an amplitude controller in the intermediate frequency band, which is easier to implement, can be used.

Although the present invention uses an intermediate frequency, it differs from the conventional heterodyne method which also uses an intermediate frequency in that it does not have the problem of image frequency response. In other words, in the conventional heterodyne method, in addition to the original input signal, there is a response to the image frequency, which is a frequency that is away from the local oscillation frequency by an intermediate frequency. To avoid this, frequency conversion is performed several times. It was also necessary to set restrictions on the intermediate frequency settings. At the same time, the variation range of the center frequency of the received modulated wave is restricted. In the present invention,
The intermediate frequency need only be used once, and its frequency setting can be chosen to be any frequency suitable for the frequency discriminator used. The latter feature is effective when the detection sensitivity increases as the intermediate frequency is lowered, such as in a pulse count type frequency discriminator. Further, even if the center frequency of the received modulated wave changes greatly, this can be easily handled by simply changing the oscillation frequency of the local oscillator 31, so a wideband receiver having a large number of channels can be easily realized. Another advantage of the present invention is that a low pass filter can be used as the channel selection filter, thereby eliminating the need for a band pass filter in the intermediate frequency band. This is particularly effective since it is difficult to implement a bandpass filter when the band of the modulated wave becomes narrow. As explained above, the present invention provides a direct conversion type FM receiver and a heterodyne type FM receiver.
It has the effect of having only the advantages of both receivers.

In the embodiment of the present invention, the phase of the local oscillation signal is made to differ by 90 degrees as a method of performing quadrature detection, but it is well known that the phase of the input modulated wave may be made to differ by 90 degrees instead. This is what is happening. Further, even if an adder circuit is used in place of the subtractor circuit 6, the same effect can be obtained, only by changing the polarity of the detection output.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention. In these figures, 1 is the input terminal, 21, 2
2, 23, 24 are mixers, 31, 32 are local oscillators, 41, 42 are 90° phase shifters, 51, 52 are low-pass filters, 6 is a subtraction circuit, 7 is an amplitude limiting circuit, 8 is a frequency discriminator , 9 are output terminals.

Claims (1)

[Claims] 1 a frequency modulated wave as an input signal, and a first signal having a frequency approximately equal to the center frequency of the frequency modulated wave.
a quadrature detector that uses the output of the local oscillator as a local oscillation signal, and first and second low-pass filters that each receive the two outputs of the quadrature detector as inputs;
a second local oscillator, a phase difference separation circuit that branches the output of the second local oscillator into two with a 90° phase difference, and a local oscillator that outputs one of the branched outputs of the phase difference separation circuit. A first frequency converter that converts the frequency of the output of the first low-pass filter as a signal, and a second low-pass filter that uses the other branched output of the phase difference separation circuit as a local oscillation signal. a second frequency converter that frequency converts the output of the first frequency converter, a circuit that adds or subtracts the outputs of the first and second frequency converters, and an amplitude limiting circuit that receives the output of the addition or subtraction circuit; 1. An FM receiver comprising: a frequency detector that receives the output of the amplitude limiting circuit as an input, and receives a signal obtained from the output of the frequency detector as a reception output.