Anritsu HFE1102_Resonators 电路图.pdf

50High Frequency Electronics High Frequency Design HIGH-Q RESONATORS C eramic coaxial resonators have become an everyday sight in engineering development labo- ratories and manufacturing produc- tion lines. These components use the size-reducing effects of high diectric constant ceramic materials to make the smallest possible resonators for VCOs and filters operating from VHF to microwave frequencies. Figure 1 shows the typical con- struction of a commercially available ceramic resonator. The configuration is coaxial, with an approximately square cross-section outer conductor and a round (cylindrical) center con- ductor. Physical dimensions for a par- ticular coaxial element are W, d and l. Together with the dielectric con- stant of the ceramic material, r, the approximate coaxial line characteris- tic impedance can be calculated by: Commonly-used products typically have Z0in the range of 5 to 15 ohms. The electrical length of a coaxial line is affected by ras follows: Since the ceramics used in these coaxial elements have rfrom 10 to more than 100, so we can see from the above equation that the electrical length can be reduced by a factor of ten or more from the free space length. For example, a 300 MHz 1/4- wavelength line section with rof 90 has a length of 1.037 in. (26.3 mm), compared to a free space length of 9.84 in. (250 mm). The typical resonator consists of a shorted /4 line section, although an open-circuit /2 line may be used for some applications.Typically, a ceram- ic coaxial element can be obtained in a specified length, with one end plat- ed to “short-circuit” the center con- ductor to the outer conductor. Resonator Q can vary significant- ly with different materials, frequen- cies and size, but will have a value in the hundreds (150 to 500). Tuning the Resonator For VCO applications the res- onator must be tuned over a signifi- cant frequency range. The simplest method for doing this is to select a coaxial element with a self-resonant frequency (SRF) that is 20 to 30 per- cent higher than the operating fre- quency, where it will present an inductive reactance. Parallel capaci- tance (e.g. a varactor tuning diode) can then be added to obtain reso- nance at the desired frequency. The inductive reactance of a coax- ial line can be approximated using: where Z0is the characteristic impedance of the line, and is its electrical length in radians. ( = 2l/eff, where eff is the wavelength in the dielectric at the operating fre- quency.) This value of inductance (actually, a range of values, since it varies with frequency), can be used to design the desired VCO circuit. Tuning of resonators for filters does not necessarily mean production line tweaking. Often, resonators are tuned with fixed capacitors to com- pensate for the shift in resonance due to different coupling coefficients between filter sections. This enables a single resonator type to be used in a typical “stagger-tuned” filter. Cautions and Caveats There are physical limitations in the ceramic resonator manufacturing processes. Each manufacturer will have a recommended range of SRF for each product, which is governed by practical lengths for each cross- section profile. Consult with the sup- plier if you want a resonator at the limits of the recommended sizes. High reactance values can be obtained close to resonance. If such use is considered, examine such fac- tors as temperature stability (of the resonator and the surrounding cir- cuitry) and manufacturing tolerances from piece-to-piece and lot-to-lot. Finally, remember all the stray and parasitic reactances and resis- tances: the effect of plating on the shorted end, the inductance of the inner conductor connecting tab, and the effects due to the PC board mounting method and proximity to adjacent components. XZ L= ( ) 0tan effective freespace = r Z W d r 0 60 1 08= ln. Basic Data on High-Q Ceramic Coaxial Resonators Conductors silver-plated on ceramic body: Outer Inner (hollow) Ceramic dielectric Figure 1 Construction of a ceram- ic coaxial resonator and its physi- cal dimensions. W d l = length From November 2002 High Frequency Electronics Copyright 2002, Summit Technical Media, LLC RadioFans.CN 收音机爱 好者资料库