Saw oscillator with antenna

An oscillating circuit includes a resonator for outputting a signal, an amplifier for amplifying the signal output from the resonator, and a radiator for transmitting a radio wave by resonating the signal amplified by the amplifier. The amplifier has a transistor connected to the resonator and a fixed inductor applying an alternating current load to the transistor. The transistor outputs a signal to be adjusted by the fixed inductor to finely adjust the frequency of the radio wave.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an oscillating circuit. More particularly, 
the invention relates to an oscillating circuit which improves the 
stability characteristics of the oscillating output signal. Applications 
of such an oscillator includes incorporation of the oscillator in radio 
wave key systems used for the remote control of automobile equipment. 
2. Description of the Related Art 
Recently, the use of radio systems utilizing radio waves for short distance 
transmission of information has increased dramatically. One example of 
this in the field of automobile related technology is the so-called "radio 
wave key system". Such systems typically operate by emitting a radio wave 
in the ultra high frequency (UHF) band from a transmitter incorporated in 
the ignition key. An automobile installed receiver then receives the radio 
wave and enables the control of such operations as, for example, door 
locking, engine starting, etc., at a location remote from the automobile. 
The transmitters of these types of radio wave key systems typically 
incorporate an oscillator 52 that uses a crystal resonator 51 as shown in 
FIG. 3. The oscillator 52 oscillates the radio wave of several tens of 
MHz. A frequency multiplier 53 multiplies the oscillating frequency of the 
radio wave to obtain a desired high frequency (several hundreds of MHz). 
The high frequency thus obtained is amplified by an amplifier 54 in order 
to radiate it from an antenna 55. 
Since the transmitter used in a radio wave key system is designed to be 
incorporated in the ignition key, it is desirable to keep the size of the 
transmitter as small as possible. More particularly, the number of the 
transmitter's component parts should be minimized and the structure of 
circuits should be kept simple in order to minimize the costs associated 
with the transmitter's manufacture. One response to the need for such a 
transmitter has in the past been the production of oscillators that 
utilize a surface acoustic wave resonator (hereinafter abbreviates as SAW 
resonator). SAW resonators have the advantage of small size and low cost 
over conventional electromagnetic structure. 
The one port SAW resonator shown in FIG. 5 comprises a comb-shaped 
electrode 20 formed by a thin metallic film on a substrate made of a 
piezoelectric material, and a pair of grating reflectors 21 and 22 
positioned on both sides of the electrode. An external terminal 23 is 
connected to the comb-shaped electrode 20. The electric signals input to 
the comb-shaped electrode 20 through the external terminal 23 are excited 
by the comb-shaped electrode 20, and transformed into elastic surface 
waves, and propagated along the surface of the substrate. Accordingly, the 
elastic surface waves are reflected by the grating reflectors 21 and 22 to 
generate the standing waves from the elastic surface waves. The standing 
waves are again converted by the comb-shaped electrode 20 into electric 
signals. In this way, it is possible to acquire sharp resonant 
characteristics for the resonator. 
In oscillators utilizing SAW type resonators, a simple one transistor 
circuit may be used to oscillate a radio wave at a frequency of more than 
several hundreds of MHz. One advantage of using this type of one 
transistor circuit, is that the frequency multiplier 53, like that shown 
in FIG. 3, can be eliminated. As a result, it is possible to design a low 
cost extremely small transmitter by use of the oscillator which 
incorporates a SAW type resonator. 
FIG. 4 is a view showing a conventional oscillator which uses a SAW 
resonator. The conventional oscillator 61 comprises an resonator SAW, a 
high frequency NPN transistor Tr, resistors R1 to R3, a variable capacitor 
(trimmer) C1, a fixed capacitor C2, and an antenna ANT. 
The transistor Tr is grounded through the emitter resistor R3, and is 
biased at its base by means of resistors R1 and R2. The antenna ANT is 
connected to the collector side of the transistor Tr. When the collector 
current of the transistor Tr flows through the antenna ANT, radio waves 
are radiated from the antenna. This antenna ANT is often formed on a 
printed circuit board as a printed pattern. The capacitor C1 feeds back 
the emitter potential of the transistor Tr to the base. This design allows 
fine adjustments to be made to the oscillating frequency by varying the 
capacitance the capacitor C1. 
The capacitor C2 feeds back the collector potential of the transistor Tr to 
the emitter. The inductance of the antenna ANT serves as alternating 
current load, to fluctuate the collector potential of the transistor Tr, 
assuming that the transistor Tr forms a common emitter grounded amplifier. 
This fluctuation is fed back to the emitter through the capacitor C2. 
Consequently, in order to achieve a stable feedback operation, it is 
necessary to acquire optimal antenna inductance as the AC load. 
Now, the oscillator 61 tends to generate harmonics (secondary harmonics) 
at, two times that of the fundamental wave's desired oscillating 
frequency. The high level of the secondary harmonic may in fact create a 
problem, especially given regulations of some governmental agencies. For 
example, Japanese Radio Law requires that when a particular transmitter 
generates a signal at a frequency in excess of 320 MHz, the field 
intensity generated thereby should undergo a rapid decrease. 
For example, if the fundamental wave is set at 160 MHz or more, the 
secondary harmonics (320 MHz) would not be within the regulated range of 
the field intensity in some cases. In particular, should a fundamental 
wave at approximately 300 MHz be transmitted from a radio wave key system, 
the secondary harmonic (at approximately 600 MHz) would most likely exceed 
the regulated field intensity. 
Also, as described above, it is necessary to acquire optimal antenna 
inductance for the conventional oscillator 61. Consequently, the design of 
the oscillator 61 should allow the oscillating frequency to be fine tuned. 
Antennas of the type described above, however, are usually arranged on a 
printed circuit board as a printed pattern. Consequently, it is impossible 
to modify the inductance once the printed circuit board is completed. One 
solution to this is to use a plurality of printed circuit boards each 
containing an antenna ANT having a different inductance. The circuit 
boards could then be prepared before assembling the oscillator. This, 
unfortunately, takes a great deal of time and work. Moreover, for those 
antennas which are arranged in patterns on printed circuit boards the 
actual inductance value of the antenna ANT is restricted by the 
configuration and size of the printed circuit board to be used. As a 
result, it is oftentimes difficult to design an antenna that can serve as 
an AC load and that provides for the fine tuning adjustment of the 
oscillator's oscillating frequency. 
SUMMARY OF THE INVENTION 
The present invention is designed to solve the above-mentioned problems. It 
is an object of the invention to provide an oscillator having very stable 
characteristics, which produces smaller harmonics, is easy to design and 
whose oscillating frequency can be fine tuned. 
In order to achieve the above-mentioned object, an oscillator of the 
present invention is arranged wherein a resonator transmits electric 
signals. The signals are amplified by an amplifier and supplied to a 
radiator. The radiator outputs radio waves by resonating the signals from 
the amplifier. The amplifier is provided with a transistor coupled to the 
resonator, and a fixed inductor which provides an alternating current load 
for the transistor. The frequency of radio waves can be finely adjusted by 
using the fixed inductor to adjust the output of the transistor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A radio wave key system for an automobile according to one embodiment of 
the present invention will now be described with reference to FIG. 1. 
An oscillator 11 of the present invention comprises a Surface-Acoustic-Wave 
(SAW) resonator SAW, a high frequency NPN transistor Tr, resistors R1 to 
R3, a variable capacitor (trimmer) Cl, fixed capacitors C2 to C4, a fixed 
inductor L, and an antenna ANT. The resonator SAW of the present 
embodiment is of the same structure as the one shown in FIG. 5. 
The combination of the resonator SAW, transistor Tr, resistors R1 to R3, 
variable capacitor C1, fixed capacitor C2, and fixed inductor L form a 
transmitter O. The fixed capacitors C3 and C4, and antenna ANT constitute 
a radiator E. All of the elements of the transmitter O, excepting the 
resonator SAW, form an amplifier 20. Further, the capacitor C4 and antenna 
ANT constitute a resonator circuit R. 
The transistor Tr is grounded through the emitter resistor R3, and is 
biased at its base by the resistors R1 and R2. A power supply B is 
connected to the base of the transistor Tr through the resistor R1. The 
antenna ANT, formed as a pattern on a printed circuit board, is coupled to 
the emitter of the transistor Tr by means of the capacitor C3. The antenna 
ANT radiates radio waves when the emitter current of the transistor Tr 
flows through the antenna ANT. 
The coupling of capacitor C1 to the transistor Tr provides the transistor's 
emitter potential as feedback to the base of the transistor Tr. Thus by 
varying the capacitance of the capacitor C1, it is possible to fine tune 
the oscillating frequency. Moreover, the capacitor C2 feeds back the 
collector potential of the transistor Tr to its emitter. 
The oscillator 11 of the present embodiment differs from the conventional 
oscillator 61 in the following items (1) and (2). 
The first item (1) is that instead of the antenna ANT being directly 
coupled to the transistor's collector, as in the conventional oscillator 
61, a fixed inductor L is inserted on the collector side of the transistor 
Tr. 
Assuming that the transistor Tr constitutes a typical grounded emitter 
amplifier, the fixed inductor L can be used to supply an alternating 
current load to the amplifier. A further distinguishing advantage of using 
the fixed inductor L is that it provides a series inductance to the 
resonator SAW. The specific use of the fixed inductor L and variable 
capacitor C1 as described provides improved fine tuning adjustment 
capability to the oscillator 11. 
This advantage is enhanced by the fact that it is possible to use an 
inductor widely available on the market (a chip inductor, for instance) 
for the fixed inductor L. As a result, the inductance can be modified 
easily even after the completion of the printed circuit board. As compared 
with the conventional oscillator 61 provided with the antenna ANT formed 
on the printed circuit board, design freedom is enhanced according to the 
present invention of the oscillator 11. 
In addition, the installation of the fixed inductor L as contemplated by 
the present invention is not restricted by the configuration or size of 
the printed circuit board. Consequently, it is easier to design the 
inductor L to function as the AC load. This enhances oscillation 
stability. 
The second item (2) in which the present embodiment differs from 
conventional oscillator designs is that the oscillating output is sourced 
from the collector of the transistor Tr through the capacitors C2 and C3, 
i.e., to the resonator R. It is thereby possible to significantly reduce 
the secondary harmonics radiating from the antenna ANT by adjusting the 
resonant frequency of the resonator R to coincide with that of the 
fundamental wave of the oscillator 11. Contributing to this reduction is 
the fact that the antenna ANT is formed as a pattern on the printed 
circuit board. Moreover, the capacitor C3 is designed with a sufficiently 
small capacitance in order to minimize its influence on the oscillating 
output of the oscillator 11. 
An additional benefit of incorporating fixed inductor L and capacitors C3, 
C4 is that the size of the oscillator circuit 11 need not be increased by 
any significant amount in order to achieve the advantages as described in 
items (1) and (2). Likewise, increases if any to the manufacturing costs 
of the present invention over that of conventional oscillators is vastly 
outweighed by the enhancement in the performance characteristics of the 
present invention. The advantage provided by conventional oscillators 
respecting their use of SAW type resonators, i.e., making extremely small 
transmitters, is enhanced by the present invention. 
As described in the item (1) above, since oscillation stability is 
enhanced, it is possible to adjust the variable range of the oscillating 
frequency. Also, as described in the item (2) above, the secondary 
harmonics radiating from the antenna ANT can be significantly reduced. 
Consequently, according to the oscillator 11 of the present invention, it 
is possible to keep the secondary harmonics (320 MHz or more) within the 
range of the regulated field intensity even when the fundamental wave is 
set at 160 MHz or more. For example, in a transmitter of a radio wave key 
system, the secondary harmonics (approximately 600 MHz) can be easily 
suppressed to be within the range of the regulated field intensity if the 
oscillator 11 of the present invention is used even when the fundamental 
wave is set at approximately 300 MHz. 
Moreover, the structure as described in the items (1) and (2) above, allows 
the oscillator O and the radiator E to be formed as separate components. 
This allows even further flexibility in the design and manufacture of the 
oscillator 11. 
It should be noted that the present invention is not limited to the 
above-mentioned embodiments, and it may be implemented as follows: 
Rather than providing the oscillating output directly from the emitter, the 
output can be provided indirectly from the emitter of the transistor Tr 
through the capacitors C2 and C3 as in FIG. 2. To achieve the same 
enhancements, as described above in the first embodiment, the capacitor C3 
of this embodiment should have a smaller capacitance than it has in the 
first embodiment. Minimizing the capacitance of C3 in this way reduces any 
interference produced by C3 on the oscillatory output. 
In place of the resonator SAW, it may be possible to use a two port 
resonator SAW2 as shown in FIG. 6. The two port resonator SAW2 comprises a 
pair of comb-shaped electrodes 31 and 32 connected in parallel to the 
external terminal 30, and a pair of grating reflectors 33 and 34 each 
arranged outside the comb-shaped electrodes 31 and 32, respectively. In 
general, however, it is easier for the single port resonators to enhance 
the stability of an oscillatory signal due to the fact that resonance 
deterioration or loss is less than that with two port resonators. One port 
resonators, moreover are capable of functioning with less feedback. 
The present invention may further be designed with power supplied to the 
fixed inductor L and the resistor R1 from different power supplies, 
respectively. Also, the resonator R comprising the capacitor C4 and 
antenna ANT may be replaced with a resonator formed independently of the 
antenna ANT. 
Accordingly, the present examples and embodiments are to be considered as 
illustrative and not restrictive and the invention is not to be limited to 
the details given herein, but may be modified within the scope of the 
appended claims.