JPS6253081B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement of an FM receiver in which a local oscillation frequency is made to follow the frequency shift of an intermediate frequency signal using a phase lock loop method.

Conventionally, there are various types of receivers that use phase lock loop (hereinafter abbreviated as PLL) control systems to stabilize reception and improve playback characteristics. This is a conventional example of an FM receiver.

In the figure, 1 is an input terminal into which an FM high-frequency signal f1 (frequency is f1) captured by an antenna and amplified by a high-frequency amplifier is input, and 2 is a local signal that is output from the signal f1 and a voltage-controlled local oscillator 10 (described later). A mixer that mixes the oscillation frequency signal f2 (frequency is f2) and outputs an intermediate frequency signal f (frequency is f=f1-f2); 3 is an intermediate frequency filter that determines the passband width of the intermediate frequency signal; 4 is an intermediate frequency filter that determines the passband width of the intermediate frequency signal; An intermediate frequency amplifier (with a limiter); 6 is a phase comparator that outputs the intermediate frequency signal f as a reference frequency signal, which will be explained below, as a voltage fluctuation amount by the phase shift from f ₀ ; 7 is the center frequency of the intermediate frequency filter; ₈ is a low-pass filter, 9 is _a low-frequency amplifier, and 10 is a voltage-controlled local oscillator that is controlled by the demodulated signal demodulated by the phase comparator. , 14 indicate demodulated signal output ends, respectively.

The PLL in this one conventional example is composed of a mixer 2, an intermediate frequency filter 3, an intermediate frequency amplifier 4, a phase comparator 6, a low-pass filter 8, a low-frequency amplifier 9, and a voltage-controlled local oscillator 10, and is configured to generate an FM high-frequency signal. Considering the case where f1 is only a carrier wave, the voltage controlled local oscillator 1 is set so that the frequency of the intermediate frequency signal f and the frequency of the reference frequency signal F0 are the same frequency.
It is designed to control 0. In addition, the output terminal 1 responds to changes in the instantaneous phase of the FM high frequency signal f1.
Considering the output voltage response from 4, the output voltage is
It can be seen that this is the product of the value obtained by differentiating the instantaneous phase of the FM high-frequency signal f1, that is, the value of the FM detection output, and the value of the frequency characteristic of its demodulated output.

In such a configuration, the FM high frequency signal f1
It is known that the higher the modulation signal frequency, the faster the loop response speed, and the larger the frequency deviation, the larger the loop gain. The loop response speed is mainly determined by the intermediate frequency filter 3 and the low pass filter 8, and the rape gain is determined by the phase comparator 6, the low frequency amplifier 9, and the voltage controlled local oscillator 10. (Note that the loop response speed also changes depending on the magnitude of the loop gain.) By the way, FM receivers generally use intermediate frequency filters 3 with sharp attenuation characteristics in order to obtain high selectivity. In this intermediate frequency filter 3, the intermediate frequency signal f undergoes a considerably large phase rotation, so that the PLL becomes unstable and becomes more likely to oscillate. In order to prevent this oscillation, conventionally, the time constant of the low-pass filter 8 is increased to perform phase compensation, or the gain of the low-frequency amplifier 9 is decreased to lower the loop gain.

However, if this phase compensation is performed in the PLL, the loop response speed will drop significantly, so the upper limit of the modulation signal frequency of the FM high-frequency signal will be limited and will decrease, while the loop gain will decrease.
The width of the frequency deviation of the FM high-frequency signal is limited and becomes small. In other words, in the conventional configuration as described above, the condition of improving reception selectivity and the FM
The conditions of expanding the upper limit of the modulation signal frequency of high-frequency signals and expanding the width of its frequency deviation are contradictory to each other.
Receivers were extremely difficult to obtain.

An object of the present invention is to provide an FM receiver that eliminates the conventional drawbacks as described above.

Hereinafter, a first embodiment of the present invention will be explained based on FIG. 2.

In the figure, the same reference numerals as in FIG. 1 indicate the same components, and the explanation thereof will be omitted.

5 is the amplified intermediate frequency signal f (frequency is f=f1-f2) output from the intermediate frequency amplifier 4 by 1/
A 1/n frequency divider that divides the frequency by n to produce a 1/n frequency signal F is shown. In this circuit, the reference frequency oscillator 7 is configured to output a reference frequency signal F ₀ (frequency is F 0 = f 0 /n), and the phase comparator 6 is configured to output a reference frequency signal F 0 (frequency is F ₀ =f ₀ /n), and the phase comparator 6 is configured to output a reference frequency signal F 0 (frequency is F 0 =f 0 /n), and the phase comparator 6 is configured to output a reference frequency signal F _{0 (frequency is F 0} =f 0 /n). This is a configuration for comparing.

In the above configuration, if the intermediate frequency signal f is f=cosωt...(1), then the signal passed through the intermediate frequency filter 3
f' becomes a signal that has undergone phase rotation, such as f'=cos(ωt-θ) (2) θ...the phase rotation determined by the filter 3. Further, this signal f' passes through an intermediate frequency amplifier 4 and is input to a 1/n frequency divider 5 where it is frequency-divided to become a 1/n frequency signal F.

F=cos(ωt/n−θ/n) (3) In other words, the frequency of the intermediate frequency signal f becomes 1/n, and at the same time, the phase rotation also becomes 1/n. Therefore, the phase margin of the PLL increases and the stability also increases.

Here, the phase margin is the phase rotation of −π at the frequency where the loop gain of the open-loop characteristic is zero.
(rad), and if this rotates by more than -π (rad) and the open loop gain is more than 1, the closed loop will oscillate. In order to suppress this oscillation, the phase rotation, the open loop gain, or both should be made as small as possible. (That is, increasing the phase margin.) To do this in an actual circuit, the time constant of the low-pass filter 8 should be made as large as possible and the gain of the low-frequency amplifier 9 etc. should be reduced. If this is done, the loop response speed becomes extremely slow, which is disadvantageous, as explained in the prior art example.

In the case of the circuit of the present invention, the phase margin calculated from equation (3) is the margin of -θ/n with respect to -π, that is, -θ/n-(-π)=π-θ/n. On the other hand, the phase margin in the case of the conventional circuit shown in FIG. 1 (a circuit without phase compensation) can be determined by considering the signal f' that has passed through the intermediate frequency filter 3 and undergone phase rotation ( 2) From formula, −θ−(−π)=π−
becomes θ. Here, since π-θ/n>π-θ can be said, it can be seen that the phase margin of the circuit of the present invention is increased compared to that of the conventional circuit. In other words, a sufficient phase margin can be obtained without performing any particular phase compensation. Therefore, the loop response speed does not decrease at all, and the loop gain only decreases to a small extent in the 1/n frequency divider 5, so it is sufficient to compensate for the decrease using the low frequency amplifier 9 or the like.

Therefore, even if an intermediate frequency filter with a sharp attenuation characteristic is used to improve reception selectivity, sufficient phase margin can be obtained without phase compensation, and the loop response speed and loop gain will not decrease significantly. It will disappear.

Note that the frequency division ratio 1/n of the 1/n frequency divider 5 can be made smaller as the phase rotation θ increases, but the frequency of the signal F after frequency division is the maximum frequency of the modulation signal of the FM high frequency signal f1. It is necessary to decide so that it does not become lower.

FIG. 3 shows a second embodiment of the invention.

In this example, a multiplier 12 and a low-pass filter 13 are provided in the configuration of the first embodiment, and the output signal F of the 1/n frequency divider 5 and the output signal F ₀ of the reference frequency oscillator 7 are multiplied. Although the basic operation is the same as that of the phase comparator of the first embodiment, only the demodulation operation is separated from the phase comparator 6 and a new multiplier 12 is used to perform the demodulation signal. It is.

FIG. 4 shows a third embodiment of the invention.

This example is a modification of the second embodiment, in which the signal f' is multiplied by the signal f ₀ = nF ₀ obtained by multiplying the reference frequency F ₀ by n by the n multiplier 11 by the multiplier 12. , and is used as a demodulated signal through a low-pass filter 13. The basic operation is completely the same as that of the first embodiment.

FIG. 5 shows a fourth embodiment of the invention.

In this example, instead of using the reference frequency oscillator 7 in the first embodiment, the intermediate frequency signal f obtained by the lower heterodyne method and the intermediate frequency signal obtained by the upper heterodyne method are each 1/n. The phase comparator 6 compares the phase of the frequency-divided signal F and the signal. Here, 10' is a voltage controlled local oscillator that outputs a local oscillation frequency signal f3 (frequency is f3), and 2' is
A mixer that mixes the FM high frequency signal f1 and signal f3 and outputs an intermediate frequency signal (frequency = f3 - f1), 3' is an intermediate frequency filter, 4' is an intermediate frequency amplifier (with a limiter), and 5' is an intermediate frequency signal. Each figure shows a 1/n frequency divider that divides the signal into a 1/n frequency signal to produce a 1/n frequency signal. The basic operation is completely the same as that of the first embodiment.

As described in detail above, according to the FM receiver according to the present invention, even if an intermediate frequency filter with a sharp attenuation characteristic is used, sufficient phase margin of the PLL can be obtained without phase compensation, so that the loop response The speed and loop gain will not be significantly reduced, and therefore the upper limit of the modulation signal frequency and the frequency deviation width of the FM high-frequency signal will no longer be restricted. In other words, it has excellent reception selectivity, and has a wide frequency response and dynamic range of reproduced sound.
This makes it possible to realize a PLL type FM receiver. Furthermore, since the configuration is as simple as inserting a frequency divider after the intermediate frequency amplifier, the present invention is easy to manufacture and inexpensive.
FM receivers have many excellent features.

[Brief explanation of the drawing]

FIG. 1 is a circuit block diagram showing a conventional example of a phase-lock loop type FM receiver. 2 to 5 show embodiments of the FM receiver according to the present invention, FIG. 2 is the first embodiment, and FIG. 3 is the second embodiment.
4 is the third embodiment, and FIG. 5 is the fourth embodiment.
3 is a circuit block diagram of each embodiment; FIG. 5, 5'...1/n frequency divider.

Claims (1)

[Claims] 1. A mixer that mixes an FM high frequency signal and a local oscillation frequency signal output from a voltage controlled local oscillator to output an intermediate frequency signal, an intermediate frequency filter that determines the passband width of the intermediate frequency signal, and an intermediate frequency amplifier that amplifies the output of the frequency filter; a phase comparator compares the phases of the intermediate frequency signal outputted from the intermediate frequency amplifier with a reference frequency, etc.; and controls the voltage controlled local oscillator using the output signal thereof; In a phase locked loop FM receiver in which the local oscillation frequency follows the frequency shift of the intermediate frequency signal, by providing a frequency divider between the intermediate frequency amplifier and the phase comparator, 1. An FM receiver characterized in that the frequency-divided intermediate frequency signal and a reference frequency are phase-compared in the phase comparator.