JPS59196629A - Fm receiver - Google Patents

Abstract

PURPOSE:To obtain a direct conversion system FM receiver not requiring a differentiating device and a multiplier by providing two orthogonal detectors taking an output of a local oscillator having a frequency nearly equal to an FM center frequency as a local oscillating signal. CONSTITUTION:An input FM modulating wave is divided into two and they enter mixers 21, 22. An output of the local oscillator 31 having an oscillating frequency nearly equal to the center frequency of the FM modulating wave is divided into two, and they are applied to mixers 41, 42 as signals having a phase difference of 90 deg.. An output of the mixers 21, 22 passes through LPFs 51, 52, it is mixed with the 2nd local oscillating signal by mixers 23, 24 and subtracted by a subtraction circuit 6. The result of subtraction is inputted to a frequency discriminator 8 via an amplitude limiter 7. Then, the FM receiver of the direct conversion system not requiring any differentiating device and multiplier is attained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an FM receiver, and particularly to an improvement of a receiver using a direct conversion method.

The direct conversion method uses a received modulated wave as a local oscillation signal, which is a signal obtained from the output of a local oscillator with a frequency approximately equal to the center frequency, and performs orthogonal detection.In order to directly convert the received modulated wave to the baseband, an intermediate frequency is used. Compared to the heterodyne method, it has the following advantages: it can 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 simply by changing the local oscillation frequency. have. For example, a frequency detector using the direct conversion method is described in the British patent, 15
30602 “Demodulator for FM
Signa! A method such as that shown in s J 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; the output of the second local oscillator is 90 Hz;
a phase difference separation circuit that branches into two with a phase difference of °;
One output of the phase difference separation circuit is used as a local oscillation signal,
a second frequency converter that converts the frequency of the output of the frequency converter; a circuit that adds or subtracts the outputs of the first and second frequency converters; and an amplitude that receives the output of the addition or subtraction circuit as input. The above object can be achieved by having a limiting circuit and a frequency detector which receives the output of the amplitude limiting circuit as an input, and by using the signal obtained from the output of the frequency detector as the 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 the reception input terminal l is split into two and input to mixers 21 and 22, respectively. This FM modulated wave can generally be expressed as follows.

V'r(t) = A cos (ωct 0P(t
) :] (1) Here, A is the amplitude, ωG is the central angular frequency of the modulated wave, here, ωm is a constant indicating the modulation degree,
8(t) is a transmission signal. The local oscillator 31 has an 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 frequency-converted by mixers 21 and 22 using these signals as local oscillation signals, and then
The signal is input to a bandpass filter 57.52. This low-pass filter is used to remove noise and interference waves of different frequencies. 51.52 of low pass filter
The two output baseband signals are respectively expressed as follows.

V, (t) = B cos (P(t)+ΔQ(f
) ) (3) V2 (t) = B sin
(P(t)+ΔQ(t)l (4) Here,
B is a constant proportional to the person, and 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.

Two baseband signals + (t) l V2 (t
) divides the output of the local oscillator 32 with an angular frequency of 1 into two, passes one half through a 90° phase shifter, and uses the obtained signal as a local oscillation signal, which is frequency-converted by the mixers 23 and 24. The resulting signals vs (i) y V4 (t) are respectively expressed as follows, 2 Vs (t) = E cos (P(t) + ΔQ
(t)) cos (ω, 10i) = Committee B cos (ω
it 10P(t)+ΔQCt)+θI)+iB co
s (ωHt −P(t)−ΔQ(t)+θ;1(51
V4(t) = B s in (P(t)+ΔQ(t
)) sin(ωit+θi)=ABcos(ω1t
+P(t)+ΔQ(t)+θ1) (6) Here, θi is a phase constant.

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

v, (t) = VB<t)-・V4 (t)=B
cos (' 7 t + P (t) + ΔQ, (t)
+0 mouth (7) Since this signal is a frequency modulated wave with a center angular frequency ωl, after passing it through the amplitude limiter 7,
By inputting it to the frequency discriminator 8, the received signal V a(t) -A is output at the output terminal 9. −(P(t)+ΔQ(t)+θ1
)i = Ao (5(t) + Q(t) l
(8) t is obtained. Here, Ao is a constant, -Q, and (t) is the difference between the center frequency of the received t modulated wave and the oscillation frequency of the local excavator 31, which is chosen small and can be ignored. The output becomes the transmission signal 5(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 same conventionally required automatic gain control circuit, an amplitude limiter in the intermediate frequency band, which is easier to implement, can be used.

Although the present invention uses an intermediate frequency,
This differs from the conventional heterodyne method, which also uses an intermediate frequency, in that there is no trench or image frequency response problem. In other words, in the conventional hetero gain method, in addition to the original input signal, there is a response to the image frequency, which is a frequency separated from the local oscillation frequency by an intermediate frequency on the opposite side.To avoid this, frequency conversion is 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. Also, in the received modulated wave, IB
Even if the frequency changes greatly, it 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, eliminating the need for a band pass filter in the intermediate frequency band. This is particularly effective since it is difficult to achieve bandpass filtering when the band of the modulated wave becomes narrow. As explained above, the present invention has the advantage of having only the advantages of both the direct connection type FM receiver and the hetero gain type FM receiver.

In the embodiment of the present invention, as a method of performing quadrature detection, the phases of the local oscillation signals are made to differ by 90 degrees, but it is also possible to make the phases of the input modulated waves differ by 90 degrees instead of this. , is well known. Also, the subtraction circuit 6
Even if an adder circuit is used instead of 9, the same effect can be obtained, only the polarity of the detected output changes.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention. In these figures, 1 is an input terminal, 21°22.23. '
24 is a mixer, 31 and 32 are local oscillators, 41 and 42 are 90° phase shifters, 51 and 52 are low-pass filters, 6 is a subtraction circuit, 7 is an amplitude limiting circuit, 8 is a frequency discriminator, and 9 is an output It is a terminal. Figure 1' 姶I

Claims (1)

[Claims] A quadrature detector whose input signal is a frequency modulated wave and whose local oscillation signal is the output of a first local oscillator having a frequency approximately equal to the center frequency of the frequency modulated wave, and two outputs of the quadrature detector are respectively input. first and second low-pass filters, 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; a first frequency converter that converts the output of the first low-pass filter into a frequency by using one branched output of the phase difference separation circuit as a local oscillation signal; a second frequency converter that frequency converts the output of the second low-pass filter using the output as a local oscillation signal; a circuit that adds or subtracts the outputs of the first and second frequency converters; or has an amplitude limiting circuit that receives the output of the subtraction circuit as input, and a frequency detector that receives the output of the amplitude limiting circuit as input;
An FM receiver characterized in that a signal obtained from the output of the frequency detector is used as a receiving output.