JPH05114921A - Modulation characteristic correction circuit - Google Patents

Abstract

PURPOSE:To automatically adjust also a correction circuit by means of modulation degree adjustment by controlling the variable capacity diode of a phase synchronization loop by means of the output of a potentiometer provided at the parallel arm of a lag-lead type low-pass filter. CONSTITUTION:By using a lag-lead filter as a correction circuit network 11, transmission function G2(s) of this circuit changes R2 and R3 of a potentiometer to increase an attenuation amount and at the same time, a corner frequency determined by 1/R3C is also increased. For this effect, by adjusting the modulation degree by means of the adjustment of the R2 and R3 corresponding to the dispersion of a Kv value, a total frequency characteristic can be reduced to 1/(R1+R2+R3)C. Accordingly, the original reduction cut-off frequency of the phase synchronization loop can be greatly and equally reduced by means of the correction circuit network, but direct current cannot be completely transmitted. However, the complete transmittion of a DC component is not necessary and it is sufficient as long as the frequency corresponding to a block period can be transmitted. Thus, by adjusting the reduction cut-off frequency to the frequency corresponding to the block period, the equipment can be used without problem.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a signal such as a telephone signal which does not contain a direct current component (the frequency component of which is approximately 300 Hz).
Modulation characteristic correction circuit network for making it possible to apply the PLL modulation method of directly frequency-modulating a voltage-controlled oscillator of a phase-locked loop (PLL) used for frequency modulation in the range of 3400 Hz) to digital signal transmission. It is about.

[0002]

2. Description of the Related Art There are roughly three methods for directly frequency-modulating a carrier wave with a digital signal (direct frequency shift keying, abbreviated as direct FSK method), and each method has advantages and disadvantages.

A method of multiplying the output of a phase modulator Also called an Armstrong method, the frequency-stable carrier wave obtained by a crystal oscillator or the like is phase-modulated by a signal obtained by integrating a modulation signal to equivalently perform frequency modulation. Although a signal is obtained, a large degree of modulation cannot be obtained, so a frequency multiplier of about several hundred times is required. Therefore, frequency accuracy and stability are good, but it is difficult to miniaturize the device because a frequency multiplier is required.

Method of Directly Converting FSK Frequency in Intermediate Frequency Section Modulation degree can be easily adjusted, and the intermediate frequency is low enough to affect the accuracy of the transmission frequency (transmission frequency is several hundred MHz).
If so, it is a method of reactance-modulating an oscillator of, for example, several hundreds of kHz to several MHz and converting the frequency into a required transmission frequency. This is a method that has been used for a long time together with the method of (1), and is still widely used today, but it requires an intermediate frequency section and a frequency conversion section, so it is not as good as the method of (3), but there is a limit for downsizing. , The corresponding adjustment required parts remain.

Method of Directly Modulating Transmission Frequency by Utilizing Modulator for Cordless Telephone This method is widely used in recent years. Since the transmission frequency is directly modulated, the circuit configuration is very simple and small. Suitable for However, this method requires that the conditions described below be satisfied. Figure 4
Is the circuit configuration, and the reason will be described using this.

In FIG. 4, reference numeral 1 denotes a modulation signal input terminal, 2
Is an integrator (the transfer function is K ₀ / S, 3 is a loop filter (here, a lag lead filter, and the transfer function is (S +
a) / (S + e)], 4 and 5 are synthesis circuits, 6 is a voltage controlled oscillator (transfer function is K _v / S), and 7 is a frequency divider (1
/ N frequency division), 8 a reference signal generator, 9 a phase detector (transfer function -K _D), 10 is a signal output terminal. 7,
8 and 9 are PLL (phase locked loop) synthesizer LSIs
It has been integrated and used extensively. In FIG. 4, only the integrator is added for digital signal transmission, and the other circuits are all used for telephone signals.

The input voltage vs. output signal frequency deviation of this circuit is expressed as a transfer function G (s) as follows.
(S is a Laplace transformer)

[Equation 1] Therefore, in order to transmit the digital signal, in the entire frequency range of the spectrum have of the digital signal, it is necessary to _{K 0 = K V K D /} N is established. A variable-capacitance diode is often used for the voltage controlled oscillator (VCO) 6 because of its simple circuit configuration, but since the rate of change in capacitance varies slightly among the diodes, the value of K _V varies accordingly. It will vary. Therefore, it is necessary to adjust the input signal level according to the value of K _V to match the modulation degree and to adjust the gain coefficient K ₀ of the integrator 2 so that K ₀ = K _V K _D / N holds. is there.

[0008]

As mentioned above, the PLL is used.
When an integrator is added to the synthesizer so that it can respond to a direct current component as well, the sensitivity of the integrator must be adjusted to match the sensitivity of the VCO. This adjustment is absolutely necessary because the VCO sensitivity adjustment must be adjusted to the required modulation factor for each variable capacitance diode.

An object of the present invention is to provide a modulation characteristic correction circuit in which the correction circuit is automatically adjusted at the same time by this modulation degree adjustment.

[0010]

In order to achieve this object, a modulation characteristic correction circuit of the present invention controls a variable capacitance diode in a phase locked loop using a voltage oscillator with a variable capacitance diode to directly perform frequency modulation. When doing
A lag lead type low-pass filter is provided on the phase lock loop signal input side in order to compensate for variations in cutoff frequency and modulation degree due to variation in modulation rate at low frequencies and variations in applied voltage to capacitance of the variable capacitance diode. The variable capacitance diode of the phase locked loop is controlled by the output of a potentiometer provided on the parallel arm of the lag lead type low pass filter.

First, the principle of the present invention will be described in detail. FIG. 1 is a circuit configuration diagram thereof, 11 is a correction circuit network having a transfer function as described later, and other components are shown in FIG.
Is exactly the same as. Transfer function (input voltage vs. output frequency deviation) G of the portion excluding the correction network 11 of FIG.
₁ (s) is the PLL modulator itself

[Equation 2] And the modulation frequency characteristic when S = jω is
Dumping factor

[Equation 3] There are various shapes depending on the value of, but when ξ <1, a peak generally occurs and it is not preferable in terms of transmission characteristics. Therefore, when G ₁ (s) is deformed for ξ> 1, approximately

[Equation 4] It can be seen that it has the form of a first-order high-pass filter.

Therefore, if a lag-lead filter as shown in FIG. 2 is used as the correction network 11 of FIG. 1, the transfer function G ₂ (s) of this circuit becomes

[Equation 5] However, when R ₂ and R ₃ of the potentiometer are changed to increase the attenuation amount, the corner frequency determined by 1 / R ₃ C also increases at the same time.

In the configuration of FIG. 1, this effect is (3)
Since it acts as the product of the equation and the equation (4), if the modulation degree is adjusted by adjusting R ₂ and R ₃ according to the variation in the value of K _V , the total frequency characteristic is 1 / (R ₁ + R ₂ + R ₃ ) The frequency has been lowered to the frequency of C. This pattern is shown in Figure 3.
Shown in.

In FIG. 3, A is the modulation frequency characteristic of the PLL expressed by the equation (1), B is the frequency characteristic of the correction circuit network 11 expressed by the equation (4), and the characteristics of the entire circuit of FIG. A + B has the characteristic of C. Reference character a indicates a modulation frequency characteristic when the modulation sensitivity K _V is slightly small, and the cutoff frequency decreases as the frequency shift amount decreases. b shows the characteristic when the attenuation in the correction network 11 is reduced in order to correct the decrease in the frequency shift amount, and the corner frequency is lowered at the same time as the attenuation is reduced, and as a result, the total frequency is reduced. The characteristic can be maintained at c.

By using the present invention, the original low-frequency cut-off frequency of the phase-locked loop can be reduced equivalently by the correction network, but it cannot be completely transmitted to direct current. is there. However, in the actual transmission state, most of the code transmission is a block (packet) configuration in which a transmission error is checked every several hundred bytes and is retransmitted depending on the presence or absence of an error, or the next block is transmitted.
It is not necessary to consider the transmission of the complete DC component, and it is sufficient that the frequency corresponding to the cycle of the block can be transmitted. Therefore, if the low cutoff frequency of the present invention is adjusted to this, it can be used without any problem.

[0016]

As described above in detail, in the FSK method, the method of directly frequency modulating the voltage controlled oscillator of the PLL synthesizer can directly modulate the transmission output frequency itself, and thus constitutes an extremely simple frequency modulator. be able to. However, since the PLL itself is synchronized with the low frequency component containing the DC component in the modulated signal, the telephone signal modulator cannot directly transmit the data signal containing the DC component. Although some kind of correction circuit is required to transmit the data signal, this correction circuit must match the crossover frequency with the PLL as an essential condition for waveform transmission. But the actual PLL
In many cases, a variable capacitance diode is commonly used for the modulation and the PLL. Therefore, if the modulation sensitivity is low, the low-frequency cutoff frequency of the PLL also becomes low.

According to the present invention, by using a potentiometer as part of the correction circuit, the crossover frequency with the PLL can be automatically adjusted all at once by adjusting the modulation factor, and thus any other adjustment is required. Since it is extremely easy to adjust, it is suitable for mass production.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a circuit diagram showing an example of a correction circuit network used in the embodiment of FIG.

FIG. 3 is an operation characteristic diagram for explaining the operation of the present invention.

FIG. 4 is a block diagram showing an example of a conventional modulation characteristic correction circuit.

[Explanation of symbols]

 1 modulation signal input terminal 2 integrator 3 loop filter 4,5 signal synthesis circuit 6 voltage controlled oscillator 7 frequency divider 8 reference signal generator 9 phase detector 10 signal output terminal 11 correction network

Claims (1)

[Claims] 1. When the variable capacitance diode is controlled in a phase locked loop using a voltage oscillator with a variable capacitance diode to directly perform frequency modulation, a decrease in modulation degree at a low frequency and an applied voltage to the variable capacitance diode. In order to compensate for variations in cutoff frequency and modulation due to variations in capacitance change rate, a lag lead type low-pass filter is provided on the phase lock loop signal input side, and the output of a potentiometer provided in the parallel arm of the lag lead type low-pass filter. The modulation characteristic correction circuit is configured to control the variable capacitance diode of the phase locked loop according to the above.