JPH0250642B2 - - Google Patents

Description

Detailed Description of the Invention a. Field of Industrial Application The present invention relates to an automatic frequency control device ( This invention relates to a frequency discrimination circuit (hereinafter referred to as AFC).

b. Prior Art In a pulse radar that has a receiver that uses a frequency conversion method, if the frequency band of the intermediate frequency amplifier and the intermediate frequency from the mixer output are out of sync, the sensitivity deteriorates significantly, so the local oscillator is always It is necessary to adjust the oscillation frequency to obtain a predetermined intermediate frequency. A device that does this automatically
It is AFC.

AFC consists of a frequency discrimination circuit and a frequency discrimination circuit that generate a frequency error signal that controls the local oscillation frequency of the local oscillator, whose frequency changes depending on the input voltage, so that it is always separated by a predetermined value from the transmitter's transmission frequency. It consists of a circuit that integrates the output of

When the transmitter and local oscillator frequencies vary,
A frequency discrimination circuit generates a frequency error signal corresponding to the deviation from a predetermined intermediate frequency. This error signal is integrated by an integrating circuit with an appropriate time constant, and the oscillation frequency of the local oscillator is controlled so that the difference between the transmission frequency and the local oscillation frequency always becomes a predetermined intermediate frequency.

FIG. 5 shows a configuration diagram of an AFC frequency discrimination circuit according to the prior art. A first tuned circuit 1a having a center frequency at a frequency _L lower than a predetermined intermediate frequency
a second tuning circuit 2a having a center frequency at a frequency _H higher than a predetermined intermediate frequency; a first detection circuit 3a that detects the output v ₁ of the tuning circuit; and a first detection circuit 3a that detects the output v ₂ of the tuning circuit. The second detection circuit 4a
and a subtraction circuit 5a that generates a difference V between the outputs of the first and second detection circuits 3a and 4a.

The frequency characteristics of the first and second tuned circuits show approximately equal gain and selectivity. This characteristic is shown in FIG. At this time, the output of the subtraction circuit exhibits an S-shaped characteristic and becomes approximately zero at a predetermined intermediate frequency _O.
Its characteristics are shown in FIG.

In the case of the conventional technology, as shown in Figure 8, if the intermediate frequency spectrum entering the frequency discrimination circuit is the spectrum of an ideal square pulse, the output of the subtraction circuit is the frequency of the peak point of the main lobe.
It operates so that _M becomes zero when it matches a predetermined intermediate frequency _O. This is because, at this time, the spectrum of the pulse train becomes bilaterally symmetrical, and the ratio of the side lobes to the main lobes is also sufficiently small.

c Problems to be solved by the invention Compared to the spectrum of an ideal square pulse, the spectrum of a radar transmission wave using an actual magnetron as a transmitter is considerably different due to the presence of parasitic AM and FM. It is known. Further, the transmission pulse width is normally switched depending on the distance range of the radar device. (for example,
(0.08μs, 0.25μs, 1.2μs) Figure 2 shows the spectrum of the radar transmission pulse used in an actual magnetron transmitter. The spectrum of a predetermined intermediate frequency obtained by mixed detection of the transmission frequency having such a spectrum and the local oscillation frequency from the local oscillator using a mixer has a shape substantially similar to the spectrum of the transmission pulse. This is shown in FIG. In this figure, the spectrum of the intermediate frequency signal is inverted on the frequency axis with respect to the spectrum of the transmitted pulse. This is because the local oscillation frequency is set above the transmission frequency by a predetermined intermediate frequency.

An intermediate frequency signal having a spectrum that is left-right asymmetric and in which the ratio of the side lobes to the main lobe is not sufficiently large can be handled by adjusting the frequency characteristics of the gains of the first tuning circuit and the second tuning circuit to a predetermined intermediate frequency _O. When inserted into a frequency discrimination circuit with characteristics almost symmetrical to (characteristics shown in Figure 6),
Since the influence of side lobes appears asymmetrically in the output of the subtracter, it does not become zero at the center frequency _M of the main lobe, but becomes zero at a different frequency. Figure 9 shows a state in which when an intermediate frequency signal with the spectrum shown in Figure 3 is input into a frequency discriminator circuit with the characteristics shown in Figure 6, it is locked at a frequency shifted by Δ from the predetermined intermediate frequency _O. shows.

In normal pulse radar, the pulse width is often switched depending on the distance to the target, but at this time, the spread of the spectrum and its shape also change by switching the transmission pulse width. Therefore, the amount of deviation Δ due to the sidelobe described above changes in accordance with the pulse width due to the spectrum change due to switching of the pulse width.

Therefore, even if the frequency discrimination circuit is adjusted using pulses, it will operate correctly for pulses of the same shape as the pulses used for adjustment, but in radars that switch pulse widths as described above, this will not work properly. ,
If the pulse width is changed, the frequency at which the output of the subtracter becomes zero will differ. In the above state
When the AFC is operated, the local oscillation frequency of the local oscillator is controlled to be an intermediate frequency different from the predetermined intermediate frequency _O , which causes a tuning shift and reduces the reception sensitivity of the radar.

As described above, in the pulse radar frequency discrimination circuit, the influence of the asymmetry of the side lobes of the spectrum cannot be ignored, and it is necessary to correct this.

d Means for solving the problem The above problem is caused by the first tuning circuit having a center frequency _L lower than a predetermined intermediate frequency _O , and the second tuning circuit having a center frequency _H higher than a predetermined intermediate frequency _O. , a first detection circuit that detects the output of the first tuning circuit, a second detection circuit that detects the output of the second tuning circuit, and an output of the first detection circuit and the second detection circuit. In the frequency discriminator circuit including a subtraction circuit that generates an output signal corresponding to the difference in the outputs of the detection circuits, the first tuning circuit, the second tuning circuit, or one of them is operated in conjunction with the pulse width of the pulse radar. The problem has been solved by a frequency discrimination circuit characterized in that it includes means for changing at least one parameter among the gain, selectivity, and tuning frequency of the tuning circuit.

e Effect: Like radar transmission waves or their intermediate frequency signals that are actually used, side lobes mainly exist only on one side of the main lobe, and when their pulse widths are switched, the relationship between the main lobe and side lobes changes. By changing the parameters of the tuning circuit according to the pulse width, the contribution of sidelobe energy in the output of the subtraction circuit is relatively reduced or canceled, and the frequency at which the output of the subtraction circuit becomes zero can be determined. It can be set at a point where the center frequency _M of the main lobe matches a predetermined intermediate frequency _O. As a result, the AFC using the frequency discrimination circuit according to the present invention operates so that the center frequency _M of the main lobe becomes the predetermined intermediate frequency _O.

In the following description based on embodiments, the configuration, operation, characteristics, etc. of the frequency discrimination circuit according to the present invention will be further clarified.

f Embodiment FIG. 1 is a block diagram of a preferred embodiment of a frequency discrimination circuit according to the present invention.

The input signal s is sent to the respective input terminals of the first tuning circuit 1 and the second tuning circuit 2, the output of the first tuning circuit 1 is detected by the first detection circuit 3, and the output of the first tuning circuit 1 is detected by the first detection circuit 3. The second output is detected by a second detection circuit 4, and a subtraction circuit 5 generates an output signal corresponding to the difference between the output of the first detection circuit 3 and the output of the second detection circuit.

The first tuned circuit has a center frequency _L lower than the predetermined intermediate frequency _O , and the second tuned circuit has a center frequency _H higher than the predetermined intermediate frequency _O. At least one parameter among the gain selectivity and center frequency of each tuned circuit is
is controlled by the second tuning circuit control circuit 6 or the second tuning circuit control circuit 7. That is, each tuning circuit control circuit 6, 7 is linked to, for example, a radar pulse width switching circuit 8, so that the parameters of each tuning circuit 1, 2 are set to optimal values.

Figure 4 shows that when an intermediate frequency signal has a spectrum in which side lobes mainly exist only on one side of the main lobe as shown in Figure 3, the center frequency _M of the main lobe can be adjusted by controlling the gain of the tuning circuit. FIG. 7 is an explanatory diagram showing that the output of the subtraction circuit can be made zero when the frequency matches a predetermined intermediate frequency _O. FIG.

In this case, since side lobes exist on the higher frequency side than the main lobe, subtraction can be performed by making the gain of the second tuned circuit, which has a center frequency _H higher than the predetermined intermediate frequency _O , smaller than the gain of the first tuned circuit. The contribution of sidelobe energy to the output of the circuit is reduced, and the frequency at which the output of the subtraction circuit becomes zero when an ideal square wave is input is set to a frequency _O1 higher than a predetermined intermediate frequency. As a result, when the center frequency _M of the main lobe matches the predetermined intermediate frequency _O , the output of the subtraction circuit can be set to zero.

When the spectrum of the input signal changes during pulse width switching, the relationship between the main lobe and the side lobe changes, so in the first embodiment of the present invention, the gain of the tuning circuit is controlled in conjunction with the pulse width switching circuit. As a result, when the center frequency _M of the main lobe coincides with a predetermined intermediate frequency in each pulse width, the output of the subtraction circuit is made zero.

When a side lobe exists on the lower frequency side than the main lobe, contrary to FIG. 4, the gain of the first tuned circuit is made smaller relative to the first tuned circuit.

As the tuning circuit control circuits 6 and 7 for making the gain of the tuning circuit variable, for example, a variable attenuator provided before or after a tuning circuit having a fixed gain can be used.

In the second embodiment of the present invention, by controlling the selectivity of the tuning circuit, that is, the half-width of the tuning characteristic, the contribution of side lobes to the output of the subtraction circuit is reduced, and when the ideal square wave is input to the subtraction circuit, The frequency at which the output becomes zero is shifted from the predetermined intermediate frequency _O as in the first embodiment. For example, when side lobes mainly exist only on the high frequency side of the main lobe, by increasing the selectivity of the second tuning circuit, the frequency at which the output of the subtracting circuit becomes zero when an ideal square wave is input is set to a predetermined intermediate frequency.
Make the frequency _O2 higher than _O.

In this case, the tuned circuit control circuits 6 and 7 control the resistance components of the tuned circuit using transistor switches, diode switches, etc. in conjunction with pulse width switching.

In the third embodiment of the present invention, by controlling the tuning frequencies _L and _H of the tuning circuit, the frequency at which the output of the subtraction circuit becomes zero when an ideal square wave is input can be determined as in the first embodiment. shift from the intermediate frequency _O. For example, when side lobes mainly exist only on the high frequency side of the main lobe, by shifting the tuning frequency _H of the second tuning circuit to the high frequency side, the frequency at which the output of the subtraction circuit when inputting an ideal square wave becomes zero is set. to a frequency _O3 higher than the predetermined intermediate frequency _O.

Displacing the tuning frequency of the tuning circuit can be done using known techniques. For example, varactors are used in the tuning circuits 1 and 2, and the bias voltage of the varactors is changed in conjunction with pulse width switching by the tuning circuit control circuits 6 and 7.

g. Effect of the invention When the _input signal is an intermediate frequency signal in which side lobes mainly exist only on one side _of the main lobe, the output of the frequency discrimination circuit is This can be reliably set to zero even when the pulse width is switched. Therefore, AFC using the frequency discrimination circuit according to the present invention is as follows:
Operates independently of changes in transmission pulse width and does not cause tuning deviation.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a frequency discrimination circuit according to the present invention, FIG. 2 is a spectrum of an example of an actual radar transmission pulse, and FIG. 3 is a spectrum of an example of an intermediate frequency signal corresponding to the transmission pulse of FIG. 4 is an explanatory diagram for explaining the operation of the frequency discrimination circuit according to the present invention, FIG. 5 is a block diagram of the frequency discrimination circuit according to the prior art, and FIG. 6 is a tuning characteristic diagram of the tuning circuit of the frequency discrimination circuit according to the prior art. Figure 7 is an output characteristic (S-characteristic) diagram of the subtraction circuit corresponding to the tuning characteristic of Figure 6, and Figure 8 is a diagram of the output characteristic of the subtraction circuit corresponding to the tuning characteristic of Figure 6.
9th diagram illustrating the operation when a signal in which side lobes are symmetrically present on both sides of the main lobe is input to the frequency discrimination circuit having the tuning characteristic shown in the figure.
This figure is a diagram illustrating the operation when a signal in which a side lobe is asymmetrically present on one side of the main lobe is input to the frequency discrimination circuit having the tuning characteristic of FIG. 6. 1...First tuning circuit, 2...Second tuning circuit, 3...First detection circuit, 4...Second detection circuit, 5...Subtraction circuit, 6...First tuning circuit Control circuit, 7... Second tuning circuit control circuit, 8... Pulse width switching circuit, _O ... Predetermined intermediate frequency, _L ...
...center frequency of the first tuned circuit, _M ...center frequency of the main lobe, _H ...center frequency of the second tuned circuit, v ₁ , v ₂ ...output of the tuned circuit, V ...output of the subtraction circuit , S...Input signal.

Claims (1)

[Claims] 1. A first tuning circuit having a center frequency _L lower than a predetermined intermediate frequency _O , and a second tuning circuit having a center frequency _H higher than the predetermined intermediate frequency _O ;
a first detection circuit that detects the output of the first tuning circuit; a second detection circuit that detects the output of the second tuning circuit; and a detection circuit that detects the output of the first tuning circuit and the second detection circuit. In a frequency discrimination circuit including a subtraction circuit that generates an output signal corresponding to a difference in the outputs of the circuits, the first tuning circuit and the second tuning circuit, or one of them, is connected to switching of the pulse width of the pulse radar. 1. A frequency discrimination circuit comprising parameter adjustment means for changing at least one parameter among gain, selectivity, and tuning frequency of a tuning circuit. 2. The frequency discrimination circuit according to claim 1, wherein the parameter adjustment means is a variable gain attenuator. 3. The frequency discrimination circuit according to claim 1, wherein the parameter adjustment means is a semiconductor switch. 4. The frequency discrimination circuit according to claim 3, wherein the semiconductor switch is a transistor switch. 5. The frequency discrimination circuit according to claim 3, wherein the semiconductor switch is a diode switch. 6. The frequency discrimination circuit according to claim 1, wherein the parameter adjustment means is a varactor.