Piston Attenuator Research Articles

On 30 MHz TE11 mode piston attenuator

A precision 30 MHz TE11 mode waveguide below cut-off piston attenuator has been designed for 0–100 dB attenuation range to serve as transfer standard of attenuation. The attenuator has been fabricated using a precision cylindrical copper waveguide and resonant inductive antennas with type-N connector at its input and output ports. The 30 MHz matched antennas have been incorporated in the cylindrical waveguide in such a manner that one serves as fixed transmitter and the other as moveable receiver for changing the attenuation. A Faraday screen type mode filter has also been fabricated to attenuate the amplitude of unwanted TM01 mode to an attenuation level of 60 dB. The piston attenuator has approximately 20 dB insertion loss and has been calibrated in steps of 10 dB over the 0–60 dB range. The results were found to lie within ±0.01 dB/10 dB.

A fiber-optic attenuator calibration system using high frequency substitution method

A fiber-optic attenuator calibration system based on a high frequency substitution method has been developed using a 30 MHz precision piston attenuator equipped with a laser interferometer as a standard. The uncertainty of attenuation calibration for a typical 10 dB attenuator is 0.025 dB at two standard deviation levels. The resolution of the system is 0.001 dB and the dynamic range is 25 dB in a single step.

Ultrasonic spectroscopy of liquids. Extending the frequency range of the variable sample length pulse technique

A method is described that allows the ultrasonic absorption coefficient alpha of liquids to be precisely measured between about 1 and 100 MHz. The automatic measuring procedure can be performed easily and comparatively fast. At high alpha values the accuracy of the method is clearly superior to the more time-consuming resonator technique, which is applied in this frequency range. This accuracy is achieved by careful construction and adjustment of the specimen cell, by semi empirical consideration of diffraction effects within the sample, and by precise calibration of the set-up by a specially designed below cut-off piston attenuator. The short measuring time is enabled by the particular mode of operation also described in this paper.

RF attenuation measurement system with 1-kHz voltage ratio standard

A homodyne system for measuring attenuation at 10 GHz and 30 MHz is constructed and evaluated. The radio frequency (RF) attenuation is converted to 1-kHz attenuation, and the 1-kHz attenuation is measured by an inductive voltage divider built in a 1-kHz receiver. A 1-kHz voltage ratio standard (VRS) is proposed to check the accuracy of the 1-kHz receiver for maintaining long-term accuracy. The differences between the VRS and the 1 kHz receiver are smaller than 0.0003 dB for 0-40 dB. A precision piston attenuator of 30 MHz agrees with the system within 0.0018 dB for 0-30 dB. The change in measured values at an interval of one year are less than 0.001 dB. The results demonstrate that this system can be expected to reliably maintain high accuracy.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

30-MHz attenuation standard using binary phase modulation method

A 30-MHz attenuation measurement system which uses a ratio transformer as the standard attenuator is constructed and examined. The binary phase modulation method is used. Higher accuracy at low attenuations and easier maintenance are expected in the system than in the conventional system using the piston attenuator. The estimated uncertainty of the system is 0.0015 dB for 0-20 dB. The attenuation measurements made by the system and the conventional system agree within the estimated uncertainties. >

Acoustical absorption spectroscopy of liquids between 0.15 and 3000 MHz: III. Hypersonic comparator technique

A method has been developed to measure the absolute hypersonic absorption of liquids in the frequency range from 0.3 to 3 GHz with 1% accuracy using a pulse transmission technique. Excitation of surface vibrations on small rod-shaped piezoelectric LiNbO3 crystals at variable transducer spacing is combined with a direct parallel RF substitution technique in which a below-cut-off piston attenuator is used as reference. Sensitive superheterodyne detection followed by signal averaging and multiple data recording allow for a low input power level so that heating of the small sample volume (<1 ml) is avoided. The measurements are automatically driven by a process control control.

Below cut-off piston attenuator as a calculable signal vernier for microwaves up to 15 GHz

Simple circular cylindrical piston attenuators have been constructed which, unlike commercially available below cut-off attenuators, are operated in the aperiodic TM01 mode. The instruments provide broad-band measurements up to 15 GHz over an extended range of attenuation. The accuracy offered by these TM01 attenuators is +or-0.02 dB. Calibration and correction procedures are not needed. The TM01 mode attenuator is therefore an attractive alternative to the conventional TE11 attenuator, since the latter, matched to a nominal frequency, requires extensive calibration measurements if applied over a broad frequency band. The construction of the TM01 piston attenuator is presented and test measurements are described to verify the favourable properties of the device.

Piston attenuator as RF signal vernier in the 10-300 MHz range: determination and correction of spurious mode errors

Systematic errors in H11-mode piston attenuators from unwanted mode excitation have been measured for CW, and pulse modulated RF signals as well, up to 300 MHz with a new calibration circuit based on a linear RF-IF mixer. The attenuation deviation by spurious mode influence can be calculated from the determined amplitude ratio of superposed modes and can be corrected. In this way piston attenuators, originally designed for IF application, can also be used as RF verniers with 0.01 dB resolution up to 300 MHz and possibly higher with an accuracy depending on mixer linearity in the calibration circuit.

Propagation Constant Below Cutoff Frequency in a Circular Waveguide with Conducting Medium

Exact and approximate propagation characteristics of normal modes in the cutoff region of a circular waveguide surrounded by a medium of finite conductivity are discussed. An exact solution is obtained by numerical analysis, and an approximate one is derived by expanding the characteristic equation considering the finite conductivity of the cylinder wall. The computed values are compared with experimental ones. It is shown that the attenuation of TM/sub 01/ mode at frequencies that are much lower than the cutoff frequency is constant, i.e., independent of frequency and the material constants of the external medium, and this mode is the most suitable one for realizing a precision circular piston attenuator.

Considerations of a Rectangular Waveguide below Cutoff Piston Attenuator as a Calculable Broad-Band Attenuation Standard between 1 MHz and 2.6 GHz

Theory, essential construction data, and results for the systematic uncertainties are presented for a precision rectangular waveguide below cutoff (WBCO) serving as a broad-band attenuation standard for the frequency range between 1 MHz and 1 GHz, and with a slight increase in the total uncertainty to 2.6 GHz. The contributions to the systematic uncertainty resulting from the mechanical tolerances, the uncertainties in length measurement, frequency stability, surface conductivity, ambient conditions (air refractive index), and higher mode disturbances are discussed. It is shown that under optimum conditions the lowest limit of systematical uncertainty is about 0.0001 dB/10 dB between 1 MHz and 1 GHz for an evacuated WBCO; for an air-filled waveguide under adequate laboratory conditions a total uncertainty ≦0.0005 dB/10 dB should be achieved between 50 MHz and 1.5 GHz.

An Excitation System for Piston Attenuators

The problems of mode purity and the interaction effect generally necessitate a large insertion loss in precision waveguide-below-cutoff (piston) attenuators to obtain a linear output. An attenuator excitation system, used on a precision attenuation measuring system, that reduces these problems is described. Experimental work as well as theoretical considerations are discussed and designs are presented that have produced out-put voltages of the piston attenuator greater than 1 V while still maintaining a linearity of 0.001 dB as a function of piston position. This has been accomplished by 1) operating with large input power (100-200 W), 2) development of new exciting units with improved geometry for better field excitation and utilizing water-cooled elements for high-power operation, 3) special impedance matching networks having exceptional stability with high power input, 4) use of current feedback to achieve effectively constant current excitation, and 5) a mode filter that attenuates the undesired TM 01 mode 98 dB while attenuating the desired TE 11 mode only 0.2 dB.

A c.w. comparator for precision r.f. attenuators

The most accurate piston attenuators work at frequencies below 100 MHz and standard piston attenuators are usually operated at a fixed frequency such as 30 MHz or 60 MHz. In designing a comparator in which attenuators operating at any frequency up to, say, 3 GHz can be compared with a standard over a wide range of attenuation up to 100 dB, a number of problems had to be solved. Chief among these was the need to keep the noise sufficiently below the signal level at - 107 dBm and the need to design frequency changers which are linear to much better than 0 01 dB over the range of levels - 7 dBm to - 107 dBm. The paper describes how these problems were solved in the design of a comparator which would function with a c.w. source of any frequency between 0.18 GHz and 1 GHz with an uncertainty of comparison well within ±001 dB.

Equipment for the measurement of the insertion loss of waveguide networks in the frequency range 3.8–4.2 Gc/s

The concept of insertion loss as applied to microwave networks is discussed and methods of measurement are reviewed. It is concluded that superheterodyne substitution methods enable precise measurements of insertion loss to be made at frequencies for which no standard attenuators exist. Equipment of this type for use in the range 3.8–4.2 Gc/s uses a principle developed by C. M. Burrell.An intermediate frequency of 34.5 Mc/s is employed, and the 34.5 Mc/s output from the waveguide path, into which the network being measured is inserted, is compared with a signal at 34.5 Mc/s obtained from a crystal oscillator followed by a piston attenuator. Switching between the two signals is effected at a repetition rate of 1 kc/s, and detection of the composite 34.5 Mc/s signal yields a 1 kc/s output whose amplitude is proportional to the difference between the two 34.5 Mc/s signals; detection in a phase detector gives the sense of the difference. The piston attenuator is adjusted for zero output from the phase detector, and the difference of the attenuator readings, with and without the network inserted, gives the required insertion loss.The accuracy of measurement is ±0.05 and ±0.1 dB for losses of 6 and 30 dB, respectively, and losses of up to 60 dB, can be measured with decreased accuracy.

Resistive attenuators and their calibration

The characteristics of metallized-glass-vane attenuators in rectangular waveguide are briefly reviewed. Resetting accuracies of 0.004 and 0.012 dB in 40 dB are achieved at 3 and 10 Gc/s, respectively. The glass vanes are unaffected by extreme climatic conditions, and laboratory attenuators in the 10 Gc/s band have retained their calibration to within 0.03 dB over a five-year period.The attenuators are calibrated with reference to the accurately constructed piston attenuator of a 60 Mc/s signal generator. The microwave signal is amplitude modulated and is converted to 60 Mc/s in a linear mixer. The signal generator output is also amplitude modulated, and the two 60 Mc/s signals are compared in a phase-sensitive detector. Methods are described for the determination of the errors of calibration, and it is concluded that the total inaccuracy does not exceed 0.03 dB in the range 0–50 dB in the 3 and 10 Gc/s bands.

An absolute microwave attenuator

A description is given of the construction and the method of operation of the rotary attenuator, and it is shown that, since the measurement of attenuation by this instrument is dependent inherently upon the measurement of an angle, it is an absolute device. By the use of scattering coefficients and considering the various discontinuities within the instrument, the errors that can arise in its use are analysed. It is shown that the most serious source of error is due to insufficient maximum attenuation, which can be due to a limitation of the centre vane or to misalignment between the end vanes. All other errors can be measured and made negligible by good microwave measuring practice.Finally, some details are given of the methods that can be used to check the law of the attenuator and practical accuracies, and results are presented for an instrument operating in the 3 cm waveband which has been compared with a precision piston attenuator operating at 60 Mc/s.

The excitation of plane surface waves

It is shown that a surface wave will be generated by a horizontal slot situated above a corrugated or dielectric-coated guiding surface.A physical interpretation of the analysis is suggested which explains the mechanism of launching in terms of a resonance phenomenon allied to that which arises when a piston attenuator is terminated by a reactance equal and opposite to its own (reactive) characteristic impedance.The problem of efficient launching is discussed, and it is shown that under suitable conditions a high launching efficiency is possible.Some of the theoretical predictions have been verified experimentally.

The measurement of permitivity and power factor of dielectrics at frequencies from 300 to 600 Mc/s

The paper describes a resonance method of measuring permittivity and power factor which is essentially a development of the Hartshorn and Ward apparatus, suitable for use at the higher frequencies. The method is one of capacitance variation, and the disc form of sample, the circular plate electrodes and the cylindrical form of micrometer capacitor are retained but reduced in linear dimensions. The capacitive elements are mounted in a re-entrant cavity, and the micrometer capacitor is so placed that the voltage across it is only a fraction of that across the plate electrodes; this increases the fineness of adjustment. The micrometer capacitor can be calibrated only by using the cavity as a wavemeter; the capacitance settings were therefore expressed in terms of the corresponding resonance frequencies, and the calculations of power factor then became identical in form with those of the frequency-change method, although the measurements were made at a constant frequency. After applying the appropriate corrections the permittivity can be determined to within ± 1% over most of the range. The high Q-factor of the cavity and the precision of adjustment of the micrometer capacitor make the power-factor determinations highly accurate. For values less than about 0.01 the error is estimated as being less than ± 2 × 10−5 (for measurements on solids). For higher values a superheterodyne receiver incorporating a piston attenuator is used as the detector, and the estimated limits of error for such measurements are ± 5 × 10−4.

Design Analysis of a TM-Mode Piston Attenuator

The design analysis of a transverse magnetic mode piston attenuator which may be utilized as a microwave absolute standard in the comparison method of measuring attenuation is described. The system employs a coaxial section input and a coaxial section output separated by a cylindrical waveguide section operating below cutoff. The electromagnetic coupling between the two coaxial sections is comprised of exponentially decaying field components of the TM <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">om</inf> modes associated with the waveguide section. A method of satisfying input and output boundary conditions based upon circular symmetry is detailed and applied in approximate form to obtain numerically the attenuation, input impedance, and VSWR characteristics of a model designed for a wavelength of 10 centimeters. The predicted data apply to coupling separations of 0.5 cm and higher.

Corrections to the attenuation constants of piston attenuators

Corrections to the attenuation constants of piston attenuators

Corrections to the attenuation constants of piston attenuators

The modes existing in a circular waveguide with walls of finite conductivity are investigated and expressions derived for the corrections to the attenuation constant of a piston attenuator. The correction is negligible for E-modes but may be of importance in work of the highest accuracy when H-modes are used. Similar corrections are derived for the effect of an oxide layer on the inner surface of the attenuator.