Field of invention

The present invention relates to a method for stabilizing the frequency of a fre¬ quency synthesizer. More specifically, the present invention relates to a method for stabilizing the frequency of a frequency synthesizer by means of a reference oscillator coupled to a voltage controlled oscillator (VCO) by using a phase locked loop (PLL). The present invention also relates to a frequency synthe¬ sizer.

Background of the invention

In typical modern wireless transceivers the communication fre¬ quency is derived from a reference oscillator. For example, in a wireless hand¬ set a frequency based on the reference oscillator is used to initiate the commu¬ nication with a base station. After the connection has been established, the fre¬ quency accuracy may further be improved by various synchronization methods, but the initial communication frequency must be accurate enough to enable the establishment of the initial communication. In a typical transceiver architecture, the output of a reference oscil¬ lator is phase-locked to the output of a VCO and the output of the VCO provides the desired local oscillator (LO) frequency. An example of such PLL based frequency generation is depicted in Figure l a.The output 13 of the phase comparator 12 is used to stabilize the op¬ eration of the VCO 14; the output frequency 15 of the VCO 14 will be the refer¬ ence oscillator frequency 11 multiplied with the dividing factor N of the divider chain 16. The basic PLL based frequency generation utilizes an integer counter chain as frequency divider, but the fractal version uses a fractal divider which alternates between two different dividing modulo, typically under control of a sigma-delta converter, to approximate a continuous output frequency range. The PLL based frequency generation depicted in Figure 1a can be developed into the frequency synthesizer depicted in Figure 1 b if the dividing factor N is arranged to be dynamically changeable. In addition to the requirement to provide long-term frequency accu¬ racy for the VCO, a low reference oscillator phase noise is needed because the less phase noise a LO generated carrier contains, the better the communication channel will be suitable to provide error free information transfer. Reference oscillators are used as precise and stable frequency ref¬ erences in frequency synthesizers and to provide a precise and stable refer¬ ence frequency signal to enable the generating of a variable but stable fre¬ quency that can be used as a tunable LO-frequency. The critical requirements for the output frequency signal of a fre- quency synthesizer is stability, low phase noise and a high thermal stability, e.g. having low thermal coefficient, and additionally the requirement that a precise value for the output frequency can be selected rapidly. In wireless communication devices the reference oscillator has conventionally been based on quartz crystals. The stable and precise mechani- cal vibration of quartz resonators suits well for creation of an oscillator that has excellent long term (drift and aging) and short-term (phase noise) stability. Fur¬ thermore, by proper quartz preparation methods (e.g. AT-cut) the temperature dependence of the resonance frequency can be reduced to a low value (less than a few ppm for the typical operation temperature range). The central disad- vantages of quartz crystal reference oscillator modules are their rather bulky size and difficulty for monolithic integration with the transceiver module that is otherwise typically based on highly integrated solutions. Modern micromachin- ing makes it possible to fabricate minituarized mechanical resonators (Micro- ElectroMechanical Systems = MEMS) with resonance frequencies ranging from several kHz up to the GHz range. Examples of such microresonators based on surface or bulk micromachining of silicon are presented in H. J. De Los Santos, "RF MEMS Circuit Design for Wireless Communications", Artech House, Bos¬ ton/London, 2002. The advantages of microresonators include small size, low power consumption, and possibility for increased integration level between the resonator, the oscillator electronics, and the device package. Both monolithic integration and the system-on-chip approach are viable solutions for increasing the integration level of a reference oscillator. Monolithic integration of micro- machined resonators and integrated circuits facilitates more complicated micro- electro-mechanical circuits, and can provide complete on-chip frequency syn- thesizers. However, a fundamental complication in using silicon-based micro¬ resonators in frequency synthesizers arises from their large temperature coeffi- cient, typically the df/dT from -10 to -30 ppm/K. Such a temperature depend¬ ence is far too large for a reference application if left unaccounted for. Compen¬ sation of this temperature dependency is therefore required to make microreso- nators suitable as frequency references for frequency synthesizers.

Summary of the present invention

It is an object of the present invention to eliminate the disadvan¬ tages of the prior art and to provide an improved method for stabilizing the fre- quency of a frequency synthesizer, and an improved frequency synthesizer. The present Invention provides a LO frequency synthesizer archi¬ tecture in which a MEMS reference oscillator unit having a non-zero tempera¬ ture coefficient is used for frequency stabilization, and where the temperature- dependence of the reference frequency is accounted for at the synthesizer level. The present invention is based on the principle that the resonator temperature is measured in order to electronically compensate for the tempera¬ ture dependency and the resulting temperature dependent frequency shift. By measuring the temperature T of the MEMS resonator as depicted in Fig. 1c and by using the known frequency vs. temperature function fr(T), the resonance fre¬ quency becomes a precisely defined quantity that can be used to improve the stability of the prior art frequency synthesizer according to Fig. 1b. In a preferred embodiment the temperature measurement is based on exciting two modes in a MEMS resonator having different temperature coef- ficients. By looking at the frequency shift of the two modes, the change in tem¬ perature can be measured and electrically compensated. This method has the following advantages: 1) no temperature sensor is needed eliminating tempera¬ ture transient hysteresis associated with temperature difference of the sensor and the resonator, 2) frequency measurement is accurate and is straightforward to realize digitally, and 3) no additional sensors are needed as the resonator is also the sensor. This simplifies fabrication and reduces costs. Characteristic features of the present invention are in detail pre¬ sented in the enclosed claims. The main advantage of the methods presented in the present in- vention is that because the rather large (but predictable) temperature depend¬ ence of a MEMS-oscillator is taken into account directly in the frequency syn¬ thesis, the long-term stability and low phase noise for the reference oscillator can be better optimized as the oscillator itself can be left running uncompen¬ sated. Using the described methods, the MEMS-reference oscillator can be (monolithically) integrated as a part of a wireless transceiver module.

Brief description of drawings

The foregoing, and additional objects, features and advantages of the present invention will be more clearly understood from the following detailed description of preferred embodiments of the present invention, taken in conjunc- tion with accompanying drawings, in which: Fig. 1a is a block diagram of a typical PLL based frequency synthe¬ sizer where the VCO generates the LO frequency, Fig. 1 b is a simplified block diagram of a frequency synthesizer, Fig. 1c is a simplified block diagram of a frequency synthesizer ac- cording to the invention, Fig. 2a is a block diagram of a frequency synthesizer, according to a preferred embodiment of the invention, Fig. 2b is a block diagram of a simplified frequency synthesizer ac¬ cording to an embodiment of the invention, Fig. 2c is a block diagram of a frequency synthesizer according to another embodiment of the present invention, Fig. 3a and 3b illustrates two vibration modes observed for a square plate resonator having different temperature coefficients, Fig. 4 discloses measured frequency coefficients for two square plate vibration modes, Fig. 5a, 5b illustrates two methods to simultaneously detect the two square plate vibration modes, and Fig. 6 illustrates the preferred embodiment of the invention imple¬ mented with the temperature information extracted from two mode measure- ment.

Detailed description of the invention

The present invention relates to a method for stabilizing the fre- quency of a frequency synthesizer by using a reference oscillator coupled to a voltage controlled oscillator (VCO) using a phase locked loop (PLL) or a fre¬ quency comparison control, wherein a reference MEMS oscillator is utilized for stabilization of the VCO, whereby by measuring the temperature T of the MEMS resonator and by using its known frequency vs. temperature function fr(T), the output frequency becomes a precisely defined quantity that can be used as a reference in frequency synthesizers, In the following three stabilization methods are presented for MEMS-oscillator-based frequency synthesizers. According to a first method, with a block diagram depicted in Fig. 2a, the VCO 24 generates the LO-frequency (e.g. at 1GHz) fL0. The MEMS- reference oscillator 21 is typically operated at a significantly lower frequency (e.g. at 10 MHz). The VCO output frequency is divided in the divider 25 in order to make the output frequency 28 of the divider equal to the MEMS-reference oscillator frequency. Their relative phase is detected by the phase detector 22 and after low pass filtering (LPF) 23 the result is used to stabilize the VCO 24 This loop forms a basic PLL, as described earlier. The frequency of the MEMS reference oscillator 21 is not stable with temperature, but its temperature-dependent frequency offset can be com¬ pensated by modifying the divider 25. A previously known fractional-N divider technique that is continu¬ ously tunable can conveniently be used to adjust the counting of the divider chain 25. It can be realized by using e.g. a sigma-delta modulator technique that continuously adjusts the count module N of the frequency divider stage. Using the sigma-delta modulator 27 to adjust 26 the divider 25, the VCO fre¬ quency becomes N+x[n] times the reference frequency and can be made to be nearly continuously adjustable. N denotes the module integer setting of the fre- quency divider and x[n] is the phase detector output signal 28 that is used to control the sigma-delta modulator 27. Based on the measured 29 temperature T of the MEMS reference 21 and the T-dependency f(T) of the MEMS reference 21, a LOGIC circuitry 30 is used to generate a x[n] 32 signal that makes the delta-sigma modulator 27 compensate the temperature-induced output frequency offset of the MEMS ref¬ erence oscillator 21. The said compensating is by means of the delta-sigma modulator 27 adjusting the modulo N of the divider chain 25. The LOGIC circuitry 30 can advantageously use a look-up table 31 that provides the needed correction con- trol signal x(n) selected using the measured temperature T of the reference os¬ cillator 21. The fabrication tolerance offset in reference frequency can as well be calibrated and used to additionally adjust x[n] in the same way by using for ex¬ ample a two dimensional look-up table 31 or a suitable combining algorithm. The MEMS reference oscillator 21 is realized using techniques de¬ scribed later. Long-term stability and low phase noise for the reference oscillator can be provided using a single MEMS-oscillator based on bulk acoustic wave (BAW) operation. If necessary, an improved performance for the reference os¬ cillator can be realized using two or more MEMS components whose properties are selectively combined. For example, the long-term stability and low phase noise for the reference oscillator can be derived by combining the properties of two MEMS-components. The block diagram of the second method is depicted in Fig. 2b. It illus¬ trates the case where the VCO 44 is capable of generating a signal with the required spectral purity (phase noise) by itself and the reference oscillator 41 is needed only for providing the long-term frequency stability. In this case, the VCO feedback loop can be made very narrow band, and a constant integer-N division can be used in the divider 43 to generate the an output to be mixed in the mixer 42 with the reference frequency from the ref¬ erence oscillator 41. The mixer 42 outputs the sum and difference frequencies of the mixed frequencies but after low pass filtering 48 only the low frequency difference frequency fbeat= fr - fVCO/N (or fVCO/N - fr) between the reference frequency fr and the divided VCO output frequency NCOIH remains, and can be used to generate a control signal for the VCO that adjusts the VCO to output the desired frequency. It is important to note that the VCO output signal does not have to be constant multiple of the reference signal. By proper selection of the difference frequency fbeat, fine tuning of the output frequency fVCO = N ■ (fr - fbeat) is possible. This can be used to digitally compensate the temperature induced change in the reference oscillator, The VCO control signal 49 is generated by the logic circuit 45 and to this control signal further correction control voltages can be added in order to generate a tuning voltage for the VCO that additionally corrects for the refer¬ ence oscillator temperature dependent frequency offset and/or for offsets found during calibration as well. The described mixing method can only be used if the relative frequency sense of the frequencies does not change. Another more general method is to determine the frequency difference and its sense by directly counting the two relative low frequency signals fr and fVCO/N using one common signal to gate the counters. This clock can for example be either of the said signals. The logic circuit 45 outputs a tuning control voltage 49 that depends on the frequency difference in such way as to make the difference frequency between the divided VCO output frequency and the reference frequency closer to the desired frequency difference. A voltage depending on the measured oscillator temperature T and any calibration correction can additionally be arranged to adjust the tuning volt¬ age for the VCO 44, advantageously by simply using look up tables and a DAC. In the same way as previously described for the first method, the look-up tables can contain both calibration correction values and correction values that depend on the known temperature dependent behaviour of the MEMS reference oscilla¬ tor. An advantageous embodiment of this method is to use the fre¬ quency difference to control the VCO by analog methods and digitally adjust the integer counter chain for temperature and calibration correction using the logic circuit 45 and the control signal 50 to change the dividing modulo of the divider counter chain 43. Whenever a synthesizer divider is dividing using integer values (e.g. for channel selection), the logic circuit can rapidly provide the channel se¬ lection if it is fed information of the desired channel, in the example of Fig. 2b the LOGIC circuit 45 could directly provide the temperature corrected N needed for the desired channel and thus provide rapid channel selection. This method for rapid channel selection in a transceiver is not lim¬ ited to the method depicted in Fig.2b but can be used whenever correction val¬ ues are arranged to modify the counter chain. This method can thus be used with the preferred embodiment as well. In a favourable embodiment, not shown, a three dimensional table is used instead of a two dimensional correction table in order to provide the correction values for all combinations of fabrication cali¬ bration correction, reference oscillator temperature correction and the channel selection frequency offsets. Providing a temperature, calibration or channel corrected N value to a counter chain can be accomplished many other ways obvious to a person skilled in the arts. For example in addition to changing the modulo of the counter chain or using fractal methods, the count value of the counter itself can be modified by adding or subtracting counts at a regular rate in order to in- crease or decrease the output frequency. This method, commonly used in phase accumulators to give a fixed phase offset, can easily be modified for con- tinuous phase change, which in effect provides controlled increase or decrease in output frequency. Fig. 2c is a block diagram of frequency synthesizer according to another embodiment of the present invention where an offset synthesizer is used for temperature stabilization of the VCO 55 output frequency. The VCO generates the LO-frequency (e.g. at 1GHz) fι_o- The MEMS-reference oscillator 51 provides the reference frequency, typically 10 MHz. The VCO output fre¬ quency is divided in the divider 54 and the divided VCO output is mixed with the reference oscillator using the mixing phase detector 52. The mixer output is fil- tered with a low pass filter 53 to obtain f1 = fVCO - fr. The second mixer 57 is used to add an offset frequency foffset provided by a oscillator 58 to the signal. After filtering with the second low pass filter 56 the resulting frequency f2 = f 1 — foffset is obtained. When the phase-loop is locked the frequency f2 is zero and the VCO output equals fVCO = N ( fr + foffset ). The frequency of the MEMS reference oscillator 51 is not stable with temperature, but its temperature-dependent frequency offset can be com¬ pensated by adjusting the foffset. Based on the measured temperature T of the MEMS reference and the T-dependency f(T) of the MEMS reference 51 , a LOGIC circuitry 60 and a look-up table 59 is used to control the offset oscillator 58. The offset oscillator may be a VCO or a MEMS oscillator with a wide tuning range (e.g. flexural oscillator). According to another embodiment of the present invention a silicon resonator is excited simultaneously in two modes, each mode having a different temperature coefficient. An example of a useful resonator structure showing these two types of modes is shown in Fig. 3. Figure 3a illustrates extensional mode vibration (at fO = 13.1 MHz, Q = 120 000) in a plate and Fig. 3b the Lame mode vibration (at fO = 12.1 MHz, Q = 60000). The extensional vibration mode is characterised as a 2-D plate expansion that preserves the original square shape. The Lame mode is characterized as plate bending that preserves the plate volume. The component can be made by deep reactive ion etching of sili- con-on-insulator (SOI) wafer. The electrical contact to the resonator can be done with corner anchoring (T-type corner anchoring) so that the entire device can be fabricated with one mask. The extensional mode has additionally been shown to be very suitable to obtain low phase noise V. Kaajakari, T. Mattila, A. Oja, J. Kiihamaki, H. Kattelus, M. Koskenvuori, P. Rantakari, I. Tittonen and H. Seppa: "Square-Extensional Mode Single-Crystal Silicon Micromechanical RF- resonator", paper to be published in Transducers'03 (Boston, June, 2003). Fig. 4 shows the measured temperature dependency for the two modes, which is different from each other. Simultaneous and independent detecting of the two vibrations is depicted in Fig 5a and Fig 5b. In Figure 5a and 5b the acoustic mode (BAW) silicon resonator is shown comprising a square plate, electrodes ELE1 - ELE4 providing capacitive coupling on all sides of the plate, voltage sources Ujn and Ubias connected to the electrodes and output voltage. With the differential elec¬ trode configuration shown in Fig. 5a, the two modes can be detected with the same electrodes. This gives maximum signal amplitude for both modes but complicates the oscillator electronics. The configuration shown in Fig 5b uses different electrodes with different bias (differential polarity for the other Lame- electrode) to excite and/or excite the Lame- and extensional-modes. The driving electronics is simpler and the two modes have good isolation, but the configura¬ tion is not optimal for signal power. For simplicity, the electrode size for the two modes are shown equal but a practical implementation may use larger elec¬ trodes for the mode that is used to generate the reference frequency and smaller electrodes for the mode that is used for generating the temperature in¬ formation. An alternative to detecting the two modes in the same resonator is to fabricate two resonators in close thermal contact (e.g. on a same substrate), driving one resonator in plate extensional mode and driving the other in Lame mode. Since these resonators are on same silicon substrate, the resonator temperatures are highly correlated. The benefit of this configuration is that the modes are electrically and mechanically isolated and the oscillator electronics is simplified. Other microresonator configurations can also be used to generate the temperature information. For example, it is possible to obtain the tempera¬ ture information from torsional and flexural beam vibration modes as these two modes have different temperature dependency as well. Alternatively, two reso- nator made of different materials with different temperature dependency can be used. The temperature information that is needed for the frequency com¬ pensation can be generated as follows: The two modes resonate at frequencies f1 and f2. Both modes are excited simultaneously and the pulses generated by the two resonances are detected as earlier described and counted in a counter 1 and 2, respectively. If the counter 1 has stored N1 cycles, then the counter 2 has stored N2 = f2/f1 -N 1 cycles in the same time. Keeping N1 fixed, the tem¬ perature can thus be obtained. Such temperature information can be used by the present invention to correct the output frequency of a frequency synthesizer according to the first method, its simplified form and the third method. Other temperature measuring methods can be used as well to implement frequency correction, but the two- mode method is particularly suited for use with MEMS reference oscillators. This two-mode method can directly be used to make a reference oscillator used for a high frequency synthesizer as depicted in Fig. 6. The de- picted synthesizer uses a reference oscillator and a fractional-N phase-locked loop.. The reference oscillator 71 has two frequency outputs 72 and 73 that have different temperature dependency. The frequency output 72 is designed to have low phase noise. These outputs 72 and 73 are counted with COUNTER1 74 and COUNTER2 75. The counter outputs are used by the logic circuitry 77 to calculate the oscillator temperature as earlier has been described.. The tem¬ perature information together with stored calibration information in the memory 76 is used calculate the correct division factor N in the divider 83 that yields the desired LO frequency given by fVCO = N-f1. Since the f1 depends on tempera¬ ture, the required division N factor may be fractional and not an integer. The desired frequency is synthesized using a VCO 80 to generate the LO frequency . The VCO output frequency is divided in the divider 83 in or¬ der to make the output frequency 81 of the divider equal to the MEMS-reference oscillator frequency 72. The divisional factor in the divider 83 is changed by the fractional sigma-delta modulator 82 in a previously described manner. The LOGIC 77 circuitry is used to control the sigma-delta modulator to obtain frac¬ tional division. The relative phase between the reference oscillator signal 72 and the divided signal 81 is detected by the phase detector 78 and after low pass filtering in the LPF 79, the phase comparison result is used to stabilize the VCO 80. This loop forms a basic PLL, as described earlier. The reference oscillator 71 is designed so that the output signal 72 primarily has low phase noise and only secondarily high temperature stability. The desired LO frequency is synthesized using a voltage controlled oscillator that has a low quality factor, and is therefore locked to the reference oscillator for good stability in order to provide a low jitter LO signal, needed for carrier sig- nals and other uses. Any reference oscillator frequency shift is corrected using fractional division of the VCO frequency using a fractional-N modulator. Stored calibration values and the known temperature dependency is used by the logic circuitry to control the fractional-N modulator. The fractional-N PLL can advantageously be implemented using a sigma-delta modulator. This technology offers nearly continuous frequency tun¬ ing and has been demonstrated to be able to satisfy the GSM phase noise specifications. It is obvious to the person skilled in the art that different embodi¬ ments of the invention are not limited to the example described above, but that they may be varied within the scope of the enclosed claims.