Calibration Times November 2010


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Calibration Times November 2010

  1. 1. CALIBRATION TIMESVo l u m e 1 Issue 5 November 2010 Dear Readers, Widespread proliferation of cellular networks and affordable feature rich mobile phones has brought about a communications revolution worldwide. These cell networks have also brought with it a huge demand for RF test equipments that need to be maintained and calibrated periodically. In RF and Microwave frequencies, spectrum analyzers replace oscilloscopes as the monitoring and diagnostic tool of choice. In fact, a study of recalibration workloads reveal that spectrum analyzers top the list of equipments that need periodic calibration alongside RF & Microwave power meters. A traditional RF & Microwave calibration lab faces a huge challenge to address this demanding commercial workload. Spectrum analyzers have been traditionally difficult instruments to calibrate requiring a rackful of expensive RF test equipment with complex cabling and characterizations that only a very experienced RF engineer can pull off. Not only were these calibrations time consuming, but complex setups required patience and skills that are hard to come by in these times. Recalibration of standards in the rack also cost a fortune besides loss of revenue due to downtime. Engineers at Fluke thought long and hard to come out with a simple and elegant single box solution to this challenge that also cost a fraction of a traditional setup. Suddenly, RF calibration is no longer a science that was out of reach of common calibration labs. Fluke put to good use its rich experience of designing the worlds most advanced oscilloscope calibration systems to design the 4 GHz 9640A Reference Source that combined complex signals needed for RF calibration into a single box. No more complex setups, special cabling, characterizations or switching. This month we focus on the technology that went into the worlds first RF Reference Standard calibrator from Fluke. There is also a short guide to RF connector care. Enjoy the articles. Good Reading! Joey Joseph Editor & Publisher T h e M o n t h l y N e w s M a g a z i n e o n M e t ro l o g y f ro m T T L Te c h n o l o g i e s
  2. 2. MEASUREMENT METHODS FOR A NEW RF & MICROWAVE CALIBRATION SOURCE inherent accuracy without the need to monitor or characterise the output with additional equipment during use. (For example, measuring of the output amplitude with a power splitter and power sensor, monitoring of the achieved modulation levels with a modulation analyzer, etc). In order to facilitate delivery of the output signal direct to the load or Unit Under Test (UUT) input and A traditional old style The new way minimize performance degradation due to cabling and RF calibration setup interconnections, the instrument has an external leveling head (see figure 1). Signals are generated in theThe signal sources used in RF and Microwave calibration mainframe and fed to the leveling head containing theapplications are usually general purpose signal level detector and attenuator Circuits.generators, not optimised for calibration requirements.Many generators widely regarded as best suited for Obtaining high signal purity is important for manycalibration use are now obsolete. A new signal source, the calibration applications, often requiring the use ofFluke model 9640A, has been specifically developed for external filters. In this design low levels of harmonicRF and Microwave calibration applications, providing and spurious content are achieved by the use ofhigh-purity precision level outputs covering a wide appropriate filters within the output signal path. It alsoamplitude range over an extended bandwidth, with features internal analog modulation capability. FM isprecision modulation capability. Performing generated within the frequency synthesizer at rates upmeasurements to evaluate the performance achieved to 300kHz and AM is generated within the output amplification and leveling circuits at rates up to Reference Source UUT Spectrum Analyzer 220kHz.against aggressive target specifications duringdevelopment and also testing products in the Measurement Methodsmanufacturing process present a variety of technicalchallenges: Measurement requirements include Level (RF Power), Output VSWR, and Modulation. Level. RF level is• Identifying methods capable of providing low measured as an absolute value at a reference frequency, uncertainties over the range of values required. followed by frequency response (flatness)• Lack of traceability for certain parameters. measurements relative to the value at the reference• Identifying alternative techniques suitable as frequency. For the reference point (100kHz), an AC cross-checks to validate the chosen methods. Voltage Measurement Standard measures the RMS voltage developed across an accurately known 50O Design Architecture and Features termination, and the corresponding power level is calculated. An RF power meter and power sensor head is used for the high frequency measurements. This is aThe 9640A frequency range is from 10Hz to 4GHz at commonly used technique, capable of extremely lowamplitudes from +24dBm to -130dBm. It is designed to uncertainty traceable measurements if appropriatelygenerate the signals necessary for the most common RF calibrated power sensors with correction data are used.& Microwave calibration applications and provide TTL Technologies Pvt. Ltd. To t a l S o l u t i o n s i n I n s t r u m e n t a t i o n
  3. 3. signal with an accurately known frequency, and as the Output VSWR modulation index value is determined mathematically by the Bessel function, this technique can be used toHaving knowledge of the output impedance of the 9640A accurately determine the FM deviation of a signal and(source match) is important not only to confirm hence evaluate the measurement performance of bothspecifications, but also to allow users to estimate mis- the measurement demodulator and the 9640A. Formatch uncertainty contributions in their applications. example, tests of FM deviation accuracy carried out withThe measurement techniques generally used for passive a 100MHz carrier modulated at 125kHz deviation atcomponents cannot be used for an active leveled source. If 23.113kHz rate (corresponding to the third carrier null)those methods are used, they are likely to give erroneous gave agreement between the Bessel Null method, themisleading results. In operation, any reflections caused 9640A, and the measurement demodulator of < a mismatch between the source and the load will be atthe signal frequency, and the leveling circuit willdetect thecombined effect of the output signal and The reflection.The methods appropriate for passive devices do not giveappropriate conditions to correctly measure theequivalent source match.ModulationAM and FM design goals of accuracy better than 0.1% anddistortion <0.05% (-66dB) are difficult to measure withtraditional methods. Measurements are made using a MetCal with 9640A saves timespectrum analyzer featuring a measurement Practical Implementationdemodulator (the Rhode & Schwarz FSMR). Thedemodulator uses digital signal Processing to extract the Fluke factories operate modern world-class practices,required signal characteristics from data obtained by which include individual manufacturing cells for eachdigitizing the IF Signal, By sampling (digitization) at the product line. All of the activities required to assemble andIF and digital downconversion to the baseband (I/Q), the test the product take place in the cell, staffed by a team ofdemodulator achieves maximum accuracy and operators and technicians trained and experienced in alltemperature stability. Accuracy and stability are the relevant processes and equipment. In the case of themaximized by minimizing the analog circuitry prior to the 9640A, calibration also takes place in the manufacturingIF analog to digital converter, for example by using only cell using the methods described in this articlethe widest bandwidth (10 MHz) IF filter. implemented in automated test and calibration systems. Identical automated test and calibration systems are alsoAnalysis of the inherent error sources within the used for service repair and re-calibration activities.spectrum analyzer and measurement demodulator Precision measurement systems usually require thesuggest extremely accurate modulation measurements tightly controlled environmental conditions of ashould be obtained, much better than its manufacturer’s calibration laboratory, and the 9640A test andpublished specifications suggest. There is a method for calibration system is no different. However, freedomFM deviation measurement based on FM theory which is from this location constraint is achieved by creating apotentially very accurate - the Bessel Null or Disappearing self-contained controlled environment for the calibrationCarrier method - and can be used to validate standards and the unit under test (UUT), allowing themeasurement capability. The amplitude of an FM signal systems to be readily deployed in the manufacturing arearemains constant, but the amplitude of the spectral or service centres. An easy to move customized rackcontent at the carrier and sideband frequencies is related enclosure contains the system instrumentation and hasto the modulation index by the Bessel function. At certain an integral environmental control and monitoringvalues of modulation index the amplitude of the signal at system with the coolers, heat exchangers and airthe carrier frequency and of the sidebands nulls to zero, circulation units mounted in module attached to the sideand this condition can be observed on a spectrum of the rack.analyser. It is generally easy to provide a modulating TTL Technologies Pvt. Ltd. To t a l S o l u t i o n s i n I n s t r u m e n t a t i o n