JPS62216519A - Automatic frequency control circuit - Google Patents

Abstract

PURPOSE:To avoid a trouble increasing excessively a loop time constant by extracting only a DC offset formed by eliminating a digital signal component from a reception signal E and using the value as a control voltage of an oscillation frequency of a local oscillator. CONSTITUTION:The reception signal E is inputted to a sample-holding circuit 7 through an LPF 6, where the signal is subjected to sample-holding by using a sampling pulse S. An identification circuit 8 generates two-bit binary signals D1, D2, and a multi-value signal generation circuit 9 generates reference 4-value voltages V21, V22, V23, V24. The difference between voltage V1n and V2n becomes a signal x3 by a subtraction circuit 10, fed to a control input of an oscillation frequency of a local oscillator 13 and a negative feedback loop to decrease the difference signal x3 is formed. It is not required to eliminate a base band component of a digital signal as a conventional circuit, the cut-off frequency is far increased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an automatic frequency control (AFC) circuit for a local oscillator in a digital radio.

[Conventional technology]

Generally no digital &! In communications, the center frequency of the IF multiplied signal changes due to a shift in the center frequency at the transmitter or a shift in the oscillation frequency of the local oscillator at the receiver, which increases the error rate of the IF multiplied signal. To increase.

In other words, in order to obtain the same error rate, a higher reception input is required, resulting in a decrease in sensitivity.

To prevent this, conventional techniques include using a highly stable oscillator as a signal source for the transmitter and as a local oscillator for the receiver, and controlling the oscillation frequency of the local oscillator by changing the DC component of the discriminator output. A so-called AFC that applies feedback to keep the center frequency of the IF multiplied signal constant.
Methods using the L route were rare.

[Problem that the invention seeks to solve]

The problems of the method using the highly stable oscillator and the method of taking out the DC component of the discriminator output and multiplying it by g will be explained.

First, the former method of using a highly stable oscillator has problems especially when the frequency becomes high. Now, assuming that the stability of the oscillator is ±3 ppm, the amount of change at the radio frequency of 900 MHz is at most ±2.7 KHz.

Here, when transmitting a 10KHz digital signal, I
The passband width of the F filter is about the same <10 KHz, and the above value cannot be ignored, resulting in an increase in the error rate. However, it is difficult to obtain an oscillator with a stability of about 1 ppm, which requires advanced stabilization technology and aging, which increases cost and mounting space.

Next, problems with the latter method of extracting the DC component from the discriminator output will be explained.

In general, digital signals have baseband components that are effective up to near DC due to their nature. In order to remove this component and extract only the DC component, it is necessary to use a low-pass filter with a very low cutoff frequency. However, if a low-pass filter with a low cutoff frequency is inserted into the negative feedback loop, the loop time constant will be long and the IF
As a result, it takes several seconds to track the frequency. This is clearly not an appropriate method considering that a single reception time in a short conversation lasts only a few seconds.

[Means for solving problems]

The AFC circuit of the present invention is configured to control the oscillation frequency of the local oscillator so as to keep the center frequency of the IP' signal constant, and includes first means for sampling the eye pattern of the received signal at the center point thereof. , a second means for generating a multilevel signal based on the eye pattern of the received signal or the output of the @i means for each CM, and the outputs of the first means and the second means are and a third means for outputting the respective differences between sections where the same information is transmitted, and the oscillation frequency of the local oscillator is controlled by the output of the third means.

Another aspect of the present invention is the above-mentioned device, further comprising a fourth means for supplying the output of the third means only during a period when a signal is being received at the oscillation frequency control input terminal of the local oscillator. I'm going to make it happen.

[Effect]

In the present invention, only the DC offset obtained by removing the digital signal component from the received signal is extracted, and this value is used as the control voltage for the oscillation frequency of the local oscillator.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail based on the drawings.

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

In the figure, 1 is a high frequency filter and a high frequency amplifier;
2 is a mixer that mixes the output of this high frequency filter and high frequency amplifier 1 with the local signal oscillated by the local oscillator 13 and converts it into an IP signal, and 3 is an IF that receives the output of this mixer 2 as input. Filter, 4 is this IF
IF amplifier connected to the output side of filter 3; 5 is a discriminator whose input is the output of this IF amplifier 4;
6 is a low-pass filter (LPF) which inputs the output of this discriminator 5; 7 is a sample-and-hold circuit that samples and holds the signal from this LPF 6; this sample-and-hold circuit T sets the eye pattern of the received signal at its center; It constitutes a sampling means for sampling at the point in time.

Reference numeral 8 designates an identification circuit that receives the output of this sample and hold circuit T as an input and generates a 2-bit 200 million signal, and 9 represents a multi-value signal generation circuit that receives the output of this identification circuit 8 as an input and generates a four-value voltage as a reference. , these constitute means for generating a multilevel signal based on the eye pattern of the received signal or the result identified from the output of the sampling means.

Reference numeral 10 denotes a subtraction circuit which inputs the output from the sample hold circuit T and the output from the multi-value signal generation circuit 9 and obtains the difference between the two outputs. Difference output means is configured to output differences between sections whose outputs represent the same information. 11 is an LPF which inputs the difference signal from this subtraction circuit 10, 12 is a circuit connected to the output side of this LPFl 1, and this circuit 12Fi is a terminal to which a signal indicating that a signal is being received is inputted. It is configured to be controlled by a signal from 14. 1ft this circuit 1
This is a local oscillator whose oscillation frequency is controlled by the output of 2. The circuit 12 constitutes means for supplying the output of the difference output means only during the period when a signal is being received at the oscillation frequency control input terminal of the local oscillator 13.

Next, the operation of the embodiment shown in FIG. 1 will be explained with reference to FIG. 2. This FIG. 2 is an operation explanatory diagram showing the operating waveforms of each part of FIG. 1, where S indicates a sampling pulse and E indicates a received signal. And x
l indicates the east line, x3 indicates a broken line, and X indicates a difference signal. In addition, in this FIG. 2, an example of 4 m is shown as a multilevel signal.

First, a received wave captured by an antenna passes through a high frequency filter and a high frequency amplifier 1, and is converted into an IP multiplied signal by a local signal emitted by a local oscillator 13 and mixers 2 and K, and the IP multiplied signal is converted into an IP multiplied signal by an IF filter 3 and an I
After passing through the F amplifier 4, it becomes a baseband reception signal E at a discriminator 5.

This received signal E is input to the sample and hold circuit T through the LPF 6, and sampled and held by the sampling pulse S. Here, the timing of this sampling pulse S is selected at the time when the eye pattern of the received signal E is open.

For this reason, the output of the sample-and-hold circuit 7 has a rectangular shape like the real 1ix□ shown in FIG. And this second
Each voltage shown in the figure v11, v12, V1llv14
are J + '4 + E3 of the above received signal E, respectively.
The value is a sample of +E4.

On the other hand, the identification circuit 8 outputs a 2-bit binary signal D1. D
, is generated, and in the multi-value signal generation circuit 9, the reference four-value voltage v2□+ vll 11 Ivm8 , v2
Generates 4.

Kokote, this four-value voltage vl I Ivm 11 +”1
1 B lv24 is each voltage v1 when the IF multiplied signal center frequency is equal to the design value.

It is equal to V engineering m1v181v14. The broken line x2 in FIG. 2 shows this waveform.

Note that in FIG. 2, the points at which the voltage changes are slightly shifted. This may be due to the identification circuit 8 and the multi-value signal generation circuit 9, but the identification circuit 8 and the multi-value signal generation circuit 9 do not include a large time constant and can be ignored compared to the sampling interval. Therefore, in reality, it can be regarded as simultaneous.

Therefore, the difference between the voltage vln and the voltage V□ is applied to the difference signal X by the subtraction circuit 10 to the oscillation frequency control input terminal of the local oscillator 13, thereby reducing the voltage of the difference signal x8. A negative feedback loop is formed.

A low-pass filter (LPF) 11 is inserted to an extent that does not increase the time constant of the loop. In this case, there is no need to remove the baseband component of the digital signal, which was a problem when directly converting the output of the discriminator 5 to direct current, as in the past, so it is sufficient to simply suppress the noise. , the cutoff frequency can be made higher than that of the former.

Next, in the embodiment shown in FIG.
An actual case of C circuit is also shown.

That is, when there is no received signal, a large noise voltage appears in the difference signal x3. Therefore, there is a possibility that the local oscillation frequency becomes unstable when there is no signal and during transmission. In order to avoid this, the negative feedback loop is activated only during the period when the signal is being received, and the voltage remains constant during other periods.
For example, it is conceivable to fix the control voltage to ground level.

A circuit 12 shown in FIG. 1 is an example of such a means, and a signal is applied to a terminal 14 indicating that a signal is being received. Furthermore, the value in the last signal reception period may be held during periods other than the signal reception period.

Moreover, although this FIG. 1 uses a single super receiver as an example to simplify the explanation, the application of the present invention is not limited to this, and the application of the present invention is equally applicable to a double super receiver. In this case, the feedback may be applied to either the first station or the second station.

〔Effect of the invention〕

As explained above, according to the present invention, only the DC offset obtained by removing the digital signal component from the received signal E is extracted, and this value is used as the control voltage for the oscillation frequency of the local oscillator. Problems such as an extremely large time constant can be avoided, and there is no cost problem since an expensive crystal oscillator or aging is not required.

As described above, the AFC circuit of the present invention is significantly superior to conventional circuits of this type in terms of both performance and cost, and is unique as a local oscillator circuit for digital radio equipment.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention, and FIG. 2 is an operation explanatory diagram showing operation waveforms of each part of FIG. 1. 5...Discriminator, T...Sample hold circuit, 8...Discrimination circuit, 9...Multi-value signal generation circuit, 10...Subtraction circuit, 12... circuit,
13...Local oscillator.

Claims (2)

[Claims] (1) In an automatic frequency control circuit that controls the oscillation frequency of a local oscillator so as to keep the center frequency of an intermediate frequency signal constant, a first means for sampling an eye pattern of a received signal at its center point; a second means for generating a multilevel signal based on the result identified from the eye pattern of the signal or the output of the first means; and the outputs of the first means and the second means contain the same information. an automatic frequency control circuit comprising: third means for outputting respective differences for the sections represented; and an oscillation frequency of the local oscillator is controlled by the output of the third means. (2) In an automatic frequency control circuit that controls the oscillation frequency of a local oscillator so as to keep the center frequency of an intermediate frequency signal constant, a first means for sampling an eye pattern of a received signal at its center; a second means for generating a multilevel signal based on the result identified from the eye pattern of the signal or the output of the first means; and the outputs of the first means and the second means contain the same information. third means for outputting respective differences for the sections represented, the oscillation frequency of the local oscillator is controlled by the output of the third means, and the control input for the oscillation frequency of the local oscillator is provided. An automatic frequency control circuit comprising: fourth means for supplying the output of the third means only during a period when a signal is being received at the end of the automatic frequency control circuit.