SAW STABILIZED MICROWAVE GENERATOR ELABORATION

Transcription

1 SAW STABILIZED MICROWAVE GENERATOR ELABORATION Dobromir Arabadzhiev, Ivan Avramov*, Anna Andonova, Philip Philipov * Institute o Solid State Physics - BAS, 672, Tzarigradsko Choussee, blvd, 1784,Soia, tel , iavramov@issp.bas.bg Technical University o Soia, FETT, 8 Kl. Ohridski blvd., 1779 Soia tel , dna@ecad.tu-soia.bg, ava@ecad.tu-soia.bg, i_9@yahoo.de In order to investigate parameters and design methods o SAW stabilized microwave generators a methodology is elaborated. The generator is divided into our dierent blocks: SAW resonator, power divider, attenuator, and ampliier. This solution allows each block to be measured and analyzed separately. The amplitude and phase condition or generation can be calculated and the resonant requency o the oscillator can be predicted. The measurements o the microwave oscillator are done in time and requency domain by requency meter and spectrum analyzer. The jitter and the spectrum are observed. The output signal is analyzed, observing the requency, phase noise (at given oset) and power o the main harmonic and the magnitude and requency o the other harmonics. 1. INTRODUCTION Keywords: SAW, microwave generator, oscillator Surace Acoustic Wave (SAW) devices are used widely in the housekeeping, industrial and military areas [1]. This is due to their unique properties small size and weight, low price, high reliability and stability in the time. In certain applications they cannot be replaced by other elements because o their unique properties. SAW resonator utilizes SAW, and is able to be applied to high requency circuit where conventional crystal, ceramic resonators are not available, as SAW resonator oscillates stably with its undamental mode over requency range rom 50 MHz to around 1,5 GHz. SAW one port resonators have been widely used in oscillators or keyless entry systems [2] These SAW resonators show a very sharp admittance Attenuator STWR Delay Line Adjustable elements Power Ampliier Wilkinson power splitter ig. 1. Schematic o the SAW oscillator Output character- ristic due to their high Q-actor. 2. PROBLEMS SAW usage requires their characteristics to be known, but suitable design kits by which they will be studied, are still missing. The development o a

2 design kit is presented in this article. It is a SAW-based oscillator (фиг.1) [3]. The design kit allows the dierent modules to be disconnected. By this way the modules can be analyzed separately each other. SAW resonators has generally two types o one-port type and two-port type. Oneport SAW resonator is a kind o two terminal resonant device utilizing piezoelectricity, like quartz crystal bulk wave resonator or ceramic resonator. The equivalent circuit o one-port SAW resonator is same to that o quartz or ceramic resonator, and thereore, equivalent circuit, basic structure and temperature characteristics o the SAW resonator is as shown in ig.2. This type o SAW resonator has high oscillation requency stability, good temperature characteristics and is developed with SAW technology. It is also stable against peripheral circuit or supply voltage luctuation, and is basically ree ig. 2. SAW resonator rom adjustment. It can be applied to many types o high requency devices including RF remote controls, demodulators and oscillators [4]. Two-port SAW resonator is a kind o very narrow, low loss band-pass ilter. Oscillation circuit is mostly like a RF ampliier with eedback loop. 3. RESULTS The exercise is divided into two parts and starts with measurement and characterization o the parameters o the built-up blocks, and ater that starts the measurement and analyses o the oscillator Measurement and characterization o the parameters o the built-up blocks The design kit is divided into our modules, and each o them is basic a RF component. Those are: broadband ampliier it serves to ensure the necessary gain,

3 SAW module the object under test, attenuator serves to limit the amplitude o the oscillations, and Wilkinson power divider serves to carry out a part o the power in the oscillator to external devices, and it preserves the oscillator rom load variance. The RF signals are transmitted between the modules by coaxial cables and or this purpose SMA connectors are provided. The overall view is shown on ig. 3. Thanks to the modulus structure, the students will be able to know in details how each module works. This will enlarge their knowledge in the RF ield. C oa x. ca bl In order to start oscillation in a e +6.5V Output eedback circuit, two conditions must be satisied the gain and the phase condition. The irst one states that the sum o the gain coeicients o all elements in the chain must be greater than one, and the second condition says that the overall phase delay must be multiple o In mathematical expression those conditions are (1), (2): ig. 3. Overall picture o the design (1) G PA + GPS + GSAW 0 kit (2) ϕpa + ϕps + ϕsaw 2. π. n, where Gi - the gain coeicients o the dierent blocks, ϕi - the phase distortion in the dierent blocks. The exercise starts with measurement o the amplitude and phase characteristics o the SAW resonator. The purpose is that the students have to extract the resonant requency, the attenuation at the resonant requency, the phase distortions, the parasitic capacitance and the quality actor (Q) o the resonator, rom the measurements. The amplitude and phase characteristics o the resonator are shown on ig. 4. The quality actor can be determined by three dierent ways, and the students will have to decide the disadvantages which method is better. The calculation o Q by the amplitude characteristics is perormed by determination o the bandwidth at - 3dB gain (3): res res (3) Q = = + 3dB 3dB Q can be also determined ig. 4. Gain and phase characteristics o measuring the change o the phase at a the SAW resonator given requency change(4): res ϕ (4) Q =. 2 STWR PA ATT Adjustable elements R R WPS 100

4 But the change o the phase is nonlinear and that will lead to errors in the Q determination. That is why the time delay o the signal is measured and by that Q is calculated (5) [3]: (5) Q = πτ ig. 5. Gain and phase characteristics o the broadband ampliier The exercise proceeds with examination o the gain and phase distortion o the broadband ampliier. From the shown diagrams (ig.5) the gain and the phase at the resonant requency are read. This measurement has to be done very careully, in order to save the equipment. That is why at the input o the ampliier a very low amplitude has to be applied, so the output signal to be less than the maxium permissible or the measurement tool. A measurement o the Wilkinson power divider ollows [2,3]. Measurement between the input and Output2 and Output3 is done. Ater that a measurements between Output2 and Output3. The results are shown on ig.6 and 7. ig. 6. Amplitude characteristic o the Wilkinson power divider input-output1 ig. 7. Amplitude characteristic o the Wilkinson power divider Output1- Output2 Ater the measurements are inished, a veriication o the condition (1) ollows. In case that the overall gain is signiicant, an attenuator in series must be connected, so that the gain decreases up to 1-2dB. Ater that the modules must be connected in series and the overall gain and phase must be measured (ig.8). In case that the phase condition is not met, additional phase delay or advance must be added. This can be done with minimal eorts by selecting the correct the overall phase. The cable length

5 calculation must be perormed by the students. Ater that the loop is closed and the observing o the oscillations at the output p the oscillator starts. 3.2 Measurement and analyses o the oscillator. The oscillator measurement is done in time and requency domain. Its purpose is to show the students the main techniques or measurement o high-requency signals Measurement o the phase noise and the spectral purity Those measurements are done in requency domain [4, 5]. The ig. 8. Overall gain and phase characteristic output o the oscillator must be connected to spectrum analyzer and the behavior o the spectrum near the resonant requency must be observed. (ig. 9). The spectral purity can be determined by measuring the amplitude o the harmonics, which originate rom the non-sinusoidal orm o generated signal (ig.10). ig. 9. Phase noise measurement ig. 10. Spectral purity measurement 3.3. Time domain measurement At this measurement the short time instability o the output signal is calculated. The output o the oscillator is connected to high requency counter. The measurements must be perormed according the recommendations o the IEEE subcommittee or requency stability [6]. They state, that M samples o the requency have to be taken or interval τ. To simpliy the calculations, τ is 1 sec. Ater that the short-term instability can be calculated by (6):

6 1 2 M (6) ( ) =. σ y τ ( yi+ 1 y) 2( 1), where M i= 1 y i is the value o the measured requency. In act this is the mean value o the requency averaged or τ seconds. Equation (6) is known as the Allan variance. 4. Conclusion In this article, the development o design kit or analyses o SAW resonators is shown. The block scheme o the kit, its built-up modules and method o work with them are shown. The sequence o the exercise and the dierent methods or measurement o the characteristics o the modules are described. The methods or phase noise measurement are shown as well as the main equations by which it can be calculated. The made sample o high-stabilized generator with SAW resonator is useul or designing o integrated SAW based oscillators. 5. REFERENCES [1] Gulyaev Y.V., Hickernell F.S., Acoustoelectronics: New Ideas or a New Era, IEEE International Ultrasonics Symposium, pp , [2] Eichinger L., Accurate Design o Low-Noise High Frequency SAW Oscillators, European Microwave week, Sept., England, [3] Ivan D. Avramov, High-Perormance Surace Transverse Wave Resonators in the Lower GHz Frequency Range, International Journal o High Speed Electronics and Systems, Vol. 10, No.3, [4] L. Eichinger, F. Sischka, G.Olbrich, R. Weigel, Accurate Design o Low-Noise SAW Oscillators, 2000 IEEE Ultrasonics Symposium, pp , [5] C.A. Greenhall, Oscilator-Stability Analyzer Based on a Time Tag Counter, NASA Tech Bries, NPO-20749, May, p , [6] Allan, D.W., Time and Frequency (Time-domain) Characterization, Estimation, and Prediction o Precision Clocks and Oscillators, IEEE Transactions UFFC-34, pp , = NIST Technical Note 1337 (NTIS #PB ), pp. TN121-TN128.