A device incorporating a tunable thin film bulk acoustic resonator for performing amplitude and phase modulation

Abstract: A Bulk Acoustic Wave (BAW) resonator (20) is provided which comprises a piezoelectric layer (22); a first and second protective layer (38b,a), a first electrode (24), a second electrode (26), a bridge (also referred to as a "membrane" (28), a pair of etch windows (40a,b), an air gap (34), and a substrate (36). A portion of the piezoelectric layer (22) is positioned atop the first electrode (24), and the second electrode (26) is positioned atop the piezoelectric layer (22); thereby forming a parallel plate structure between which the piezoelectric layer (22) is allowed to resonate or vibrate. The piezoelectric layer (22) comprises, by example, zinc-oxide (ZnO), and has a thickness of 1.7 µm. The electrodes (24,26) comprise, by example, gold (Au) and have thicknesses of 0.1 µm. The membrane (28) comprises two layers (30,32) namely a top layer (30) and a bottom layer (32). The top layer (30), which preferably has a thickness of 0.6 µm and comprises poly-silicon, has a top surface which is in contact with the first electrode (24) and portions of the piezoelectric layer (22). The top layer (30) is situated atop a portion of the bottom layer (32), which preferably has a thickness of 0.4 µm and is comprised of silicon-dioxide (SiO 2 ). A portion of a bottom surface of the membrane (28) is situated adjacent to the air gap (34), which separates this portion of the membrane's bottom surface from a portion of the substrate (36). This air gap (34), which is typically filled with air but any suitable material may be used, is formed by etching a portion of the substrate (36). The air gap (34) is bounded by the etch windows (40a,b), by a portion of the first protective layer (38b), by the bottom surface portion of the membrane (28), and by inner faces (36b,c,d) of the substrate (36). The air gap (34) functions to isolate acoustic vibrations created by the piezoelectric layer (22) from the substrate (36).

Description: The present invention relates to phase modulator circuits and methods for phase modulating a signal. In accordance with the invention, circuits for phase modulating RF carrier signals are provided. A first circuit may include a bulk acoustic wave (BAW) resonator connected in parallel with two circuit branches. A first one of the branches may comprise a first generator for generating an RF carrier signal, a bandpass filter, and a low frequency blocking capacitor. The second branch may comprise a second generator for generating a modulating signal, a low pass filter, and an RF-choke. The resonator may have parallel and series resonances which shift in frequency in response to an applied low frequency signal. Although outside the scope of the claims, the circuit may be employed to amplitude modulate an RF signal, in which case the circuit operates as follows. The generator generates an RF carrier signal having a frequency that is preferably within a frequency range in which the resonator exhibits a minimal phase shift response. After the RF carrier signal is generated, it is filtered by the bandpass filter, and is then applied to a low frequency blocking capacitor, which blocks any attendant DC signals. The second generator, which is a variable voltage source, generates a low frequency, DC modulating signal having a voltage which varies over time. This signal is then filtered by the lowpass filter, and is thereafter applied to an RF-choke which blocks any attendant RF signals. The RF-choke then provides the low frequency signal to the remaining portion of the circuit comprising the resonator. As the low frequency signal traverses the remaining portion of the circuit, the resonator has a time-varying voltage applied across its electrodes as a result of the signal. The voltage creates a varying electric field within a piezoelectric material located between the electrodes. The electric field causes the piezoelectric material to compress or expand depending on the polarity of the voltage, causing a time-varying frequency shift of the series and parallel resonant frequencies and of the impedance exhibited by the resonator. The shifts cause the strength of the RF carrier signal to be attenuated by varying amounts over time (i.e., the signal experiences a time-dependent changing insertion loss), resulting in an amplitude modulation of the carrier signal. A second circuit comprises similar elements to those of the first circuit, except that the resonator is series-connected within the circuit, and an additional coil (e.g., an RF-choke which provides grounding for the low frequency signal) is included within the circuit. The second circuit also amplitude modulates signals. Within the scope of the invention, the circuits may operate in a manner similar to that described above in connection with amplitude modulation. However, for the first circuit, the RF carrier is generated to have a frequency which preferably is approximately equal to the parallel resonant frequency of the parallel-connected resonator. This frequency is one at which the resonator may yield its maximum phase shift response as a result of the effect of the modulating low frequency signal on the resonator. As such, an application of the carrier signal to the resonator causes a phase shift to occur to the signal. Similarly, for the second circuit, wherein the resonator is connected in series, an RF signal is generated which has a frequency that preferably is approximately equal to the series resonant frequency of the resonator. This is the frequency at which the resonator may yield a maximum phase shift in response to the modulating low frequency signal. In this manner an application of the RF carrier signal to the resonator to cause a phase shift to occur to the carrier signal. The invention provides phase modulator circuits as claimed in claims 1 and 5. The invention also provides methods for phase modulating signals as claimed in claims 3 and 7. The above set forth and other features of the invention are made more apparent in the ensuing Detailed Description of the Invention when read in conjunction with the attached Drawings, wherein: Figure 1a illustrates a circuit incorporating a parallel-connected Bulk Acoustic Wave (BAW) resonator for amplitude modulating and phase modulating signals. Figure 1b illustrates a circuit including a series-connected BAW resonator for amplitude modulating and phase modulating signals. Figure 2a shows a frequency response of a two port BAW resonator which is connected in parallel. Figure 2b illustrates a frequency response of a two port BAW resonator which is connected in parallel. Figure 2c illustrates a frequency response of a two port series-connected BAW resonator. Figure 2d illustrates a frequency response of a two port series-connected BAW resonator. Figure 3a illustrates a frequency response of a series-connected resonator in response to applied DC-bias voltages of +28V, OV, and -28V. Figure 3b illustrates a measured spectrum of the resonator of Figure 14a in response to an RF carrier signal having a frequency of 951 MHz and a low frequency modulating signal having a frequency of 600KHz and an amplitude of 8 Vpp. The inventor has determined that when a frequency signal (e.g., in the KHz to MHz range) signal is applied to a bulk acoustic wave (BAW) resonator, the resonator may be utilized to amplitude modulate or phase modulate higher frequency RF carrier signals of interest. This can be appreciated in view of Figure 1a, which illustrates a device, namely a circuit 91 incorporating a BAW resonator 102 for one of amplitude modulating on phase modulating an RF carrier signal. Within the circuit 91, the resonator 102 is connected in parallel with two circuit branches. A first one of the branches comprises a generator 90 for generating an RF carrier signal, a bandpass filter 92, and a low frequency blocking capacitor 94. The second branch comprises a generator 96 for generating a modulating low frequency signal, a low pass filter 98, and an RF-choke 100. The resonator 102 may be any known type of resonator. In an exemplary application in which the circuit 91 is being employed to amplitude modulate an RF signal, the circuit 91 operates in the following manner. Generator 96 generates a modulating low frequency signal which varies within a predetermined voltage range (e.g., between -30 volts, 0 volts, and +30 volts) over a specified time period. After the low frequency signal is produced, it is filtered by lowpass filter 98, and is thereafter applied to the RF-choke 100 which blocks RF signals. The RF-choke 100 then provides the low frequency signal to the remaining portion of circuit 91 comprising resonator 102. As the low frequency signal passes through the remaining portion of circuit 91, the resonator 102 has a voltage applied across its electrodes 102a and 102b as a result of the signal. The voltage creates an electric field within piezoelectric material 102c between the electrodes 102a and 102b of the resonator 102. The electric field magnitude varies as a function of the time-varying voltage applied to the resonator 102 in response to the low frequency signal. The electric field effects the piezoelectric material 102c in a manner similar to that described above, causing it to vibrate, which causes a frequency shift of the resonator's series and parallel resonant frequencies and a shift of the resonator's impedance. The amounts by which the respective series and parallel resonances and the impedance are shifted are a function of the voltage variation of the low frequency signal and of the electric field variation. The bandwidth bounded by the parallel and series resonant frequencies remains substantially constant during the frequency shifts. The generator 90 generates an RF carrier signal having a frequency that is between the series and parallel resonant frequencies exhibited by the resonator 102 in response to the applied modulating low frequency signal. Preferably, the generated RF carrier signal has a frequency which is within a frequency range in which the resonator exhibits a minimum phase shift in response to the applied modulating low frequency signal. By example, for a resonator having a frequency response such as the one shown in Figure 2b, which will be described below in more detail, a minimum phase shift occurs at approximately 963MHz. Similarly, for a resonator having a frequency response as that illustrated in Figure 2d, which also will be described below, a minimum phase shift occurs at approximately 978MHz. Thus, ideal carrier signal frequencies for these cases are approximately 963MHz and 978MHz, respectively. After the RF carrier signal is generated by generator 90, it is filtered by the bandpass filter 92, and is then applied to low frequency blocking capacitor 94 which blocks any attendant low frequency signals. Thereafter, the RF carrier signal is supplied to the remaining portion of the circuit 91 comprising the resonator 102. The shifting of the impedance and of the series and parallel resonant frequencies exhibited by the resonator 102 in response to the modulating low frequency signal cause a portion of the RF carrier signal traversing the circuit 91 between node 95 and output node 96 to pass through resonator 102. The amount of this portion of the RF carrier signal depends upon the varying impedances and resonant frequencies exhibited by the resonator 102. As a result, the strength of the RF carrier signal travelling between node 95 and output node 96 is attenuated by time-varying amounts which are a function of the voltage variation of the low frequency signal (i.e., the signal experiences a time-dependent, varying amount of insertion loss), thus resulting in an amplitude modulation of the carrier signal. By example, in an exemplary case wherein a resonator having a frequency response such as that of Figure 2b is employed for performing an amplitude modulation of a 963MHz carrier signal via a 600kHz modulating signal having voltages varying between -V, 0V and +V, the carrier signal experiences a time-varying insertion loss of approximately + 2dB (see discussion of Figure 2b below). Circuit 93 illustrated in Figure 1b shows another embodiment of a circuit for performing amplitude modulation. This circuit 93 comprises similar elements to those of circuit 91, except that resonator 102' is series-connected within the circuit 93, and a coil 104 is shown connected to ground. Similar to the operation of circuit 91, the generator 96 generates a low frequency modulating signal and generator 90 generates an RF carrier signal having a frequency that is between the series and parallel resonant frequencies of the resonator 102'. The low frequency signal is then applied to elements 98 and 100, and the RF carrier signal is applied to elements 92 and 94, which perform in the same manner as described above. The low frequency signal is then applied to resonator 102' which, in response thereto, experiences an electric field between electrodes 102a' and 102b' within piezoelectric material 102c'. The electric field causes an impedance shift and a shift of the series and parallel resonant frequencies of the resonator 102' as a function of the voltage variation of the low frequency signal, in a similar manner as described above. As such, after the RF carrier signal traverses circuit 93 from nodes 95 to resonator 102' and is applied to the resonator 102', the signal then experiences a varying, time-dependent attenuation, and becomes an amplitude modulated signal. The amplitude modulated signal is then supplied to output node 96. The low frequency signal is directed to a low frequency ground via coil 104. As described above, the circuit 91 of Figure 1a may also be employed to phase modulate a carrier signal of interest. For this application, the circuit 91 is operated in a manner similar to that described above, except that the generator 90 generates a carrier signal having a frequency that is preferably approximately equal to the parallel resonant frequency of the shunt-connected resonator 102. This is the frequency of which the resonator 102 resonates and yields its maximum phase shift response as a result of the modulating low frequency signal being applied to the resonator 102. As such, an application of a signal having this frequency to the resonator 102 results in a phase shift occurring to the signal by an amount which is a function of the modulating low frequency signal voltage variation. By example, for an exemplary case in which the resonator 102 has frequency response curves like those shown in Figure 13b, and wherein the low frequency signal applied to the resonator 102 has a voltage which varies between -V, 0V, and +V, a phase shift of approximately ± 28 degrees occurs to a carrier signal having a frequency of approximately 977.5MHz. Circuit 93 of Figure 1b may also be employed to phase modulate a carrier signal of interest. For this application, the circuit 93 is operated in a manner similar to that described above except that generator 90 generates a carrier signal having a frequency that is preferably approximately equal to the series resonant frequency of the series-connected resonator 102'. This is the frequency of which the resonator 102' resonates and yields its maximum phase shift response as a result of the effect of the low frequency signal on the resonator 102'. As such, an application of the carrier signal to resonator 102' causes a phase shift to occur to the signal by an amount which is a function of the modulating low frequency signal voltage variation. The phase shifted carrier signal is then supplied to output node 96, and the low frequency signal is directed to a low frequency ground via coil 104. It should be noted that whether circuit 91 or circuit 93 is best suited for a particular application depends upon the impedances of the elements forming the respective circuits. By example, the shunt connected resonator configuration (Figure 1a) performs most effectively for a high impedance circuit. The BAW resonators of this invention can perform amplitude or phase modulation to RF carrier signals having frequencies in the range from approximately 500MHz to at least 3GHZ, and can operate at an RF power level which is at least ±30dBm. Moreover, the resonators can perform these modulations in response to low frequency modulating signals having voltages of at least ±30 volts and frequencies within the KHz to MHz range. Also, at least a 5% amplitude modulation of a carrier signal has been observed in an application employing a +28 volt, 600KHz modulating low frequency signal. However, at least a 30% amplitude modulation of an RF carrier signal is achievable. The invention can also phase shift a carrier signal by at least one degree per volt of the modulating low frequency signal. Figures 2a-2d illustrate frequency responses for various exemplary BAW resonators connected either in series or in parallel, and having two ports of specified impedances. For each Figure, the three gain response curves represent the amplitude versus frequency responses to negative, zero, and positive bias voltages, respectively. Similarly, the three phase response curves represent the phase versus frequency responses to negative, zero, and positive bias voltages, respectively. Ideal carrier signal frequencies for achieving amplitude modulation and phase modulation for the particular resonators are also indicated. Referring to Figure 2a, frequency response curves of an exemplary two port BAW resonator which is shunt-connected are illustrated. The ports have high impedances of 1k0hm. For this example, the resonator has a layer of ZnO that is 2060nm thick, a layer of SiO2 that is 400nm thick, and lateral dimensions of 330mm x 330mm. Figure 2b illustrates a frequency response of a two port BAW resonator which is shunt-connected. The ports have high impedances (e.g., 1kOhm). For this example, the resonator has a layer of ZnO that is 1650mm thick, a layer of SiO2 that is 1140nm thick, and lateral dimensions of 330mm x 330mm. Also, as described above, the application of a low frequency (e.g., 600kHz) signal to the resonator causes a time-varying insertion loss of approximately ±2dB to be experienced by a 963MHz carrier signal. Figure 2c illustrates a frequency response of a two port BAW resonator which is series-connected. The ports have low impedances (e.g., 20 Ohm). For this example, the resonator has an area of 170mm x 170mm, and has a high impedance. Figure 2d illustrates a frequency response of an exemplary two port BAW resonator which is series-connected. The ports have low impedances (e.g., 50 Ohm). For this example, the resonator has an area of 232mm x 232mm, corresponding to an impedance of approximately 50 Ohms. Also, for this example, an application of a low frequency (e.g., 600KHz) signal to the resonator causes a time dependent insertion loss of approximately +3dB to be experienced by a carrier signal having a frequency of 978MHz. Figure 3a illustrates a frequency response of a series-connected resonator in response to an applied DC-bias voltage of +28V, OV, and -28V. Figure 3b illustrates a measured gain response of the resonator in response to an RF carrier signal having a frequency of 951MHz and a low frequency modulating signal having a frequency of 600KHz and an amplitude of 8 Vpp. Sideband spikes occur at 600kHz as shown in Figure 3b. While the invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope of the invention.

Claims: A phase modulator circuit, comprising: a variable voltage signal generator (96) for generating a low frequency signal having a time-varying voltage; a tunable resonator (102), said tunable resonator yielding a maximum phase shift at a parallel resonant frequency in response to a modulating low frequency signal output by said variable voltage signal generator; and means (90, 92, 94) for generating an RF carrier signal; wherein, in response to both of a modulating low frequency signal being output by said variable voltage signal generator (96) and an RF carrier signal being output by said means for generating an RF carrier signal, said tunable resonator causes the RF carrier signal to be phase shifted by an amount that is a function of a variation of the modulating low frequency signal voltage, thereby forming a phase-modulated signal. A phase modulator circuit as claimed in claim 1, in which the RF carrier signal has a frequency that is approximately equal to said parallel resonant frequency. A method for phase modulating a signal, comprising the steps of: applying a modulating low frequency signal having a time-varying voltage to a tunable resonator (102), the tunable resonator yielding a maximum phase shift at a parallel resonant frequency in response to the modulating low frequency signal, wherein the amount of phase shift yielded is a function of a variation of the voltage of the modulating low frequency signal; and applying an RF carrier signal, wherein in response thereto, the tunable resonator phase shifts the RF carrier signal by the amount of phase shift yielded by the tunable resonator, thereby phase modulating said RF carrier signal. A method as claimed in claim 3, in which the RF carrier signal has a frequency that is approximately equal to said parallel resonant frequency. A phase modulator circuit, comprising: a variable voltage signal generator (96) for generating a low frequency signal having a time-varying voltage; a tunable resonator (102), said tunable resonator yielding a maximum phase shift at a series resonator frequency in response to a modulating low frequency signal output by said variable voltage signal generator; and means (90, 92, 94) for generating an RF carrier signal; wherein, in response to both of a modulating low frequency signal being output by said variable voltage signal generator and an RF carrier signal being output by said means for generating an RF carrier signal, said tunable resonator causes the RF carrier signal to be phase shifted by an amount that is a function of a variation of the modulating low frequency signal voltage, thereby forming a phase-modulated signal. A phase modulator circuit as claimed in claim 5, in which the RF carrier signal has a frequency that is approximately equal to said series resonant frequency. A method for phase modulating a signal, comprising the steps of: applying a modulating low frequency signal having a time-varying voltage to a tunable resonator (102), the tunable resonator yielding a maximum phase shift at a series resonant frequency in response to the modulating low frequency signal, wherein the amount of phase shift yielded is a function of a variation of the voltage of the modulating low frequency signal; and applying an RF carrier signal having a frequency that is substantially equal to said series resonant frequency, wherein in response thereto, the tunable resonator phase shifts the RF carrier signal by the amount of phase shift yielded by the tunable resonator, thereby phase modulating said RF carrier signal. A method as claimed in claim 7, in which the RF carrier signal has a frequency that is approximately equal to said series resonant frequency.

Inventors: ELLA JUHA, ELLA, JUHA
Applicants: NOKIA CORP, NOKIA CORPORATION
Application year: 1997
Main classification: H03H9
IPC codes: H03H9, H03C3, H03C1, H04R17, H03H3