JPH0786983A - Afc initial pull-in circuit for demodulator in direct spread spectrum communication system - Google Patents

Abstract

PURPOSE:To shorten a time required for initial pull-in by differentiating the output correlation value of a matched filter with the frequency in digital mode, and detecting the change point of the sign to set it as the maximum value of the correlation value. CONSTITUTION:The correlation value of the output of the matched filter 500 which performs inverse spread of spectrum by finding the correlation between the output signal of a frequency conversion part 100 and a built-in spread code pattern is differentiated with respect to the frequency by a maximum value detecting means 110, and the maximum value of the correlation value is found by detecting the change point of the sign in the differentiated result. The sweep of a sweeper 800 is stopped at a time when the maximum value of the correlation value is found, and the output frequency of a local oscaillator 200 is fixed, and an original signal outputted from the matched filter 500. Therefore, it is possible to suppress the effect of a noise to a minimum and to shorten a time required for the initial pull-in by performing frequency sweep once,.

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 initial pull-in circuit (hereinafter referred to as an AFC initial pull-in circuit) of a demodulator in a spread spectrum communication system.

[0002]

2. Description of the Related Art FIG. 8 shows an output v of an example of a matched filter.
It is a s frequency characteristic figure. FIG. 9 is a block diagram of an AFC initial pull-in circuit of a demodulator in a conventional direct spread spectrum communication system.

In FIG. 9, a mixer 1 includes a reception spread spectrum signal (SS reception signal) converted from a high frequency signal to an intermediate frequency signal (for example, IF = 70 MHz) and a voltage controlled oscillator (hereinafter referred to as VCO) 2. An output (eg, 70 MHz) is added to output a baseband SS signal. LP of this baseband spread spectrum signal
By passing through F3, the harmonic component and aliasing noise (aliasing noise) generated in the mixer 1 are cut.

The output of the LPF 3 is A / D-converted by sampling at a predetermined clock frequency in the A / D converter 4, and becomes, for example, an 8-bit digitized reception spread spectrum signal. The digitized received spread spectrum signal is input to the matched filter 5, and the spectrum is inversely spread by calculating the correlation between the input signal and the built-in spread code pattern, and the correlation value is output.

For example, the output of the sweeper 8 composed of a counter is converted into an analog voltage through a D / A converter 9 and V
In addition to CO2, the output voltage of the sweeper 8 is sequentially increased within a fixed range to sweep the frequency, so that the matched filter 5 obtains a correlation value vs frequency characteristic as shown in FIG. The maximum value detector 6 finds the maximum value of this correlation value, and the sweep is stopped assuming that the initial pull-in of the AFC is completed at the output frequency (fc) of the VCO 2 when the maximum value of the correlation value is obtained, and the matched filter 5 To output demodulated data.

Generally, in the AFC initial pull-in circuit as described above, the maximum value detector detects the maximum value of the correlation value. Therefore, considering the influence of noise, in order to stop the sweep at the center frequency to be pulled in, the frequency sweep is performed. Had to be reciprocated 2-3 times.

[0007]

As described above, in the prior art, the frequency sweep must be reciprocated 2-3 times in consideration of the influence of noise, and it takes time to complete the initial pull-in of the AFC. There was a problem.

The present invention minimizes the effects of noise,
It is an object of the present invention to provide an AFC initial pull-in circuit that shortens the time required for the initial pull-in of AFC by performing such frequency sweep only once and prevents pseudo pull-in due to noise or the like.

[0009]

[Problems to be Solved by the Invention] The above problems are caused by the problems shown in FIGS.
This is solved by the circuit configuration shown in. In FIG. 1 (Claim 1), in the AFC initial pull-in circuit of the demodulator in the spread spectrum communication system, the sweeper 800 that controls to sweep or fix the frequency of the output of the local oscillator, and the sweeper are used. A local oscillator 200 whose output frequency is controlled, a received high frequency signal or an intermediate frequency signal modulated by a signal obtained by applying spread spectrum to the original signal, and the output of the local oscillator are input, and the spread spectrum is applied to the original signal. Frequency converter 100 that outputs the signal
And a matched filter 500 that outputs the correlation value by performing spectrum despreading by obtaining the correlation between the output signal of the frequency conversion unit and the built-in spreading code pattern,
There is provided a maximum value detection unit 110 that obtains the maximum value of the correlation value by differentiating the correlation value of the output of the matched filter with respect to the frequency and detecting the change point of the sign of the differentiation result.

Then, when the maximum value of the correlation value is obtained by the maximum value detection unit, the sweep of the sweeper is stopped, the output frequency of the local oscillator is fixed, and the original signal is output from the matched filter. Configure to output.

In FIG. 2 (claim 2), a sweeper 800 for controlling the frequency of the output of the local oscillator to be swept or fixed in the AFC initial pull-in circuit of the demodulator in the direct spread spectrum communication system, A local oscillator 200 whose output frequency is controlled by a sweeper, a received high frequency signal or an intermediate frequency signal modulated by a signal obtained by subjecting an original signal to spread spectrum, and the output of the local oscillator are input, and the original signal is input. A frequency conversion unit 100 that outputs a signal subjected to spread spectrum and an analog / digital conversion unit that digitizes and outputs the output of the frequency conversion unit.
400, the output signal of the analog / digital converter,
A matched filter 500 that performs spectrum despreading by outputting the correlation value by obtaining the correlation with a built-in spreading code pattern, and the correlation value of the output of the matched filter is differentiated with respect to frequency, and the differentiation result is the code. Together with the frequency discriminating means 150 that outputs the change point of the sign of the output of the frequency discriminating means, and outputs a write signal for storing the output data of the sweeper in the storage means 340 which will be described later. At the time when it is confirmed that the code does not change after a predetermined time elapses from the time when the change point of the code is detected, of the data stored in the storage means, the frequency corresponding to the maximum value of the correlation value or its vicinity And a protection means 180 which outputs a read signal for reading data giving a frequency and outputs a sweep stop signal for stopping the sweep of the sweeper. , Storage means 340.

Then, the sweep of the sweeper is stopped by the sweep stop signal, the data read from the storage means is added to the local oscillator as fixed output data of the sweeper, and the original signal is output from the matched filter. To configure.

[0013]

In FIG. 1, the maximum value detecting means 110 differentiates the correlation value of the output of the matched filter 500 with respect to the frequency, detects the change point of the sign of the differentiation result, and obtains the maximum value of the correlation value. Maximum frequency (fc) or f
Frequencies near c can be clearly distinguished, and the frequency sweep can be performed only once to shorten the time required for initial pull-in.

In FIG. 2, when the protection means 180 detects the change point of the sign of the output of the frequency discriminating means 150 and confirms that the sign does not change after a lapse of a predetermined time from the detection time, Of the data stored in the storage means 340, the data that gives the frequency (fc) corresponding to the maximum value of the correlation value or a frequency in the vicinity thereof is read out, and the sweep of the sweeper is stopped. The frequency can be reliably detected, and pseudo pull-in due to the influence of noise or the like can be prevented.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 3 is a block diagram (No. 1) of an AFC initial pull-in circuit according to an embodiment of the present invention. FIG. 4 is a configuration diagram (No. 2) of the AFC initial pull-in circuit according to the embodiment of the present invention.

FIG. 5 is a conceptual diagram of AFC initial pull-in based on the embodiment. FIG. 6 is a diagram showing a correlation value and its frequency discrimination characteristic when it is affected by noise. FIG. 7 is a time chart for explaining the operation of the embodiment.

In FIG. 3, the circuit at the preceding stage (not shown)
For example, after removing the noise component from the reception spread spectrum signal (SS reception signal) converted from a high frequency signal such as microwave into an intermediate frequency signal (for example, IF = 70 MHz) through the BPF 10, the signal is branched and the quadrature detector is used. Make up 2
Add to mixers 1-1 and 1-2. In the mixers 1-1 and 1-2, the output signal of the VCO 2 (the frequency of which is 70 degrees) which has been converted into signals having phases different from each other by 90 ° by the hybrid circuit 39.
MHz) is also added to obtain the baseband spread spectrum signal. This baseband spread spectrum signal output contains harmonic components generated in mixers 1-1 and 1-2 and unnecessary components such as aliasing noise (aliasing noise), so pass LPF3-1 and 3-2 respectively. By
Cut unnecessary components.

The outputs of LPF3-1 and 3-2 are A / D converters 4-1.
, 4-2, A / D-converted by sampling at a predetermined clock frequency to form, for example, an 8-bit digitized reception spread spectrum signal. The digitized received spread spectrum signal is matched filter 5
The spectrum is inversely spread by calculating the correlation with the built-in spreading code pattern, and the correlation value is output. At the same time equal to the clock frequency of the demodulated data
It also outputs the det pulse.

For example, the count output of the sweep counter 8'is converted into an analog voltage via the D / A converter 9 to convert it to a VCO.
In addition to 2, the count output of this sweep counter 8'is shown in FIG.
As shown in (2), the frequency is sequentially increased within a certain range and the frequency is swept, so that the matched filter 5 can see the
The correlation value characteristic as shown in is output.

In the present invention, the difference between the output correlation values is digitally obtained, that is, the output correlation value is digitally differentiated with respect to the frequency, and the change point of the sign of the differentiation result is detected (see FIG. ), (4)), and the frequency near the frequency (fc) at which the differential value becomes 0, as shown by the shaded area in (2), and the correlation value corresponding to this frequency is defined as the maximum correlation value. To do. The details will be described below.

First, by the initial reset signal generated by turning on the sweep start switch 41 shown in FIG. 3, the sweep counter 8 ', the flip-flop circuit (hereinafter referred to as FF) 19, FF28, FF29, memory shown in FIG. The address counter 25 and the like that form the access unit are initialized.

Then, the sweep counter 8'starts counting, and the matched filter 5 outputs a correlation value consisting of, for example, 8 bits and applies it to the frequency discriminator 11. Frequency discriminator
Reference numeral 11 is composed of two FFs 12 and 13, a subtractor 14, and a code detector 15. The correlation value (for example, B) at a certain point is first latched by the FF 12 in the first stage, and this correlation value is obtained by the det pulse described above. Is latched by the FF13 of the second stage and the correlation value (A) with respect to the next count value is latched by the FF12 of the first stage. Further, the subtractor 14 obtains the difference (AB) between the correlation values (A, B) latched by the two FFs 12 and 13 by the next det pulse.

The difference characteristic (frequency discrimination characteristic) obtained in an actual circuit is not a smooth curve due to noise as shown in FIG. 6 (2). Especially in the vicinity of fc, it fluctuates in a zigzag manner around 0 as shown in FIG. For this reason, fc is not easily determined, so in the present invention, it was determined by the method described below.

An absolute value of the difference result (AB) is obtained by an absolute value circuit (ABS) 26 shown in FIG. 4 and added to a difference signal register 27 which will be described later. On the other hand, the difference result (AB)
Is detected by the code detector 15, and a sign bit of "L" level for positive and "H" level for negative is output (see (3) in FIG. 7) and added to the protection circuit 18.

The protection circuit 18 is composed of an FF 19 having a shift register function, an AND circuit 20, an inverter 21, an FF 22, a NAND circuit 23, and an exclusive OR circuit (hereinafter referred to as EX-OR circuit) 24. The FF 19 has a function of, for example, a three-stage shift register, sequentially inputs the outputs of the code detector 15 described above, and sequentially shifts and outputs from the three output terminals as shown in (4) of FIG. .

The EX-OR circuit 24 finds the exclusive OR of the first two outputs of the FF 19, and adds the output to the enable terminal (E) of the address counter 25. As shown in (6) of FIG. 7, the EX-OR output becomes "H" only when the sign bit changes, and the address counter 25 starts counting (see FIG. 7).
(Refer to (8)), count value ("00", "01", "10", ...)
And outputs as an address signal from an output terminal Q _A to Q _C is added to the memory RAM 34.

Further, the NAND circuit 32 obtains the NAND of the output of the EX-OR circuit 24 and the phase-inverted clock (INV CLK), adds it to the WE terminal of the RAM 34 as a write enable signal (WE), and adds the address counter 25 described above. To the address ("00", "01", "10", ...) Specified by the output of the sweep counter 8'at that time count output ("7F", "80", "81",
...) is written through the 3-state buffer 35 (see (9) and (10) in FIG. 7).

On the other hand, the absolute value of the output of the subtracter 14 of the frequency discriminator 11 is sequentially obtained by the absolute value circuit (ABS) 26,
As shown in (11) of FIG.
The values are latched by the FFs 28 and FFs 29 formed of stages, shifted and added to the comparator (CMP) 30, and the magnitudes of two consecutive absolute values of the correlation value difference result are compared.
(Note that FF28 and FF29 operate with the same clock as the above-mentioned WE clock.) (3) of FIG. 6 is an enlarged view of the vicinity of the change point fc of the sign of (2) of the same figure. If and are shown in (a) and (b) of 3) respectively correspond to the inputs a and b of the comparator, in the case of (a), <and a <b. In this case "01"
Is output and added to the A terminal of the address counter 25. (In case of (b), it is> and a> b, so "L"
("00") is output).

On the other hand, when all three outputs of the FF 19 of the protection circuit 18 become "H", the output of the AND circuit 20 becomes "H" and the sweep stop of "L" converted through the inverter 21 is stopped. Output a signal. At the same time, the output of the AND circuit 20 is shifted directly and via the FF 22 and added to the NAND circuit 23 to obtain the "L" level address counter load signal (see FIG. 7).
(See (7)) to the L terminal of the address counter 25, and the output of the comparator 30 (for example, "01") added to the above-mentioned A terminal to Q
Output from the _A to Q _C terminal, RAM 34 as an address signal
Add to.

In the RAM 34, the data "80" stored at the address "01" is read by the output enable signal of the sweep stop signal "L" described above, and is read into the sweep counter 8'through the 3-state buffer 36. The same load signal (L signal that is loaded and added to the address counter 25 described above)
According to 1), the sweep counter 8'adds a fixed count value to the VCO 2 via the D / A 9. As a result, the sweep frequency is fixed near fc, and the AFC initial pull-in is completed.

When the AFC initial pull-in is completed, the demodulated data is sent from the matched filter 5 to the subsequent phase pull-in circuit (A
FC2) 40 and the sweep stop signal described above are applied to the phase pull-in circuit 40 via the inverter 37, whereby the phase pull-in circuit 40 starts operating. That is, the phase pull-in circuit 40, adder 38, D / A 9, VCO 2, hybrid circuit 39, mixers 1-1, 1-2, LPF3-1, 3-2, A
/ D4-1, 4-2, and the phase pull-in circuit is performed by the PLL circuit composed of the matched filter 5.
Output more stable demodulated data from 40.

In this embodiment, when a> b in the comparator 30,
"L" is output, and when "a <b", "H" is output.
The method is not limited to this method. For example, when a> b
The configuration may be such that "H" is output and "L" is output when a <b.

Further, the FF 19 of the protection circuit 18 has a three-stage shift register function, but it is not limited to the three-stage shift.

[0034]

As described above, according to the present invention, the maximum value detecting means differentiates the correlation value of the output of the matched filter with respect to the frequency, detects the change point of the sign of the differentiation result, and detects the correlation value of the correlation value. By configuring to find the maximum value, the maximum frequency (fc) or the frequency near fc can be markedly identified, and the frequency sweep can be performed only once, and the time required for the initial pull-in can be shortened. .

Further, when the protection means detects the change point of the sign of the output of the frequency discriminating means and confirms that the sign does not change after a predetermined time has passed from the detection time point, it is stored in the storage means. Since the data that gives the frequency (fc) corresponding to the maximum value of the correlation value or a frequency near it is read out from the data and the sweep of the sweeper is stopped, the frequency near fc or fc can be reliably detected. Therefore, it is possible to prevent the pseudo pull-in due to the influence of noise or the like.

[Brief description of drawings]

FIG. 1 is a principle diagram of the first invention,

FIG. 2 is a principle diagram of the second invention,

FIG. 3 is a configuration diagram (1) of an AFC initial pull-in circuit according to an embodiment of the present invention,

FIG. 4 is a configuration diagram (part 2) of an AFC initial pull-in circuit according to an embodiment of the present invention,

FIG. 5 is a conceptual diagram of AFC initial pull-in based on an embodiment,

FIG. 6 is a correlation value diagram when affected by noise and its frequency discrimination characteristic diagram,

FIG. 7 is a time chart for explaining the operation of the embodiment,

FIG. 8 is an output vs. frequency characteristic diagram of an example of a matched filter,

FIG. 9 is a configuration diagram of an AFC initial lead-in circuit of a demodulator in a conventional direct spread spectrum communication system.

[Explanation of symbols]

100 is a frequency conversion unit, 110 is a maximum value detection means, 150 is a frequency discrimination means, 180 is a protection means, 200 is a local oscillator, 34
0 is a storage means, 400 is an analog / digital converter, 50
0 indicates a matched filter and 800 indicates a sweeper.

Claims (2)

[Claims] 1. A sweeper (800) for controlling the frequency of the output of a local oscillator to be swept or fixed in an AFC initial pull-in circuit of a demodulator in a spread spectrum direct communication system, and an output of the sweeper (800). Frequency controlled local oscillator (2
00), a received high frequency signal or an intermediate frequency signal modulated by a signal obtained by performing spread spectrum on the original signal, and the output of the local oscillator, and outputs a signal obtained by performing spread spectrum on the original signal. A conversion unit (100), a matched filter (500) for performing spectrum despreading by outputting the correlation value by calculating the correlation between the output signal of the frequency conversion unit and the built-in spreading code pattern, and the matched filter And a maximum value detecting means (110) for obtaining the maximum value of the correlation value by differentiating the correlation value of the output with respect to the frequency and detecting the change point of the sign of the differentiation result. The sweep of the sweeper is stopped when the maximum value of the correlation value is obtained, the output frequency of the local oscillator is fixed, and the original signal is output from the matched filter. AFC initial pull-in circuit of demodulator in spread spectrum direct communication system. 2. A sweeper (800) for controlling the frequency of the output of the local oscillator to be swept or fixed in the AFC initial pull-in circuit of the demodulator in the spread spectrum direct communication system, and the output of the sweeper (800). Frequency controlled local oscillator (2
00), a received high frequency signal or an intermediate frequency signal modulated by a signal obtained by performing spread spectrum on the original signal, and the output of the local oscillator, and outputs a signal obtained by performing spread spectrum on the original signal. The conversion unit (100), the analog / digital conversion unit (400) that digitizes and outputs the output of the frequency conversion unit, and the correlation between the output signal of the analog / digital conversion unit and the built-in spreading code pattern. A matched filter (500) for despreading the spectrum to output a correlation value, and a frequency discrimination means (150) for differentiating the correlation value of the output of the matched filter with respect to frequency and outputting the differentiation result together with its sign. ) And a writing point for detecting the change point of the sign of the output of the frequency discriminating means and storing the output data of the sweeper in the storage means (340) described later. At the time of outputting the signal and confirming that the code does not change after a predetermined time has passed from the time when the change point of the code is detected, the maximum value of the correlation value among the data stored in the storage means is set. A protection means (180) which outputs a read signal for reading data giving a corresponding frequency or a frequency in the vicinity thereof and outputs a sweep stop signal for stopping the sweep of the sweeper.
And a storage means (340) for stopping the sweep of the sweeper by the sweep stop signal, adding the data read from the storage means to the local oscillator as fixed output data of the sweeper, and adding the matched filter. The AFC initial lead-in circuit of the demodulator in the direct spread spectrum communication system, characterized in that the original signal is output from