JPH0261170B2 - - 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 having a receiver using a frequency conversion method, sensitivity deteriorates significantly when the frequency band of the intermediate frequency amplifier and the intermediate frequency from the mixer output are different from each other. For this reason, it is necessary to always adjust the oscillation frequency of the local oscillator so that a predetermined intermediate frequency is obtained. AFC is a device that automatically does this.

AFC controls the local oscillation frequency of a local oscillator, whose frequency changes depending on the voltage, so that it is always separated from the transmission frequency of the transmitter by a predetermined value. For this purpose, it is comprised of a frequency discrimination circuit that generates a frequency error signal corresponding to the difference between the intermediate frequency and the predetermined frequency, and an integration circuit that integrates the output of the frequency discrimination circuit.

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 with an appropriate time constant in an integrating circuit, and the oscillation frequency of the local oscillator is controlled so that the difference between the transmission frequency and the local oscillation frequency is always a predetermined intermediate frequency.

FIG. 6 shows a general configuration diagram of an AFC frequency discrimination circuit. A first tuning circuit 10 having a center frequency at a frequency _L lower than a predetermined intermediate frequency o
, a first detection circuit 11 that detects the output of the tuned circuit, and a frequency higher than the predetermined intermediate frequency o.
a second tuned circuit 12 having a center frequency at _M ;
a second detection circuit 13 that detects the output of the tuned circuit;
and a subtraction circuit 14 that generates a difference between the outputs of the first and second detection circuits.

In the prior art, the first and second tuning circuits 1
The frequency characteristics v ₁ and v ₂ of 0 and 12 have approximately equal gain and selectivity, as shown in FIG. The output of the subtraction circuit 14 at this time exhibits an S-shaped characteristic as shown in FIG. 8, and becomes approximately zero at a predetermined intermediate frequency o.

When the bandwidth of the frequency spectrum A() of the input signal of the frequency discrimination circuit is narrow as shown in FIG. 7, when the center frequency _A of the spectrum A() is not equal to the predetermined intermediate frequency o, the subtraction circuit The output V corresponds to the shaded area in Fig. 8,
V≠0. The output of the subtraction circuit 14 is integrated in a subsequent integration circuit (not shown), and the output of the integration circuit is negatively fed back to the local oscillator so that the output of the subtraction circuit becomes zero. As a result, the oscillation frequency is such that the intermediate frequency is equal to the above predetermined frequency.
controlled to be o.

Also, even when the intermediate frequency spectrum entering the frequency discrimination circuit is the spectrum of an ideal square pulse, the output of the subtraction circuit will become zero when the frequency of the main lobe peak point matches the predetermined intermediate frequency o. works. This is because at this time, the spectrum of the pulse train becomes symmetrical,
This is also because the ratio of the side lobes to the main lobes is sufficiently small.

The characteristics of the frequency discrimination circuit are determined in consideration of the accuracy and pull-in range of AFC.
In a control system such as an AFC, there is a steady state error that remains after the system has settled into a steady state, that is, a voltage fluctuation dv where the output of the subtraction circuit shown in FIG. 8 is around zero volts. In order to improve the accuracy of AFC, the sensitivity of the frequency discrimination circuit, that is, the slope of the so-called S-shaped characteristic of the output of the subtraction circuit shown in Fig. 8 near a predetermined intermediate frequency o (dv/d in Fig. 8) It is necessary to increase the value and reduce the steady-state deviation. Specifically, this is achieved by increasing the selectivity of each tuning circuit and bringing their center frequencies closer together.

In this type of AFC, when an intermediate frequency signal exists between the center frequencies of each tuned circuit, the frequency of the intermediate frequency signal quickly follows the predetermined intermediate frequency o. Further, even if the frequency is lower than the center frequency of the first tuning circuit or higher than the center frequency of the second tuning circuit, the intermediate frequency signal is within the range where the output of the frequency discrimination circuit generates the voltage of the frequency error signal. If present, the frequency of the intermediate frequency signal follows the predetermined intermediate frequency o.
The range that follows this predetermined intermediate frequency is called the pull-in range.

There is a trade-off between improving AFC accuracy and expanding the pull-in range as mentioned above, and performance is selected depending on the application.

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 10 shows the spectrum of an actual radar transmission pulse. The intermediate frequency spectrum 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 almost 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 transmission pulse. This is because the local oscillation frequency is set above the transmission frequency by a predetermined intermediate frequency.

The spectra shown in Figures 10 and 11 are characteristic of magnetrons, in which the side lobes are asymmetrical on the left and right, and the side lobes on one side have a smaller level difference than the main lobe, while the side lobes on the other side have a smaller level difference than the main lobe. have a sufficiently large level difference.

The input signal to the AFC will also have the same spectrum as in the same figure, but the AFC for Palace radar is
It is necessary to operate satisfactorily even with such input signals. However, if such an input signal is conventionally conditioned for an ideal rectangular pulse,
When entering the AFC circuit, the following three problems occur.

The first problem is that when an intermediate frequency signal has a spectrum that is asymmetrical and the ratio of the side lobes to the main lobe is not large enough,
When the gains of the first tuning circuit and the second tuning circuit are put into a frequency discrimination circuit whose frequency characteristics are almost symmetrical with respect to a predetermined intermediate frequency o (characteristics shown in FIG. 7), the subtraction circuit Since the influence of side lobes appears asymmetrically in the output of the main lobe, it does not become zero at the center frequency _M of the main lobe, but becomes zero at a different frequency.

Figure 12 shows that when an intermediate frequency signal with the spectrum shown in Figure 11 is input into a frequency discriminator circuit with the characteristics shown in Figures 7 and 8, it is locked at a frequency shifted by △ from the predetermined intermediate frequency o. Indicates the condition.

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.

The second problem is that with normal pulse radar,
This is due to the fact that the pulse width is often changed depending on the distance to the target. At this time, by switching the transmission pulse width, the spread of the spectrum and its shape also change. 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,
If the pulse width is changed, the frequency at which the output of the subtraction circuit becomes zero will differ.

When the AFC is operated in the above state, the local oscillation frequency of the local oscillator is controlled to be an intermediate frequency different from the predetermined intermediate frequency o, so a tuning shift occurs and the reception sensitivity of the radar decreases.

The third problem occurs when short pulses are input.
As mentioned above, the pulse width is usually switched depending on the distance range of the radar device (for example, 0.08 μS, 0.25 μS, 1.25 μS). Being a pulse wave means that the spectrum of the transmitted pulse is wide and has side lobes, and the shorter the pulse width, the wider the spectrum.

A frequency discriminator circuit whose pull-in range is set narrow in order to improve AFC accuracy will have problems if the transmission pulse width is extremely narrow and a signal with a wide spectrum is input. The main lobe frequency of the frequency discrimination circuit input signal immediately after the start of AFC operation is
When the main lobe frequency is within the above-mentioned pull-in range, AFC operates correctly, but when the main lobe frequency deviates significantly from the pull-in range, the contribution of frequency components within the pull-in range of the frequency discriminator circuit, such as side lobes, becomes dominant; The energy of the lobe goes outside the operating range of the frequency discrimination circuit and becomes irrelevant to the operation of the frequency discrimination circuit.
AFC treats the sidelobe frequency as a predetermined intermediate frequency, and sometimes mislocks.

To give a specific example, an intermediate frequency signal with a spread spectrum, such as a pulse width τ = 0.08μS (in this case, the spectrum spread is 1/τ = 12.5MHz), is input, and the local oscillator is When AFC operation is started from a frequency where the frequency of O is significantly different from the local oscillation frequency at which the predetermined intermediate frequency is obtained, AFC captures the sidelobes of the spectrum of the intermediate frequency as the predetermined intermediate frequency o (for example, 45 MHz) and subtracts it. It operates to reduce the output of the circuit to zero. This relationship is illustrated in FIG. In Figure 9, the main lobe frequency is 26.25M.
Hz, the energy of the main lobe does not contribute to the frequency discrimination operation because it is outside the operating range of the frequency discrimination circuit, and the energy from the first side lobe is dominant. For this reason,
AFC mislocks when the first sidelobe of 45MHz is used as a predetermined intermediate frequency. In other words, AFC sets the frequency of the first sidelobe to 45MHz.
It works like this. It is clear that sensitivity will be reduced in such a state.

In order to avoid this, mislock can be prevented by lowering the selectivity of each tuning circuit and setting the center frequency of each tuning circuit sufficiently apart from the predetermined intermediate frequency o to expand the pull-in range. However, in this case, the sensitivity of the frequency discrimination circuit decreases,
AFC accuracy decreases. In addition, by widening the passband width of the input signal, the signal-to-noise ratio (S/N
(ratio) deteriorates and becomes more susceptible to interference from external radio wave sources.

The present invention prevents mislock even when the power is turned on and ensures that the frequency of the input signal remains at a predetermined value when the frequency spectrum of the input signal is wide and asymmetrical, and the spectrum changes in response to pulse width switching. The purpose of the present invention is to provide a frequency discriminator circuit that can realize an AFC that can follow the AFC frequency and has a small steady-state error without causing a tuning shift with respect to a predetermined intermediate frequency despite the asymmetry of the spectrum. do.

d. Means for solving the problem The above problem is a frequency discrimination circuit that generates a frequency error signal corresponding to the difference between the main lobe frequency _M of the input signal and a predetermined intermediate frequency o.
a first filter circuit whose gain rapidly decreases in a frequency band higher than the predetermined intermediate frequency o;
a second filter circuit whose gain rapidly decreases in a frequency band lower than the predetermined intermediate frequency o;
a first detection circuit that detects the output of the first filter circuit; a second detection circuit that detects the output of the second filter circuit; an offset voltage generation circuit that generates a DC voltage; a subtraction circuit that generates a voltage obtained by adding the output of the offset voltage generation circuit to the difference between the output of the detection circuit and the output of the second detection circuit; The characteristic parameters are asymmetric, and the center frequency of the main lobe of the above input signal is
A frequency characteristic in which the output of the subtraction circuit is approximately zero when _M is approximately equal to a predetermined intermediate frequency o,
and a frequency discriminator circuit comprising means for changing at least one parameter of the first filter circuit, the second filter circuit, or one of the filter circuits in accordance with pulse width switching. , resolved.

e Effect Normally, in a pulse radar using a magnetron, the side lobes of the transmission spectrum are asymmetrical with respect to the main lobe due to the nature of the magnetron, and exhibit a very large level on one side.
On the other side, it is at an extremely small level. A frequency discrimination circuit according to the present invention that focuses on this point will be described.

For example, when the input signal is an intermediate frequency signal that mainly has side lobes only on the high frequency side of the main lobe, the offset voltage is generated to position the main lobe of the input signal in a higher frequency band than the center frequency _M of the second tuning circuit. A DC voltage, which is the output voltage of the circuit, is added to the difference between the output of the first detection circuit and the output of the second detection circuit.

As a result, the output of the integrating circuit is a voltage that makes the oscillation frequency of the local oscillator sufficiently high.

When the above input signal is input to the frequency discrimination circuit in this state, the AFC operates to lower the frequencies of the main lobe and side lobes, and locks when the center frequency of the main lobe reaches approximately the predetermined intermediate frequency. . Therefore, even when the transmitter is powered on, the frequency of the local oscillator always starts AFC operation at a higher frequency than the local oscillation frequency that obtains the predetermined intermediate frequency. operates to lock the center frequency _M of the main lobe as the predetermined intermediate frequency o without mislocking the center frequency of the first side lobe as the predetermined intermediate frequency o.

Furthermore, when the above offset voltage is added and the spectrum of the above input signal mainly has side lobes only on one side of the main lobe, and when the relationship between the main lobe and the side lobes changes when the pulse width is switched, the filter is applied according to the pulse width. By changing the characteristic parameters of the circuit, the contributions of the offset voltage and sidelobe energy in the output of the subtraction circuit are relatively reduced or canceled, and the frequency at which the output of the subtraction circuit becomes zero is set to the center frequency of the main lobe. _M is the given intermediate frequency
It can be set to a point that coincides with o.

As described above, the AFC using the frequency discrimination circuit according to the present invention operates without causing tuning deviation even when the intermediate frequency signal has a wide and asymmetric spectrum.

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 _i is divided into a first filter circuit 1 whose gain rapidly decreases in a frequency band higher than a predetermined intermediate frequency o, and a second filter circuit whose gain rapidly decreases in a frequency band lower than the predetermined intermediate frequency o. 2, and a first detection circuit 3 that detects the output of the first filter circuit 1, respectively;
A second filter circuit that detects the output of the second filter circuit 2.
The signal is sent to the input terminal of the subtraction circuit 5 through the detection circuit 4 .

A third input to the subtraction circuit 5 is the output signal of the offset voltage generating circuit 6, which is composed of an offset voltage adjusting circuit 6a and an offset voltage generating means _6b whose DC output voltage is adjusted by the offset voltage adjusting circuit 6a. given as a signal. The subtraction circuit 5 includes a first detection circuit 3 and a second detection circuit 4.
An output signal So proportional to the sum of the output voltage of the offset voltage generating circuit 6 and the difference between the outputs of the offset voltage generating circuit 6 is generated. The output So of the subtraction circuit 5 is integrated with an appropriate time constant by an integrating circuit (not shown), and becomes a voltage that makes the oscillation frequency of the local oscillator sufficiently high. The characteristic parameters of the first filter circuit 1 and the second filter circuit 2 are such that the spectrum of the intermediate frequency signal is wide and asymmetric by the first parameter adjustment circuit 7 and the second parameter adjustment circuit 8, respectively. is adjusted to an optimum value so that no synchronization occurs even when the offset voltage is added. The first parameter adjustment circuit 7 and the second parameter adjustment circuit 8
is further controlled by a pulse width switch 9, and the optimum parameter value for each pulse width is given to each filter circuit.

FIG. 2 shows an example of the spectrum of the input signal S _i before and after the start of the AFC operation, and a preferred embodiment of the characteristic curve v ₁ of the first filter circuit and the characteristic curve v ₂ of the second filter circuit. FIG. 3 is a diagram showing an example of the output characteristics of the subtraction circuit.

In the embodiment shown in FIG. 2, an example will be explained in which the input signal S _i has a main lobe whose center frequency is _M and side lobes mainly only on the higher frequency side than the main lobe.

The offset voltage generation circuit 6 applies the voltage Vo to the input of the subtraction circuit 5 even before the start of the AFC operation. This voltage is integrated by an integrating circuit (not shown), and the oscillation frequency of the local oscillator is adjusted to a predetermined intermediate frequency.
Set the frequency to be sufficiently higher than the frequency at which o is obtained. This frequency corresponds to the main lobe frequency of the input signal S _i being ₁ in FIG.

When the transmitter is powered on in this state, the frequency of the input signal S _i is always higher than the predetermined intermediate frequency o, so the AFC operates to lower the frequency of the input signal S _i . In addition, since there are almost no side lobes on the low frequency side of the main lobe, the frequency of the main lobe of the input signal S _i is always locked to a predetermined intermediate frequency o.

Since the above-mentioned offset voltage Vo is added and there are side lobes mainly on the higher frequency side than the main lobe, the gain, frequency and/or half width of the characteristic curve of the first filter circuit 1 are
be larger than those of the second filter circuit 2, and reduce the contributions of the offset voltage and sidelobe energy in the output of the subtraction circuit 5, and set the frequency at which the output of the subtraction circuit becomes zero when an ideal square wave is input to a predetermined frequency. Let the frequency o be ₁ which is higher than the intermediate frequency o. 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 switching the pulse width, when the spectrum of the input signal changes, the relationship between the main lobe and the side lobe changes, so the parameters of the characteristic curves v ₁ and v ₂ of the first filter circuit and the second filter circuit are The width is adjusted to the optimum value in conjunction with the width switching means 9. As a result, the frequency discrimination circuit of the present invention makes the output of the subtraction circuit zero when the center frequency _M of the main lobe matches the predetermined intermediate frequency at each pulse width.

Note that the characteristics of the first filter circuit 1 and the second filter circuit 2 are not limited to the unimodal tuned circuit having the characteristics shown in FIG.

FIG. 4 shows that the first filter circuit 1 is a first saturation tuned circuit whose output is not saturated at a predetermined intermediate frequency o, but whose output is saturated at least near a frequency _L lower than the intermediate frequency o, and the second filter circuit 1 is circuit 2
This embodiment is a second saturation tuned circuit in which the output is not saturated at a predetermined intermediate frequency o but is saturated at least near a frequency _M higher than the intermediate frequency o. In this case, by adjusting the saturation voltage etc. of the first saturation tuning circuit and the second saturation tuning circuit,
The contribution of offset voltage and sidelobe energy can be reduced. This embodiment has a wide pull-in range and can increase the slope of the curve near the frequency o ₁ where the two characteristic curves v ₁ and v ₂ intersect, so that the steady-state deviation is also small.

In FIG. 5, the first filter circuit 1 is a low-pass filter LPF whose gain decreases sharply in a frequency region higher than a frequency _L lower than a predetermined intermediate frequency o, and the second filter circuit 2 is a low-pass filter LPF whose gain is lower than a predetermined intermediate frequency o.
This is an embodiment configured as a high-pass filter HPF whose gain decreases rapidly in a frequency region lower than _M , which is higher than o. In this case, by adjusting the pass loss etc. of the low-pass filter LPF and high-pass filter HPF, the contributions of the offset voltage and side lobes can be reduced, the pull-in range is wide, and both characteristic curves v ₁ and v ₂ can be reduced. Since the slope of the characteristic curve near the frequency o ₁ giving the intersection point can be increased and the selectivity can be increased, the steady-state deviation of the center frequency of the main lobe can also be reduced.

Above, we have explained the case where side lobes exist mainly on the high frequency side of the main lobe.
Conversely, even if there are side lobes mainly on the lower frequency side than the main lobe, the offset voltage should be a voltage that makes the oscillation frequency of the local oscillator sufficiently lower than the predetermined intermediate frequency, and the parameters should be selected in reverse order. It can be implemented by

g Effects of the Invention Even when side lobes exist asymmetrically with respect to the main lobe, AFC can be operated to accurately match the center frequency of the main lobe to a predetermined intermediate frequency.

It is possible to eliminate malfunctions when the power is turned on.

The retraction range can be adjusted to the optimum value.

It is possible to reduce the steady-state deviation of the frequency.

[Brief explanation of drawings]

FIG. 1 is a block diagram of a preferred embodiment of the frequency discrimination circuit according to the present invention, and FIG. 2 is an example of a spectrum of an input signal and an example of a preferred embodiment of the characteristic curves of the first filter circuit and the second filter circuit. 3 is a diagram showing the output characteristics of the subtraction circuit corresponding to FIG. 2, and FIGS. 4 and 5 are other preferable characteristic curves of the first filter circuit and the second filter circuit. 6 is a block diagram of a general frequency discriminator circuit, and FIG. 7 is a diagram showing the characteristic curves of the first tuning circuit and the second tuning circuit according to the prior art, and an input signal having a narrow spectrum. 8 shows the output characteristics of the subtracting circuit corresponding to FIG. 7, FIG. 9 is an explanatory diagram showing a mislock state when the input signal has a side lobe, and FIG.
The figure shows the spectrum of an actual radar transmission pulse.
Fig. 11 shows the spectrum of the intermediate frequency signal corresponding to Fig. 10, and Fig. 12 shows the spectrum of the intermediate frequency signal corresponding to Fig. 10, and Fig. 12 shows that when the signal in Fig. 11 is used as an input signal, AFC is locked at a frequency shifted by △ with respect to the predetermined intermediate frequency o. FIG. 1...first filter circuit,
2... Second filter circuit, 3... First detection circuit, 4... Second detection circuit, 5... Subtraction circuit, 6
...offset voltage generation circuit, 6a...offset voltage adjustment circuit, 6b...offset voltage generation means, 7...first parameter adjustment circuit, 8...second parameter adjustment circuit, 9...pulse width switching means .

Claims (1)

[Scope of Claims] 1. A frequency discrimination circuit that generates a frequency error signal corresponding to the difference between the center frequency _M of the main lobe of a pulse-modulated input signal and a predetermined intermediate frequency o, a first filter circuit consisting of a unimodal tuning circuit having a center frequency in a higher frequency band; and the above-mentioned predetermined intermediate frequency.
a second filter circuit consisting of a unimodal tuning circuit having a center frequency in a frequency band lower than o; a first detection circuit that detects the output of the first filter circuit; and an output of the second filter circuit. a second detection circuit that detects a DC voltage, an offset voltage generation circuit that generates a DC voltage, and an output of the offset voltage generation circuit that is added to the difference between the output of the first detection circuit and the output of the second detection circuit. the frequency characteristic is such that the output of the subtraction circuit is approximately zero when the center frequency _M of the main lobe of the input signal is approximately equal to a predetermined intermediate frequency o; Filter circuit and the second
A frequency discriminator circuit characterized by comprising means for changing at least one parameter among the gain, tuning frequency, and half-value width of a tuning characteristic curve of a single-peak tuning circuit of the filter circuit or one of the filter circuits. 2. The frequency discrimination circuit according to claim 1, wherein the characteristic parameters of the first filter circuit and the second filter circuit are switchable in accordance with switching of the pulse width of the input signal. 3. The frequency discrimination circuit according to claim 1, wherein the offset voltage generation circuit comprises an offset voltage adjustment circuit and offset voltage generation means. 4. A frequency discrimination circuit that generates a frequency error signal corresponding to the difference between the center frequency _M of the main lobe of a pulse-modulated input signal and a predetermined intermediate frequency o, which does not saturate at the predetermined intermediate frequency o and at least A first saturated tuned circuit whose output is saturated near a frequency _L lower than the intermediate frequency o, and a second saturated tuned circuit whose output is not saturated at a predetermined intermediate frequency o and whose output is saturated at least near a frequency _H higher than the intermediate frequency o. a saturation tuning circuit, a first detection circuit that detects the output of the first saturation tuning circuit, a second detection circuit that detects the output of the second saturation tuning circuit, and an offset that generates a DC voltage. The main lobe of the input signal includes a voltage generation circuit and a subtraction circuit that generates a voltage obtained by adding the output of the offset voltage generation circuit to the difference between the output of the first detection circuit and the output of the second detection circuit. center frequency _M
has a frequency characteristic in which the output of the subtraction circuit is approximately zero when is substantially equal to a predetermined intermediate frequency o, and the gain of the first saturation tuning circuit, the second saturation tuning circuit, or one of the saturation tuning circuits. , a frequency discrimination circuit comprising means for changing at least one parameter of the tuning frequencies. 5. The frequency discrimination circuit according to claim 4, wherein the saturation gain of at least one of the saturation tuning circuits is adjustable. 6. Frequency discrimination according to claim 4, wherein the characteristic parameters of the first saturation tuning circuit and the second saturation tuning circuit are switchable in accordance with switching of the pulse width of the input signal. circuit. 7. The frequency discrimination circuit according to claim 4, wherein the offset voltage generation circuit comprises an offset voltage adjustment circuit and offset voltage generation means.