JP2001267950A - Automatic tuning antenna system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an automatic tuning antenna system that can automatically correct a change in an antenna tuning frequency and decrease in the sensitivity due to approach of an article or a person to the system so as to maintain the best sensitivity with no adjustment. SOLUTION: The automatic tuning antenna system is provided with an antenna 1 where the bias voltage of a varactor diode 5 is changed to vary the tuning frequency, a circuit where a reference signal generator 22 generates a prescribed reference carrier to be coupled with the antenna 1, a carrier strength detection circuit that detects the strength of the carrier amplified by a receiver via the antenna, and a function that records bias voltage at a tuning point at which the strength of the received carrier is maximized when a bias voltage is swept. The effect of an article approaching the antenna 1 is detected by a change in the bias tuning point of the antenna with respect to a reference signal wave with a frequency matched with or apart by a prescribed value from the frequency of a remote control carrier and the bias voltage of the varactor diode is corrected and then maintained by tuning the antenna tuning point to the frequency of the remote control carrier.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement of an automatic tuning antenna system used for a small-sized mobile radio device, and in particular, a radio device such as a keyless entry system for locking and unlocking a car door by a remote controller. It is.

[0002]

2. Description of the Related Art Conventionally, an automatic tuning antenna system for a small mobile radio device generally uses a built-in antenna for transmitting a radio signal.
When the resonance frequency changes (detunes) due to the influence of surrounding objects, the bandwidth is narrow due to the small size of the antenna, and the gain or sensitivity at the target frequency is reduced due to detuning. An automatic tuning antenna system used for a keyless entry system is, for example, 30
It consists of a portable wireless device for transmission in the 0 MHz band, a vehicle-mounted antenna, a receiver, and a control device. The reception antenna of the keyless entry system is installed close to a structure such as a metal wall. Is
As a result, the antenna tuning frequency changes by about 1 to 2% from the set value, and in some cases, there is a problem that a target signal cannot be received.

[0003] A change in the tuning frequency occurs when the built-in antenna is constituted by a resonance circuit and its electromagnetic field distribution is changed by surrounding objects. The built-in antenna generally has a small radiation resistance due to a limited volume, operates as a high-Q resonance circuit, and has a narrow band. Here, Q is an amount representing the sharpness of the resonance of the resonance circuit. The higher the Q, the higher the frequency selectivity. The tendency of the antenna to be affected by surrounding objects (same below) becomes more pronounced as the circuit loss is reduced and the sensitivity is increased. As a countermeasure against this phenomenon in the past, it is common to provide a frequency offset corresponding to a specific installation location at the time of assembling adjustment of the antenna, or to adjust to a specified frequency using a metal jig that produces the same frequency change. there were.

[0004] However, this frequency change depends on the type of vehicle and the mounting position, and even if the position is fixed, it changes to some extent due to a slight difference in the wire harness. Therefore, it is difficult to eliminate individual differences only by the above measures. there were. That is, the change in the built-in antenna tuning frequency differs depending on the antenna type. In a quarter-wave helical antenna in which a receiver board, which is very small compared to the wavelength, is used as the ground, a high-frequency current easily flows through a wire harness connected to the board. The high-frequency current flowing to the outside from the ground assists radiation, but also has a large effect on the antenna resonance frequency. This is because the high frequency impedance of the connected wire harness is not always uniform, so that the antenna tuning frequency is changed as a reaction to change the sensitivity. Due to size limitations, the miniaturized radio antenna has a low radiation resistance, and to increase the antenna efficiency that controls the antenna gain, the conductor loss and the dielectric loss to this radiation resistance must be extremely reduced. There must be. However, when there is an effect of an approaching object in the application, the antenna Q increases when the loss is reduced, and if the band required for the transmission signal is satisfied without detuning, the frequency change relative to the antenna bandwidth is relatively small. There has been a problem that the number of cells increases in practice, which is rather inconvenient in practical use.

[0005]

In view of this point, the present invention is applicable to any miniaturized antenna type, and detects the influence of a nearby object to automatically adjust the tuning frequency to a specified value. It has been made to provide an economical means.

[0006]

SUMMARY OF THE INVENTION The present invention provides an antenna in which the tuning frequency is varied by changing the bias voltage of a variable capacitance diode, and a circuit for generating a constant reference carrier from a reference signal generator and coupling the reference carrier to the antenna. And a carrier intensity detection circuit for detecting the intensity of the carrier amplified by the receiver via the antenna, and a function of recording a bias voltage at a tuning point at which the received carrier intensity becomes maximum when the bias voltage is swept. By detecting the influence of an object approaching the antenna due to a change in the antenna tuning point, the bias voltage of the variable capacitance diode is corrected so as to tune the tuning frequency of the antenna to the reference carrier or a frequency away from the reference carrier by a fixed value. It is characterized by being held.

When a reference carrier is created, it is preferable to use a harmonic of a crystal oscillator of a signal processing microprocessor of a radio as a signal source.

Further, the reference carrier is tuned so that the reference signal reception level is maximized by a periodic operation using a carrier matched with the reception signal frequency, and the carrier strength of the receiver or When the arrival of the normal transmission wave is detected based on a change in the demodulated signal of the receiver, the input of the reference carrier is preferably stopped.

Further, an antenna in which the tuning frequency is made variable by changing the bias voltage of the variable capacitance diode,
A carrier strength detection circuit for detecting carrier strength for a signal amplified by a receiver from a remote control transmitter via an antenna, and a judgment circuit for using the carrier strength as a reference signal for antenna tuning when the carrier strength is above a certain level And a function to record the bias voltage at the tuning point where the carrier intensity received when sweeping the bias voltage is maximized.The effect of an object approaching the antenna is detected by the change of the antenna tuning point, and the antenna tuning is performed. The bias voltage of the variable capacitance diode can be modified and held so that the frequency is tuned to the reference carrier or to a frequency separated by a certain value from the reference carrier.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is applicable to any miniaturized antenna type and provides an economical means for detecting the influence of a nearby object and automatically adjusting the tuning frequency to a specified value. Has been realized with a simple structure.

[0011]

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 1, a reception high-frequency module 14, and 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 a reception wave level.

The receiving high-frequency module 14 includes a low-noise amplifier, a local oscillator, a mixer, an intermediate frequency amplifier, and a demodulation circuit, similar to a normal receiver, and its functions are to amplify, detect, and demodulate a received radio signal. Then, the original transmission data is reproduced. In addition, the receiving high-frequency module 14 displays the radio signal strength indication (R
SSI) function. The control CPU 19 decodes the sender's ID code and command and outputs a control signal (Ctrl) to the terminal 29 for opening and closing the car door.

The antenna 1 is formed in a loop-like shape by a capacitor 4 inserted between a loop conductor 3 on a dielectric substrate 2 and conductor tips 6a and 6b, and a varicap 5 which is a variable capacitance diode using a barrier capacitance. Formed as a circuit. The operating frequency of the antenna is roughly determined by these circuit elements.

Next, the coupling capacitor 8 and the center conductor 9 form a matching circuit together with the antenna circuit element, and match the antenna impedance with the characteristic impedance of the feed line.

The matched antenna impedance is connected to the center conductor 11a of the coaxial cord 11 at the end of the center conductor 9. The outer conductor 11b of the coaxial conductor is grounded to the loop conductor base 10 where the high frequency voltage is the lowest on the loop conductor 3, and the coaxial cord 11
To prevent the high-frequency current from leaking to the outer conductor. The high-frequency signal from the coaxial cord 11 is transmitted to the input terminal 15 of the high-frequency receiving module 14 via the capacitor 13. 16 is a receiving harmonic module (RF-Mo)
14, a demodulated signal output terminal of the receiving harmonic module 14, and a signal output terminal of a radio frequency (signal) level detector of the receiving harmonic module 14. A DC bias voltage is applied to the varicap 5 of the antenna 1 via the center conductor 11a of the coaxial cord 11. Here, the resistors 7 and 12 for supplying a DC bias have a high resistance value of about 1 kΩ or more, so that a high-frequency current does not flow to the bias voltage supply side and no loss occurs. I do.

Here, FIG. 2 is an equivalent circuit of the antenna 1 of FIG. 1, and the principle of operation of the antenna will be described with reference to FIG. This antenna is based on Japanese Patent Application No.
This is an embodiment in which a balanced type loop antenna described in Japanese Patent No. -366229 is used as a basic type, and a varicap tuning function is given to this.

The 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 periphery of the opposed 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 and the conductor loss and the dielectric loss of the entire antenna circuit.

Since the small loop antenna has a small radiation resistance to the current flowing through the loop, the parallel resonance resistance Rsh of the equivalent circuit and the antenna impedance including the reactance component have a very large value.

FIG. 2B shows an equivalent circuit of a matching circuit for converting a high resistance into a low impedance.

This equivalent circuit is a circuit when the antenna side is viewed from the coaxial circuit connection point of the center conductor 9. The equivalent circuit of the antenna body shown in FIG. 2A has a high resistance, that is, a small conductance G1 at its resonance frequency.

Since G1 is smaller than the conductance (reciprocal of the characteristic impedance) of the power supply line, current does not flow much as it is. Therefore, the configuration of the matching circuit is as follows. First, the susceptance B1 of the capacitive element is inserted in parallel with G1, the current flowing therethrough is increased to lower the impedance, and the real part of the antenna impedance is converted to a desired low resistance. Matching is achieved by setting it to zero.

In practice, the susceptance B1 may be detuned to a higher frequency by reducing the antenna capacitance C1 of FIG. 2 (a) instead of adding a parallel susceptance element. It is equivalent to the one inserted in parallel. In FIG. 2B, C2 represents the capacitance of the coupling capacitor 8, L2 represents the inductance of the center 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 a combination of the detuning degree of the antenna main body and L2 and the coupling capacitors 8 and C2. In the above setting example, the reason that the series element is made capacitive is that a capacitor having a wide selection range of the element constant can be used. In some cases, the detuning can be reversed to make the element inductive.

In an experimental example in which a trimmer capacitor of 3 pPF is used instead of the varicap 5 for simplicity in the antenna having the type shown in FIGS. 1 and 2, the capacitance of the capacitor is changed after complete tuning. When the antenna is viewed from the feeder line, the return bandwidth of the antenna is 20 dB or less, and the tuning bandwidth is about ± 20 MHz around 315 MHz, thereby reducing the representative value of the antenna frequency change when an object approaches to -3 MHz. It turned out to be enough.

Even when an antenna different from that of the present embodiment is used, a tuning capacitor is generally used for a small antenna. Therefore, a part of the capacitor capacity is formed by a varicap. For example, any type of antenna can be varicap-tuned, and the present invention can be applied.

Next, a procedure for detecting a change in the antenna frequency due to an approaching object and automatically tuning in the wireless device of the present invention will be described.

The change in the resonance frequency of the antenna 1 when an object approaches approaches the object by sweeping the bias of the varicap 5,
The receiver's RF level detection signal for the reference carrier is examined to determine the change in tuning point relative to the reference carrier. 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.

In order to check the output of the radio signal strength indicator of the high-frequency receiving module 14 with respect to the signal from the reference signal generator 22, the reference voltage switch 27 selects the reference voltage V2 for level detection. This can be determined by using a carrier intensity detection circuit in which the voltage generated by the DA converter 26 is compared by the comparator 28. Here, the bias voltage of the varicap that maximizes the wireless intensity signal is recorded in its internal memory by the CPU 19. With this circuit function, the effect of an object approaching the antenna due to a change in the antenna tuning point is detected, and the varicap bias voltage is adjusted so that the tuning frequency of the antenna is tuned to the reference carrier or to a frequency away from the reference carrier by a fixed value. It can be modified and retained.

In this embodiment, a digital type circuit is used for the DA / AD converter, but these parts are replaced with other generally used circuit types, for example, an analog circuit using an integrating circuit. A smooth sweep voltage may be supplied.

The reference signal generator 22 has an inductance 23 and a capacitance 24 for a target harmonic of the crystal oscillator signal.
And the signal source impedance connected to the antenna 1 is high. Therefore, the voltage on the input terminal of the high-frequency module changes depending on the antenna impedance. That is, the maximum value is obtained at the time of tuning, and a short circuit is approached 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 via the center conductor 11a. A general PIN diode is used as an element, and ON / OFF control of the applied reference signal is performed by the bias voltage. Is made possible.

As another method of finding the resonance point, the frequency sweep is performed at high speed in an analog manner by using an integrating circuit, and the radio level detection signal is passed through a differentiating circuit. May be used.

The reference signal generator 22 of the present embodiment has a CPU
This is a circuit for deriving a signal from the voltage dividing capacitor 21 by using a crystal oscillator for use and extracting 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 = 15.9 MHz, and the fundamental frequency of the 19th harmonic is 31.
8 MHz is used as a reference carrier. Depending on the CPU, instead of directly providing the output of the crystal oscillator as a clock as in this embodiment, the frequency may be divided by 4 by a frequency divider and supplied to about 4 MHz.

The reason for making the fundamental frequency of the crystal relatively high is that
This is to limit the sweep width of the bias voltage so that one antenna tuning point appears, and at the same time, sufficiently cover a frequency change due to a nearby object. Also, the crystal oscillation frequency of the CPU should be such that the harmonics do not interfere with the reception frequency.

Next, it will be shown that the tuning control accuracy is not impaired even if the reference carrier does not match the prescribed reception frequency.

When the frequency changes by Δf from the original frequency f0 and the antenna capacitance changes by ΔC from C0, when the change is small, the change in the resonance frequency can be expressed by the following approximate expression. Δf / f0 = −ΔC / (2C0) The frequency change due to a nearby object can be expressed on an equivalent circuit by inserting the equivalent capacitance change ΔC calculated from the above equation into C0 in parallel. Here, it is assumed that ΔC0 is a change in capacitance when measured using the normal reception frequency.

Next, using the reference carrier shifted by Δft from the transmission signal, the antenna is tuned to this reference carrier at C1, and the value of ΔC1 of the equivalent capacitance change detected using the reference carrier is calculated. And compare it to the correct value above. That is, C1 = C0 (1-2Δft / f0) Δf1 / f1 = −ΔC0 / C1 = −ΔC0 / (2C0)
・ (1 + 2Δft / f0), the resonance accuracy of the antenna is examined, and the measurement accuracy when detecting the capacitance change ΔC0 from the original tuning capacitance is determined by the influence of the second-order minute amount on the small change in the test frequency. Has no effect. For example, when the frequency change due to a nearby object is 1% and the deviation of the signal of the reference carrier is 1%, the measured capacitance change is 0.0204C0 against the true value of 0.02C0.
C0.

It is apparent from the above-described calculation results that the error in the capacitance change amount when the influence of a nearby object is compensated for by the reference carrier is very small even if the capacitance change amount for correction when receiving a normal signal is used. .

On the other hand, for more accurate control, an accurate capacitance value change amount at the normal reception frequency is converted from a change in the tuning capacitance determined by the set test frequency using the above relational expression. It is also possible to ask for it.

In this system, the control is performed by the bias voltage of the varicap, so that the conversion of the bias voltage into a capacitance value is basically performed using a conversion table.
However, a small change is almost linear, so conversion is easy in most cases, and actual control can be easily performed.

In the case where the harmonic of the crystal oscillator is used as a reference carrier, harmonics are generated at a constant frequency interval. Therefore, by selecting the oscillation frequency of the crystal oscillator, a plurality of harmonics can be set within the tuning frequency range of the varicap antenna. Can appear. If these are used for markers,
Individual differences in antenna tuning characteristics mainly related to bias voltage versus capacitance value of the varicap can be calibrated, and the reliability of the system can be improved.

As another embodiment, the addition of a component for generating a reference carrier having the same frequency as the transmission frequency simplifies tuning control.

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 to be received. It is possible to enhance responsiveness. When a method is adopted in which the reference carrier is matched with the transmitted wave, the change in the carrier strength or the change in the received demodulated signal is judged, and when the arrival of the normal transmitted wave is detected, the reference carrier should be cut off immediately. do it.

Further, as another embodiment (claim 4), it is possible to omit the reference signal generator 22 and provide an automatic tuning function. In this case, since the signal wave from the remote control transmitter is used as it is as a reference signal for antenna tuning, the carrier intensity detection circuit and the CPU determine that the signal is equal to or greater than a predetermined value (determination circuit). The antenna is tuned to the incoming wave according to the tuning mode sequence. At this time, the antenna of the transmitter placed opposite to the antenna and the space between them act as the reference signal generator 22 of FIG.

In the case of this system, the output from the radio frequency level detector of the receiver must be sufficiently higher than the noise level so that the change can be measured accurately. 1m
Since the transmitted wave can be sent so that the carrier strength becomes extremely strong at a position very close within
The transmission wave level is high, and the timing for setting the automatic tuning mode can be easily determined. Then, the receiving high-frequency module 14 is set to have the original function of fixing the bias voltage of the variable capacitance diode, the standard mode for performing reception and demodulation, and the bias voltage at which the radio signal level detected by sweeping the bias voltage is maximized. It is only necessary to be able to switch to the sweep mode that finds.

As a procedure for updating the bias voltage using the tuning function, first, the keyless remote control transmitter transmits a signal wave a plurality of times for one control command. Then, the receiver is normally operated in the standard mode. When the receiver detects the unlock signal of the door, the receiver is unlocked and the receiver is set in the sweep mode for a certain period of time. When the optimum bias voltage is found from the arriving wave in the sweep mode, the bias voltage is temporarily set. When the next arriving signal wave is correctly demodulated by the updated bias voltage, the temporarily set voltage is held and fixed, and when the demodulation by the updated bias voltage is not performed correctly, the original Return to the bias voltage. In this case, the operator sends an unlock signal 1 near the car door.
It is required to transmit once or twice, so that the receiving antenna can be kept tuned for best reception.

Note that the update procedure is not limited to the above, and can be set in accordance with various control sequences in consideration of reduction in processing time, easiness of operation, and reliability of operation.

[0046]

As described above, the automatic tuning antenna system of the present invention detects the influence of an object approaching the antenna by changing the antenna tuning point, and adjusts the tuning frequency of the antenna from the reference carrier or the reference carrier. Since the bias voltage of the variable capacitance diode is modified and held so as to tune to a frequency separated by a certain value,
It can be easily applied to small antennas of radio equipment, and can be used for keyless entry systems and high sensitivity and high Q for general mobile communication.
It is possible to prevent a decrease in sensitivity due to detuning by a nearby object in the antenna.

Further, it is possible to eliminate the trouble of readjustment at the time of installing the wireless unit, to improve and stabilize the performance of the wireless transmission system, and to relieve a decrease in sensitivity due to a temporal change of the high Q antenna.

Further, harmonics of a crystal oscillator for CPU, etc.
If an existing signal source or a signal source that can be easily synthesized therefrom is used, the increase in components and power consumption can be minimized.

Further, when the carrier intensity from the remote control transmitter becomes equal to or higher than a certain level, if the antenna is tuned using this carrier as a reference carrier, a reference signal generator is not required and a simple structure can be achieved. it can.

[Brief description of the drawings]

FIG. 1 is a perspective view of an antenna unit and a block diagram of an automatic tuning antenna system according to an embodiment of the present invention.

2A and 2B are equivalent circuits of an antenna unit according to one embodiment of the present invention, in which FIG. 2A is an equivalent circuit of a loop antenna viewed from conductor tips 6a and 6b, and FIG. 2B is a matching circuit that converts high resistance to low impedance. Is an equivalent circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Antenna 5 Varicap (variable capacity diode) 11 Coaxial code 14 High frequency module for reception 19 CPU 22 Reference signal generator 26 DA converter 28 Comparator

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5K062 AA01 AB03 AB10 AB14 AC02 AD04 AE02 AE06 AG01 BA06 BB13 BD02 BE06 BE12

Claims (4)

[Claims] An antenna in which a tuning frequency is varied by changing a bias voltage of a variable capacitance diode, a circuit for generating a fixed reference carrier from a reference signal generator and coupling to the antenna, and a receiver via the antenna A carrier intensity detection circuit for detecting the carrier intensity amplified by the step (a), and a function of recording a bias voltage at a tuning point at which the carrier intensity received when the bias voltage is swept is maximized. The bias voltage of the variable capacitance diode is corrected and held so as to detect the influence of an object approaching the antenna and tune the tuning frequency of the antenna to the reference carrier or to a frequency away from the reference carrier by a certain value. Automatic tuning antenna system. 2. The automatic tuned antenna system according to claim 1, wherein when the reference carrier is created, a harmonic of a crystal oscillator of a signal processing microprocessor of the wireless device is used as a signal source. 3. The reference carrier is tuned so that the reference signal reception level is maximized in a periodic operation using a frequency matched to the received signal frequency, and the carrier intensity or the carrier intensity of the receiver is adjusted. 2. The automatic tuned antenna system according to claim 1, wherein the input of the reference carrier is stopped when the arrival of the normal transmission wave is detected based on a change in the demodulated signal of the receiver. 4. An antenna in which a tuning frequency is varied by changing a bias voltage of a variable capacitance diode, and a carrier intensity detecting circuit for detecting carrier intensity for a signal amplified by a receiver from a remote control transmitter via the antenna. A determination circuit that uses the carrier intensity as a reference signal for antenna tuning when the carrier intensity is equal to or higher than a certain level, and a function of recording a bias voltage at a tuning point at which the received carrier intensity becomes maximum when the bias voltage is swept. With
By detecting the influence of an object approaching the antenna by changing the antenna tuning point, correcting the bias voltage of the variable capacitance diode so as to tune the tuning frequency of the antenna to the reference carrier or a frequency away from the reference carrier by a fixed value. An automatic tuning antenna system characterized by holding.