JP3717741B2 - Automatic tuning antenna system - Google Patents

Description

[0001]
[Technical field to which the invention belongs]
The present invention relates to an improvement of an automatic tuning antenna system used for a small mobile radio, particularly a radio such as a keyless entry system for locking and unlocking an automobile door by a remote controller.
[0002]
[Prior art]
Conventionally, a small mobile radio auto-tuning antenna system uses a built-in antenna to transmit a radio signal, and this built-in antenna has a resonant frequency that changes (separates) due to the influence of surrounding objects. Tuned), the bandwidth is narrow because the antenna is small, and there is a problem that the gain or sensitivity at the target frequency decreases due to detuning. The auto-tuning antenna system used for the keyless entry system is composed of, for example, a 300 MHz band transmitting portable radio, an in-vehicle antenna, a receiver, and a control device. The built-in antenna is generally installed close to a structure such as a metal wall. As a result, the antenna tuning frequency changes by about 1 to 2% from the set value. In some cases, the target signal is received. There was a problem that made it impossible.
[0003]
The change in the tuning frequency occurs when the built-in antenna is constituted by a resonance circuit and its electromagnetic field distribution changes depending on surrounding objects. The built-in antenna generally has a limited volume and thus has a small radiation resistance, operates as a high-Q resonance circuit, and has a narrow band. Here, Q is an amount representing the sharpness of resonance of the resonant circuit, and a high Q increases the frequency selectivity. (same as below)
The tendency of the antenna to be affected by surrounding objects becomes more prominent as the circuit loss is reduced and the sensitivity is increased. As a countermeasure against this conventional phenomenon, it is common to provide a frequency offset corresponding to a specific installation location during antenna assembly adjustment, or use a metal jig that causes the same frequency change to match the specified frequency. there were.
[0004]
However, this frequency change depends on the vehicle type and the mounting position, and also changes to some extent due to subtle differences in the wire harness even if the position is fixed. Therefore, it is difficult to eliminate individual differences only by the above measures. That is, the change in the built-in antenna tuning frequency varies depending on the antenna type. A quarter-wave helical antenna having a receiver substrate that is very small compared to the wavelength as a ground is likely to cause a high-frequency current to flow through a wire harness connected to the substrate. The high-frequency current flowing out of the ground helps radiation, but has a great influence on the antenna resonance frequency. This is because, since the high frequency impedance of the connected wire harness is not necessarily uniform, the antenna tuning frequency is changed as a reaction to change the sensitivity. Miniaturized radio antennas have small radiation resistance due to size limitations, and in order to increase the antenna efficiency that governs antenna gain, the conductor loss and dielectric loss for this radiation resistance are greatly reduced. There must be. However, when there is an influence from an approaching object in the application, the antenna Q increases when the loss is reduced, and if the frequency is not detuned, the frequency change relative to the antenna bandwidth is relative even if the band required for the transmission signal is satisfied There is a problem that it increases in number and becomes practically inconvenient.
[0005]
[Problems to be solved by the invention]
The present invention pays attention to this point, and can be applied to any miniaturized antenna type, and provides an economical means for detecting the influence of a close object and automatically adjusting the tuning frequency to a specified value. Is made as an issue.
[0006]
[Means for Solving the Problems]
The present invention includes an antenna in which a tuning frequency is varied by changing a bias voltage of a variable capacitance diode, a circuit that generates a fixed reference carrier from a reference signal generator and couples the antenna to the antenna, and a receiver via the antenna. A carrier strength detection circuit for detecting the strength of the amplified carrier and a function for recording a bias voltage at a tuning point at which the received carrier strength becomes maximum when the bias voltage is swept. By detecting the influence of an approaching object, the bias voltage of the variable capacitance diode is corrected and held so that the tuning frequency of the antenna is tuned to the reference carrier or a frequency separated from the reference carrier by a certain value.
[0007]
When creating the reference carrier, it is preferable to use the harmonics of the crystal oscillator of the signal processing microprocessor of the radio as a signal source.
[0008]
In addition, the reference carrier is tuned so that the reference signal reception level is maximized in a periodic operation using the one matched to the reception signal frequency, and the carrier strength of the receiver or the receiver demodulation is obtained. It is preferable to stop the input of the reference carrier when the arrival of the regular transmission wave is detected by the change of the signal.
[0009]
Furthermore, an antenna whose tuning frequency is variable by changing the bias voltage of the variable capacitance diode, a carrier strength detection circuit that detects the strength of the carrier with respect to the signal amplified by the receiver from the remote control transmitter via the antenna, A judgment circuit that uses the carrier strength as a reference signal for antenna tuning when the carrier strength is above a certain level, and a function of recording the bias voltage at the tuning point at which the received carrier strength is maximum when the bias voltage is swept. The bias voltage of the variable capacitance diode is corrected so that the tuning frequency of the antenna is tuned to the reference carrier or a frequency away from the reference carrier by detecting the influence of the object approaching the antenna by the change of the antenna tuning point. Can also be held.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
The present invention can be applied to any miniaturized antenna type and provides a simple structure to provide an economical means for detecting the influence of a nearby object and automatically adjusting the tuning frequency to a specified value. It was realized.
[0011]
【Example】
Embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 is a block diagram of a keyless entry system receiver to which the present invention is applied. The receiver includes an antenna l, a receiving high-frequency module 14, a control CPU (computer unit) 19, a DA converter 20, A reference voltage switch 27, a reference signal generator 22, and a comparator 28 for detecting the received wave level are included.
[0012]
The reception high-frequency module 14 is composed of a low-noise amplifier, a local oscillator, a mixer, an intermediate frequency amplifier, and a demodulation circuit, as in a normal receiver. The function is to amplify, detect, and demodulate the received radio signal. The transmission data is reproduced. The reception high-frequency module 14 includes a radio signal strength display (RSSI) function in the intermediate frequency amplification section. The control CPU 19 outputs a control signal (Ctrl) for opening and closing the door of the car by decoding the sender's ID code and command to the terminal 29.
[0013]
The antenna 1 is formed as a loop circuit by a loop conductor 3 on a dielectric substrate 2, a capacitor 4 inserted between the conductor tips 6a and 6b, and a varicap 5 that is a variable capacitance diode using a barrier capacitance. Is done. The operating frequency of the antenna is roughly determined by these circuit elements.
[0014]
Next, the coupling capacitor 8 and the central conductor 9 form a matching circuit together with the antenna circuit element to match the antenna impedance to the characteristic impedance of the feeder line.
[0015]
The matched antenna impedance is connected to the center conductor 11 a of the coaxial cord 11 at the end of the center conductor 9. Further, the outer conductor 11b of the coaxial conductor is grounded to the loop conductor base 10 where the high-frequency voltage is the lowest position on the loop conductor 3, and prevents the high-frequency current from leaking to the outer conductor of the coaxial cord 11. To do. The high frequency signal from the coaxial cord 11 is transmitted to the input terminal 15 of the receiving high frequency module 14 through the capacitor 13. Reference numeral 16 denotes a ground terminal of the receiving harmonic module (RF-Module) 14, 17 denotes a demodulated signal output terminal of the receiving harmonic module 14, and 18 denotes a radio frequency (signal) level detector of the receiving harmonic module 14. This is a signal output terminal. A DC bias voltage is applied to the varicap 5 of the antenna 1 via the center conductor 11 a of the coaxial cord 11. Here, the resistors 7 and 12 for supplying the DC bias are those having a high resistance value of about 1 kΩ or higher so that a high-frequency current does not flow on the bias voltage supply side to cause loss. To do.
[0016]
Here, FIG. 2 is an equivalent circuit of the antenna 1 of FIG. 1, and the operation principle of the antenna will be described. This antenna is an embodiment in which the balance type loop antenna described in Japanese Patent Application No. 10-366229 filed by the present applicant is used as a basic type, and a varicap tuning function is provided thereto.
[0017]
An equivalent circuit of the loop antenna viewed from the conductor tips 6a and 6b of the antenna 1 can be represented by a parallel resonance circuit shown in FIG. Here, L0 is the inductance of the loop conductor 3, and the antenna capacitance C1 is the capacitance of the capacitor 4 and the varicap 5 plus the capacitance exhibited by the opposing conductor tips 6a and 6b. The parallel resonance resistance Rsh represents an equivalent loss resistance of the loop antenna, and is a value determined by the radiation resistance of the antenna, the conductor loss of the entire antenna circuit, and the dielectric loss.
[0018]
Since the small loop antenna has a small radiation resistance to the current flowing through the loop, the antenna impedance including the parallel resonance resistance Rsh and the reactance component of the equivalent circuit has a very large value.
[0019]
FIG. 2B shows an equivalent circuit of a matching circuit that converts high resistance into low impedance.
[0020]
This equivalent circuit is a circuit seen from the coaxial circuit connection point of the central conductor 9 to the antenna side. The equivalent circuit of the antenna body shown in FIG. 2A has a high resistance, that is, a small conductance G1 at the resonance frequency.
[0021]
Since G1 is smaller than the conductance of the feeder line (the reciprocal of the characteristic impedance), current does not flow much as it is. Therefore, the configuration of the matching circuit is as follows.
First, with respect to G1, the susceptance B1 due to the capacitive element is put in parallel to increase the current flowing in to reduce the impedance so that the real part of the antenna impedance is converted to the desired low resistance, and then the imaginary part is inserted by inserting series reactance. Matching is achieved by setting it to zero.
[0022]
The susceptance B1 may actually be detuned to a higher frequency by reducing the antenna capacitance C1 of FIG. 2A instead of adding a parallel susceptance element, and an inductive element is inserted in parallel with G1. Is equivalent to In FIG. 2B, C2 represents the capacitance of the coupling capacitor 8, L2 represents the inductance of the central conductor 9, and R2 represents the loss of the matching circuit.
Eventually, the matching method is to convert the antenna impedance to an arbitrary resistance value by appropriately selecting the detuning degree of the antenna body and the combination of L2 and the coupling capacitance 8, C2.
In the above setting example, the series element is made capacitive because a capacitor having a wide selection range of element constants can be used. In some cases, detuning can be reversed to make the element inductive.
[0023]
In the experimental example in which a trimmer capacitor of 3 pPF is used instead of the varicap 5 for the sake of simplicity with the antenna having the form shown in FIGS. 1 and 2, when the capacitance of the capacitor is changed after being completely tuned, The tuning bandwidth with a return loss of 20 dB or less for the antenna viewed from the feeder line is about ± 20 MHz centered on 315 MHz, and this can sufficiently cope with the representative value of −3 MHz of the antenna frequency change when an object approaches. I understood.
[0024]
Even when an antenna different from this example is used, a tuning capacitor is generally used for a small antenna. Therefore, if a part of the capacitor capacity is created with a varicap, it is optional. This type of antenna can be varicap tuning type, and the present invention can be applied.
[0025]
Next, a procedure for detecting a change in antenna frequency due to an approaching object and automatically tuning in the wireless device of the present invention will be described.
[0026]
When the object approaches, the resonance frequency of the antenna 1 is changed by sweeping the bias of the varicap 5 and examining the detection signal of the radio frequency level of the receiver with respect to the reference carrier, and changing the tuning point with respect to the reference carrier. To decide.
In order to change the capacity of the varicap 5, the bias voltage supplied by the DA converter 26, the reference voltage switch 27, and the bias reference voltage V1 is controlled.
[0027]
Further, in order to check the output of the radio signal strength indicator of the receiving high-frequency module 14 with respect to the signal from the reference signal generator 22, the reference voltage V2 for level detection is selected by the reference voltage switch 27, and the DA converter is selected. It can be determined by using a carrier intensity detection circuit in which the voltage generated at 26 is compared by the comparator 28. Here, the bias voltage of the varicap that maximizes the radio intensity signal is recorded by the CPU 19 in its internal memory.
This circuit function detects the effect of an object approaching the antenna due to a change in the antenna tuning point, and adjusts the varicap bias voltage so that the tuning frequency of the antenna is tuned to the reference carrier or a frequency away from the reference carrier by a certain value. It can be modified and retained.
[0028]
In this embodiment, a digital circuit is used for the DA / AD converter, but these parts are replaced with other commonly used circuit systems, for example, an analog circuit using an integration circuit, so that a smoother sweep is possible. A voltage may be supplied.
[0029]
The reference signal generator 22 is tuned by the inductance 23 and the capacitor 24 with respect to the target harmonic of the signal of the crystal oscillator, and the signal source impedance connected to the antenna 1 is high. Therefore, the voltage on the input terminal of the high frequency module varies depending on the antenna impedance. That is, it is maximum at the time of tuning and approaches a short circuit with detuning. The exact tuning point can be found by taking the average of two points above a certain level at the left and right slopes of the tuning curve.
Here, the switch 25 is for applying a reference signal to the antenna 1 through the central conductor 11a, and uses a general PIN diode as an element, and on / off control of the reference signal applied by the bias voltage is used. It is made to be able to.
[0030]
Another method for finding the resonance point is to perform frequency sweep at high speed in an analog manner by using an integration circuit, etc., and know the resonance point from the inflection point by signal processing that passes the radio level detection signal through the differentiation circuit. May be used.
[0031]
The reference signal generator 22 of the present embodiment is a circuit that guides a signal from the voltage dividing capacitor 21 using a crystal oscillator for CPU and extracts a reference carrier close to the reception frequency from its harmonics. As an example, the fundamental frequency of the crystal oscillator is set to f = l5.9 MHz, and 318 MHz of the 19th harmonic is used as a reference carrier. Depending on the CPU, instead of directly supplying the output of the crystal oscillator as a clock as in this embodiment, it may be divided into four by a frequency divider to give about 4 MHz.
[0032]
The reason why the fundamental frequency of the crystal is made relatively high is to limit the sweep width of the bias voltage so that one antenna tuning point appears, and at the same time, to sufficiently cover the frequency change due to the close object. The crystal oscillation frequency of the CPU should be such that the harmonics coincide with the reception frequency and do not cause interference.
[0033]
Next, it is shown that the accuracy of tuning control is not impaired even if the reference carrier does not match the specified reception frequency.
[0034]
If the antenna capacity changes from C0 to ΔC when the frequency changes from the original frequency f0 by Δf, the change in the resonance frequency can be expressed by the following approximate expression when the change is minute.
Δf / f0 = −ΔC / (2C0)
The frequency change due to the proximity object can be expressed on the equivalent circuit by inserting the equivalent capacitance change ΔC calculated from the above equation in parallel with C0. Here, it is assumed that ΔC0 is a change in capacity when measured using the regular reception frequency.
[0035]
Next, using the reference carrier shifted by Δft from the transmission signal, the antenna is tuned with respect to this reference carrier at C1, and the value of ΔC1 of the equivalent capacitance change detected using the reference carrier is calculated. We will compare with the correct value of.
That is, C1 = C0 (1-2Δft / f0)
Δf1 / f1 = −ΔC0 / C1 = −ΔC0 / (2C0) · (1 + 2Δft / f0)
The measurement accuracy when detecting the capacitance change ΔC0 from the original tuning capacitance by examining the resonance frequency of the antenna has only the influence of the secondary minute amount on the minute change of the test frequency. For example, when the frequency change due to the proximity object is 1% and the deviation of the reference carrier signal is 1%, the measured capacitance change is 0.0204 C0 with respect to the true value 0.02 C0.
[0036]
From the above verification results, it is clear that the amount of change in the capacitance when the influence of the proximity object is compensated by this reference carrier has a very small error even if it is the capacitance change for correction when receiving the normal signal as it is.
[0037]
On the other hand, for more accurate control, the above equation is used to calculate and calculate the exact change in capacitance value at the regular reception frequency from the change in tuning capacitance determined by the set test frequency. Is also possible.
[0038]
In this system, since the control is performed by the bias voltage of the varicap, the conversion table is basically used when the bias voltage is converted into the capacitance value. However, for a small change, since it is almost linear, conversion is easy in most cases, and actual control can be simplified.
[0039]
In the case of the method in which the harmonics of the crystal oscillator are used as the reference carrier, the harmonics are generated at regular frequency intervals, so that multiple harmonics appear within the tuning frequency range of the varicap antenna by selecting the oscillation frequency of the crystal oscillator. It can also be. If these are used as markers, individual differences in antenna tuning characteristics mainly related to the bias voltage versus capacitance value of the varicap can be calibrated, and the reliability of the system can be improved.
[0040]
As another example, tuning can be controlled more easily by adding a component that generates a reference carrier having the same frequency as the transmission frequency.
[0041]
In this case, instead of sweeping the bias voltage, it is possible to keep the antenna constantly near the best tuning state by periodically fine-tuning the bias voltage while waiting for the incoming wave reception, Improving responsiveness. When the method of matching the reference carrier with the transmission wave is taken, the change of the carrier strength or the change of the received demodulated signal is judged, and when the arrival of the regular transmission wave is detected, the reference carrier is cut immediately. do it.
[0042]
In another embodiment (claim 4), the reference signal generator 22 may be omitted to provide an automatic tuning function. In this case, since the signal wave from the remote control transmitter is used as a reference signal as it is for antenna tuning, the carrier strength detection circuit and CPU determine that the signal is above a certain value (determination circuit), and a predetermined automatic The antenna is tuned to the incoming wave according to the sequence of tuning modes.
At this time, the transmitter antennas placed opposite to each other and the space between them act as a reference signal generator 22 in FIG.
[0043]
In the case of this method, the output from the radio frequency level detector of the receiver must be made sufficiently high from the noise level to accurately measure the change. In the keyless entry system, for example, the reception antenna is within 1 m. Since the transmission wave can be transmitted so that the carrier strength becomes extremely strong at a very close position, the timing at which the transmission wave level is high and the automatic tuning mode should be made can be easily determined.
Then, the receiving high-frequency module 14 has the original function of fixing the bias voltage of the variable capacitance diode, the standard mode for receiving and demodulating, and the bias voltage that maximizes the radio signal level detected by sweeping the bias voltage. It is only necessary to be able to switch to the sweep mode for finding the image.
[0044]
As a procedure for updating the bias voltage using this tuning function, first, the keyless remote control transmitter transmits a signal wave a plurality of times in response to one control command.
The receiver is normally operated in the standard mode. When the receiver detects the door unlock signal, the receiver unlocks the door and puts the receiver in the sweep mode for a certain time.
When the optimum bias voltage is found from the incoming wave in the sweep mode, the bias voltage is temporarily set.
When the next incoming signal wave is correctly demodulated by this updated bias voltage, the temporarily set voltage is held and fixed, and when the demodulated by the updated bias voltage is not correctly performed, Return to bias voltage.
In this case, the operator is required to send an unlock signal once or twice near the door of the car, thereby keeping the receiving antenna in tuned state for the best reception.
[0045]
The update procedure is not limited to the above, and can be set in accordance with various control sequences in consideration of shortening of processing time, ease of operation, and certainty of operation.
[0046]
【The invention's effect】
As described above, the self-tuning antenna system of the present invention detects the influence of an object approaching the antenna due to a change in the antenna tuning point, and sets the tuning frequency of the antenna at a frequency separated from the reference carrier or the reference carrier by a certain value. Because the bias voltage of the variable capacitance diode is modified and held so that it can be tuned to the antenna, it can be easily applied to small antennas for radios, and it has high sensitivity and high Q for keyless entry systems and general mobile communications. It is possible to prevent a decrease in sensitivity due to detuning caused by a close object in the antenna.
[0047]
In addition, it is possible to eliminate the trouble of readjustment when the wireless unit is installed, to improve and stabilize the performance of the wireless transmission system, and it is also possible to relieve a decrease in sensitivity due to a change with time of the high Q antenna.
[0048]
Furthermore, if an existing signal source such as a harmonic of a CPU crystal oscillator or a signal source that can be easily synthesized from the signal source is used, the increase in the number of components and power consumption can be minimized.
[0049]
Furthermore, when the carrier strength from the remote control transmitter exceeds a certain level, if the antenna is tuned using this carrier as a reference carrier, the reference signal generator is not required, and a simple structure can be achieved.
[Brief description of the drawings]
FIG. 1 is a perspective view of an antenna unit showing an embodiment of an automatic tuning antenna system of the present invention and a block diagram of the system. FIG. 2 is an equivalent circuit of an antenna unit showing an embodiment of the present invention. An equivalent circuit of a loop antenna viewed from the parts 6a and 6b is an equivalent circuit of a matching circuit for converting high resistance into low impedance.
[Explanation of symbols]
1 Antenna 5 Varicap (variable capacitance diode)
11 Coaxial code 14 High-frequency module for reception 19 CPU
22 Reference signal generator 26 DA converter 28 Comparator

Claims (4)

An antenna whose tuning frequency is varied by changing the bias voltage of the variable capacitance diode, a circuit for generating a fixed reference carrier from the reference signal generator and coupling it to the antenna, and a carrier amplified by the receiver via the antenna And a function for recording a bias voltage at a tuning point at which the received carrier strength is maximized when the bias voltage is swept. A self-tuning antenna system characterized in that the bias voltage of a variable-capacitance diode is corrected and held so that the influence is detected and the tuning frequency of the antenna is tuned to the reference carrier or a frequency away from the reference carrier by a certain value. 2. The self-tuning antenna system according to claim 1, wherein a harmonic of a crystal oscillator of a signal processing microprocessor of a radio is used as a signal source when creating the reference carrier. The reference carrier is tuned to maximize the reference signal reception level in a regular operation using the one matched to the received signal frequency, and the carrier strength of the receiver or the demodulated signal of the receiver 2. The automatic tuning antenna system according to claim 1, wherein the input of the reference carrier is stopped when the arrival of the regular transmission wave is detected due to the change of. An antenna in which the tuning frequency is varied by changing the bias voltage of the variable capacitance diode, a carrier strength detection circuit for detecting the carrier strength with respect to the signal amplified by the receiver from the remote control transmitter via the antenna, and the carrier strength is It is equipped with a decision circuit that uses this as a reference signal for antenna tuning when it is above a certain level, and a function to record the bias voltage at the tuning point at which the received carrier strength is maximum when the bias voltage is swept, and antenna tuning The effect of an object approaching the antenna is detected by changing the point, and the bias voltage of the variable capacitance diode is corrected and held so that the tuning frequency of the antenna is tuned to the reference carrier or a frequency away from the reference carrier by a certain value. This is an automatic tuning antenna system.

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