JPH0998071A - Method and circuit for automatic frequency control - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the AFC performance which controls the local oscillation signal on the reception side. SOLUTION: Two auxiliary signals outside the frequency band of a modulated signal are transmitted together with the modulated signal synchronously with the carrier on the transmission side. On the reception side, two auxiliary signals are taken out from the output of a frequency detector 5 by band-pass filters 13 and 14, and a signal (c) having the frequency difference is generated by a frequency mixer 15 and a band-pass filter 16. This signal (c) is used as the reference signal of a phase comparator 8 to the time of AFC operation to perform such control that a local oscillation signal (a) always has an accurate frequency.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an automatic frequency control method and its circuit, and more particularly to an automatic frequency control method and its circuit for accurately controlling the output frequency of a receiving side local oscillation circuit.

[0002]

2. Description of the Related Art FIG. 3 is a block diagram showing a circuit example of an FM receiver including a conventional automatic frequency control circuit (hereinafter referred to as an AFC circuit), which is a synthesizer type single superheterodyne type receiver. In the figure, the switch SW1 is an AFC (Automatic Frequency Control)
l) Switch between operation and normal reception operation. First, during normal reception operation, switch SW
1 is in the illustrated state. At this time, the reception signal received by the antenna is amplified by the high frequency amplifier 1, then mixed with the local oscillation signal a by the frequency mixer 2 and converted into an intermediate frequency signal (frequency f ₀ ). This intermediate frequency signal has its image component removed by a band pass filter 3, is amplified by an intermediate frequency amplifier 4, is detected by a frequency detector 5, and a demodulated signal is taken out.

Here, a VCO (Voltage Controlled Oscil)
The local oscillator signal a output from the (lator) 6 needs to have a frequency that is separated from the carrier frequency of the received signal by f ₀ . Therefore, a PLL (Phase Lock) including a VCO 6, a 1 / N frequency divider 7, a phase comparator 8, a charge pump 11 and the like.
ed Loop) controls the output frequency of the VCO 6. That is, the 6.875 MHz oscillation output from the crystal oscillator 9 is converted into a 2.5 kHz reference signal by the 1/2750 frequency divider 10 and input to one terminal of the phase comparator 8. The output of VCO 6 is also 1 / N by 1 / N frequency divider 7.
Although the frequency is divided, the frequency division ratio N is adjusted so that the output frequency of the 1 / N frequency divider 7 becomes 2.5 kHz, and is also input to the other terminal of the phase comparator 8. Here, the frequency division ratio N of the 1 / N frequency divider 7 is set by a system (not shown) according to the channel frequency (25 kHz interval) of the received radio signal. The phase comparator 8 outputs the UP pulse when the phase of the signal from the VCO is behind the reference signal, that is, when the frequency is low, and outputs the DOWN pulse in the opposite case. These are input to the charge pump 11 via the switch SW1. Charge pump 11 is UP
When a pulse is applied, the internal capacitor is charged with a constant charge to increase its output voltage, and when a DOWN pulse is applied, the constant charge is discharged to decrease the output voltage. Therefore, assuming that the VCO 6 has a characteristic that its output frequency increases as the output voltage of the charge pump 11 increases, the output frequency of the VCO 6 is about the same as that of the crystal oscillator 9 with reference to the reference signal from the crystal oscillator 9. It is possible to output a local oscillation signal with a stability of.

Next, the AFC operation will be described. This is performed so that the intermediate frequency becomes f ₀ when the frequency difference between the carrier frequency on the transmission side and the local oscillation signal a of the VCO 6 deviates from the intermediate frequency f ₀ by a certain amount or more. During this AFC operation, SW1 is switched to the position opposite to the position shown in FIG. 3, and the pulse signal from the pulse conversion circuit 13 is input to the charge pump 11.
The output of the frequency detector 5 is input to the comparator 12. The frequency detector 5 has an output voltage of 0 when the input frequency is f _0, a positive voltage when the input frequency is higher than f ₀ , and a frequency higher than f ₀ . Outputs negative voltage when low. On the other hand, the comparator 12 outputs the DOWN signal when the input voltage is positive.
When it is negative, the UP signal is output. Further, the pulse conversion circuit 13 converts the DOWN signal and the UP signal into the same DOWN pulse and UP pulse as the output of the phase comparator 8, respectively. Therefore, as in the normal operation described above, V
The output frequency of the CO 6 is controlled so that the output of the frequency detector 5 becomes 0, that is, the intermediate frequency becomes f ₀ .

[0005]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
In the circuit, when the received signal level is low, the output voltage of the frequency detector 5 drops and it becomes impossible to obtain a sufficient control gain. Therefore, it must be used when the received signal level is high to some extent, or there is the following problem even when the received signal level is high to some extent. That is, the received signal is normally modulated by voice or data by some method. Therefore, even if the frequencies on both the transmitting and receiving sides have a predetermined difference, the output voltage of the frequency detector 5 temporally changes around OV. In order to eliminate this fluctuation and form a stable AFC circuit, for example, the time constant of the charge pump 11 must be increased. But charge pump 1
When the time constant of 1 is increased, there is a problem that the responsiveness when the switch SW1 is switched to the AFC operation deteriorates, and it takes time until the frequency control by the AFC takes effect.

An object of the present invention is to provide an automatic frequency control capable of obtaining a stable local signal even when the demodulation output varies with time when receiving a modulated signal and capable of high-speed response when switching to AFC operation. A method and its circuit are provided.

[0007]

SUMMARY OF THE INVENTION The present invention is a system in which a transmitting side modulates a carrier wave with a modulating signal and transmits the modulated signal, and a receiving side converts a received signal into an intermediate frequency signal by a local oscillation signal and then demodulates it. In the automatic frequency control method of the local oscillation signal in, on the transmission side, by dividing or multiplying the output of the signal source of the transmission side carrier or a signal obtained by multiplying or dividing the signal source output, Two sub-signals that are not in the frequency band of the modulation signal are generated, the carrier wave is modulated by the combined signal of the two sub-signals and the modulation signal and transmitted, and on the reception side, the sub-signal is transmitted together with the modulation signal and demodulated. The automatic frequency control is characterized in that the two sub-signals are extracted, a signal having a difference frequency between the two sub-signals is generated, and the frequency of the local oscillation signal is controlled by the generated signal. How to disclose.

[0008]

Embodiments of the present invention will be described below. 1 and 2 are block diagrams showing examples of a receiver circuit and a transmitter circuit including an AFC circuit according to the present invention,
The same reference numerals as those in FIG. 3 in FIG. 1 denote the same circuits. Further, the characteristic part of the present invention is the part surrounded by the dotted line in both FIGS.

In the transmitter circuit shown in FIG. 2, a modulated signal such as voice or data is amplified by an amplifier 21 and FM
The modulator 22 performs FM modulation on the transmission carrier wave b. The carrier wave b at this time is generated by dividing the 6.875 MHz signal from the crystal oscillator 28 by the 1/2750 frequency divider 29.
It is generated from a PLL configured by a VCO 25, a 1 / N frequency divider 26, a phase comparator 27, and a charge pump 30 with a signal of 5 kHz as a reference. This PLL operation is similar to that described in FIG. The FM-modulated signal of the transmission frequency has its image component removed by the low-pass filter 23, is amplified by the amplifier 24, and is transmitted.

The first frequency divider 31 and the second frequency divider 32, which characterize the present invention, output the output signal of the crystal oscillator 28 to 1/27.
Divided into 500 and 1/2500, 250Hz each
, And 2750 Hz sub-signals are generated, added, and added to the modulation signal at the output point of the amplifier 21. A normal voice signal has a frequency band of 300 to 2700 Hz in a telephone or the like, and assuming that data is also used as a signal within this band, the above sub-signal exists outside both ends of this modulation signal band, and these are modulated. Adding to the signal does not hinder it.

In the receiver circuit of FIG. 1, when the switch SW is in the position shown in the figure, the configuration is almost the same as that of the conventional FIG. That is, with reference to the signal of the crystal oscillator 9, the VCO 6, 1
The local oscillation signal a is output from the PLL including the / N frequency divider 7, the phase comparator 8 and the charge pump 11, whereby the received signal is converted into an intermediate frequency signal and then detected by the frequency detector 5. However, since this detection output contains the above-mentioned sub-signal in addition to the modulated signal, a band-pass filter 12 for removing this sub-signal is provided, and the signal of this filter 12 serves as the demodulation output. Become.

On the other hand, the output of the frequency detector 5 is also led to the dotted line portion, which is a feature of the present invention, in which the two band pass filters 13 and 14 are used for the above-mentioned 2750 Hz and 2
The sub-signal of 50 Hz is taken out, but if the difference between the carrier frequency on the transmission side and the local oscillation signal a is δ than the predetermined intermediate frequency,
If it is deviated by only 275, the detected side signals will be 275 each.
It deviates from 0 ± δ (Hz) and 250 ± δ (Hz) (decoding same order). Then, when the difference frequency signal between these two sub-signals is taken out by the frequency mixer 15 and the bandpass filter 16, the above transmission / reception deviation δ is canceled out and always 2
A signal c of 750-250 = 2500 Hz is obtained. However, in this operation, the frequency band of the band pass filters 13 and 14 is limited to ± 50 Hz because it is necessary to block the modulation signal. Therefore, the relative error of the transmission / reception frequency that may actually occur also needs to be within ± 50 Hz in this case, but this condition is satisfied.

As described above, the receiving side obtains the frequency signal c which is equal to the frequency difference of 2.5 kHz between the two sub-signals on the transmitting side. This signal c always has the same frequency as the 2.5 kHz signal input to the phase comparator 27 of FIG. 2 regardless of the value of the relative error δ of the transmission / reception frequency. Therefore, when performing the AFC operation, if the switch SW is connected in the opposite direction to the one shown in the figure, the band pass filter 16
Output to the output phase comparator 8 and is controlled by the PLL so that the local oscillation signal a maintains an accurate frequency. Moreover, in this method, since the level fluctuation due to the modulation signal is irrelevant, it is not necessary to particularly increase the time constant of the charge pump 11.

In the above description, the embodiment of the present invention has been described for the FM demodulation type transmitter / receiver, but it is obvious that the present invention can be applied to the SSB type and the like. Further, although the modulated signal is a telephone band signal, it is apparent that the modulation signal can be applied to other band signals.

[0015]

According to the present invention, it is possible to realize an AFC circuit which is always stable, accurate, and has a good responsiveness at the time of switching, without being affected by the level fluctuation of the modulation signal.

[Brief description of drawings]

FIG. 1 is a block diagram of an FM receiver circuit including an AFC circuit according to the present invention.

FIG. 2 is a block diagram of an FM transmitter circuit including an AFC circuit according to the present invention.

FIG. 3 is a conventional AFC circuit (hereinafter the same as FIG. 1).

[Explanation of symbols]

 13, 14, 16, 31, 32 Band pass filter 15 Frequency mixer

Claims (2)

[Claims] 1. An automatic frequency of the local oscillator signal in a system, wherein a transmitting side modulates a carrier wave with a modulation signal and transmits the modulated signal, and a receiving side converts the received signal into an intermediate frequency signal and then demodulates it. A control method, wherein the transmitting side divides or multiplies a signal source output of the transmission side carrier or a signal obtained by multiplying or dividing the signal source output so that it is not in the frequency band of the modulated signal. Two sub-signals are generated, the carrier wave is modulated by a combined signal of the two sub-signals and a modulation signal and transmitted, and on the receiving side, two sub-signals transmitted and demodulated together with the modulation signal are taken out, An automatic frequency control method, wherein a signal having a difference frequency between the two sub-signals is generated, and the frequency of the local oscillation signal is controlled by the generated signal. 2. A sub-signal that is not in the frequency band of the modulated signal is generated by dividing or multiplying the output of the signal source of the carrier wave on the transmission side or the signal obtained by multiplying or dividing the output of the signal source. Then, the sub-signal generating means which is configured to add the two sub-signals to the modulation signal and is installed on the transmission side, and the two sub-signals which are transmitted together with the modulation signal and demodulated are taken out. An automatic frequency generating means for generating a signal having a difference frequency, the reference signal generating means being arranged on the receiving side so as to control the frequency of the local oscillation signal by the generated signal; Control circuit.