Understanding Level Accuracy specifications and techniques in Spectrum Analysers.

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1 Understanding Level Accuracy specifications and techniques in Spectrum Analysers. Anritsu Spectrum Analyser roadshow. Feb 2018

2 Agenda Measurement fundamentals and uncertainties Amplitude level accuracy related specifications. Spectrum analyser hardware and effects on level accuracy. Novel solutions to improve level accuracy. 2 Copyright ANRITSU

3 Introduction. When specifying the level performance of a spectrum analyser, the two areas most often considered are Amplitude level sensitivity (the lowest level of signal that can be measured) Amplitude level accuracy (the level of accuracy on the actual measured value) Some typical amplitude level measurements with Spectrum Analyzers Power level of a single signal that is part of a more complex wide band modulation scheme e.g. the power of a single carrier in an OFDM signal Power of a specific narrow band signal that is isolated e.g. an oscillator or amplifier harmonic spur Total power of all signals within a given channel bandwidth e.g. total power of a wide band signal Frequency selectivity of a spectrum analyser is needed for all these measurements Wide band power meter/sensor would not be suitable. 3 Copyright ANRITSU

4 Introduction. Measuring precisely the power level of a signal requires many different settings for the bandwidth and input attenuator on a spectrum analyser Some examples: harmonic spurs of a transmitter power of sub-carriers in a complex modulation scheme These SPA settings can have an effect on level accuracy Changes in frequency Changes in resolution bandwidth, Changes in input attenuation Changes input signal level 4 Copyright ANRITSU

5 Units of measurements (International System of Units) SI Base Units Mass kilogram Length meter Time second Electric Current ampere kg m s A Temperature kelvin K Luminous Intensity candela cd Amount of Substance mole mol SI Derived Units kg m 2 s -2 Energy joule Non-SI units J recognized for use with SI day: 1 d = s hour: 1 h = 3600 s minute: 1 min = 60 s liter: 1 l = 10-3 m 3 ton: 1 t = 10 3 kg degree: 1 0 = (/180) rad minute: 1 = (/10800)rad second: 1 = (/648000)rad electronvolt: 1 ev x J unified atomic mass unit: 1 u x kg kg m s -2 Force newton N kg m s -3 Power watt W S Coordinated Time international atomic time TAI kg m-1s -2 Pressure pascal Pa s -1 Activity becquerel Bq s -1 Frequency hertz Hz kg m 2 s -3 A -1 Electric Potential volt V kg -1 m 2 s 4 A 2 Capacitance m 2 s -1 Absorbed Dose gray Gy S A Electric charge coulomb C farad F m 2 s -2 Dose Equivalent sievert Sv Health related measurement units K Celsius Temperature 0 Celsius 0 C kg m 2 s -3 A -2 Resistance ohm kg m 2 s -2 A -1 Magnetic Flux weber Wb kg -1 m 2 s 3 A 2 Conductance siemens S kg s -2 A -1 Conductance siemens S cd sr Luminous Flux lumen lm m -2 cd sr Illuminance lux lx kg m 2 s -2 A -2 Inductance henry H Electromagnetic measurement units sr: the steradian is the supplementary SI unit of solid angle (dimensionless) rad: the radian is the supplementary SI unit of plane angle (dimensionless) Power = Energy/Time Power = [Joule/second] 5 Copyright ANRITSU

6 Fundamentals of accuracy, stability, and uncertainty SI units are fixed definition and have no limitation on accuracy. The tools we used to observe them may introduce limitations into our accuracy, due to stability and uncertainty issues. Example: Using a SPA to measure power will introduce some limitations and errors Stability, accuracy and repeatability 6 Copyright ANRITSU

7 Stability and accuracy Stability indicates how well an instrument can measure the same level over a given period of time. Stability doesn t indicate whether the level measurement is right or wrong, but only whether it stays the same. In contrast, accuracy indicates how well an analyser can measure the actual level. 7 Copyright ANRITSU

8 The relationship between accuracy and stability Required accuracy True level fc, the centre frequency, is the true frequency of the signal. We then see how the measured value f varies against time. These effects can be seen also on a Spectrum Analyzer 8 Copyright ANRITSU

9 Estimating Stability Stability is estimated with statistics evaluate the level fluctuations of a measurement that occur over time. Short-term stability intervals of less than 100 seconds, Longer-term stability intervals greater than 100 seconds, usually refers to periods longer than 1 day. Never confuse accuracy with stability. Accuracy over a given averaging time can never be better than the stability over that same averaging time. Continuously calibrating an analyser can make the accuracy as good as possible The stability number shows you the limit of accuracy you can claim over a given averaging time. 9 Copyright ANRITSU

10 Uncertainty Metrologists generally use a statement of measurement uncertainty as the performance metric for a device, rather than accuracy and stability. However, accuracy and stability are the two main components that make up the uncertainty and can both be used in the uncertainty analysis. Measurement uncertainty is generally reported in this form Prescribed by the ISO Guide to the Expression of Uncertainty in Measurement (GUM): Y y U Where: Y is the nominal value of the measurand (time or frequency) y is the best estimate of Y, for example, the average measured time or frequency U is the combined measurement uncertainty The range from y U to y + U is the coverage area, normally set to 2 standard deviations so that the device under test should remain in this range about 95% of the time. 10 Copyright ANRITSU

Types of Uncertainty Type A Uncertainties that are evaluated by the statistical analysis of a series of observations. Formerly called statistical uncertainties.

Formerly called systematic uncertainties. An example would be a fixed cable delay in a time measurement.

11 Types of Uncertainty Type A Uncertainties that are evaluated by the statistical analysis of a series of observations. Formerly called statistical uncertainties. An example would be frequency stability or random noise. Type B Uncertainties that are evaluated by means other than the statistical analysis of series of observations. Formerly called systematic uncertainties. An example would be a fixed cable delay in a time measurement. (Noise picked up by the cable would be a statisticial element) Combined Type A and Type B uncertainties are combined into the quantity U. Sometimes, one type of uncertainty dominates and the other type is insignificant. red = 1 sigma (68.3% probability) red + green = 2 sigma (95.4% probability) red + green + blue = 3 sigma (99.7% probability) 11 Copyright ANRITSU

12 Defining total level accuracy, and uncertainty in a spectrum analyser. We can define as follows: Total level accuracy = fundamental uncertainty + uncorrected systematic errors. (e.g. +/- 1dB) Fundamental uncertainty is quoted as absolute level accuracy, usually measured at one specific frequency, attenuator setting, and input signal level. Used as benchmark of the fundamental uncertainty of different instruments, Does not give a clear indication of the actual measurement uncertainty that a user will face when using different instruments in the real world. By comparing the specification of different analysers at the given setting of frequency, attenuator and input level, then the absolute accuracy can be compared. Does not equate to the level accuracy when used in a real system making measurements at different levels and frequencies. The key to the real measurement uncertainty is the total level accuracy, includes uncorrected systematic errors for instrument settings outside of the conditions made for the absolute level accuracy 12 Copyright ANRITSU

13 Separating systematic errors vs true uncertainties. Systematic errors are defined as those which can be measured and compensated for with correction algorithms, as they have a deterministic value (can be measured) and characterised. These occur due to manufacturing tolerances in the individual electronic components that cause fixed errors and device parameter setting dependencies (such as frequency or level) that are repeatable characteristics of the components (usually non-linear semiconductors) in the system. True measurement uncertainties come from errors and variations that are not measured or characterised. These are typically due to electrical noise (thermal semiconductor junction effects) and uncompensated temperature fluctuations (assuming the components are not housed in a thermally stable environment, but are used in a regular laboratory environment). Aging effects on components such as oscillators and switches can also be uncertainties if they fall outside the path of the compensation circuits but are still part of the signal path. By measuring and compensating the maximum number of systematic errors, the total level accuracy of an analyser can be improved to give best measurement accuracy. 13 Copyright ANRITSU

14 Specifications important for Spectrum Analyzer amplitude/level measurement accuracy Dynamic Range Relative Amplitude Accuracy Absolute Amplitude Accuracy 14 Copyright ANRITSU

Specifications for Dynamic Range ADC Dynamic range ADC Noise floor Quantisation errors # of bits (12,14,16) +30 dbm -10 dbm MAXIMUM POWER LEVEL MIXER COMPRESSION -35 dbm THIRD-ORDER DISTORTION

15 Specifications for Dynamic Range ADC Dynamic range ADC Noise floor Quantisation errors # of bits (12,14,16) +30 dbm -10 dbm MAXIMUM POWER LEVEL MIXER COMPRESSION -35 dbm THIRD-ORDER DISTORTION CRT-DISPLAY RANGE 80 db INCREASING BANDWIDTH OR ATTENUATION MEASUREMENT RANGE 145 db SIGNAL/NOISE RANGE 105 db SIGNAL /3rd ORDER DISTORTION 80 db RANGE -45 dbm SECOND-ORDER DISTORTION 0 dbc NOISE SIDEBANDS SIGNAL/ 2nd ORDER DISTORTION 70 db RANGE SIGNAL/NOISE SIDEBANDS 60 dbc/1khz RF Frontend -115 dbm (1 khz BW & 0 db ATTENUATION) MINIMUM NOISE FLOOR 15 Copyright ANRITSU 15

16 Dynamic and Measurement Range MS2830A Typical parameters for Signal Analyzer Family Dynamic Range > GHz, 2/3 (TOI-DANL) in 1 Hz RBW Displayed Average Noise Level > 2 GHz to 5 GHz -151 dbm (1) Measurement Range DANL to +30 dbm with PreAmp OFF Measurement Range DANL to +10 dbm with PreAmp ON Amplitude accuracy 300 khz to 3 GHz typ. ± 0.5 db, NORMAL operation MS2840A Dynamic Range > GHz, 2/3 (TOI-DANL) in 1 Hz RBW Displayed Average Noise Level 1 < f < 2.4 GHz -151 dbm (2) with PreAmp OFF Displayed Average Noise Level 1 < f < 2.4 GHz -165 dbm (2) with PreAmp ON Measurement Range DANL to +30 dbm with PreAmp OFF Measurement Range DANL to +10 dbm with PreAmp ON Amplitude accuracy 300 khz to 3 GHz typ. ± 0.5 db, NORMAL operation (1) Difference between TOI and DANL as simple guide (2) Sample, VBW: 1 Hz (Video Average), Input attenuator: 0 db, no low phase noise opt. 17 Copyright ANRITSU

17 Amplitude and Frequency Accuracy types (Absolute & Relative) e.g. +/-1dB Absolute measurement Single marker (freq or power) Relative measurement Delta freq or delta amplitude Absolute Amplitude in dbm Relative Amplitude in db Relative measurements are more accurate than absolute measurements (see next slide) Frequency Relative Frequency 18 Copyright ANRITSU 18

18 Accuracy: Relative Amplitude Accuracy Uses some part of the signal as reference Relative Amplitude Accuracy is affected by: Display fidelity (ADC, detectors, FFT linearity) Frequency response These parameters have no influence (unless changed during measurement) Change of RF Input attenuator (uncertainty of setting accuracy) Change of Reference level (changes IF attenuator) Change of Resolution bandwidth (change of noise floor) 19 Copyright ANRITSU 19

19 Accuracy: Relative Amplitude Accuracy - Display Fidelity Applies when signals are not placed at the same reference amplitude Display fidelity includes Log amplifier or linear fidelity How true is the log characteristic? Relative Amplitude in db Detector linearity Digitizing circuit linearity Frequency 20 Copyright ANRITSU

20 Accuracy: Relative Amplitude Accuracy - Freq. Response Freq Response usually NOT part of level accuracy spes 50MHz) (it is a separate spec somewhere else in the data sheet) Signals in the Same Harmonic Band +1 db Relative Amplitude in db 0-1 db Frequency BAND (Hz) Frequency Example specification: ± 1 db 21 Copyright ANRITSU 21

21 Broadband OFDM signals challenge amplitude accuracy 22 Copyright ANRITSU

22 Accuracy: Absolute Amplitude Accuracy Absolute Amplitude Accuracy is affected by: Calibrator accuracy (typically 0.3dB) Signal of known amplitude Typically Reference Level is given absolute accuracy Absolute measurements are relative to the calibrator level Frequency response If frequency is different to calibrator frequency Reference level uncertainty (IF gain uncertainty) 23 Copyright ANRITSU 23

23 Accuracy: Other Sources of Uncertainty Mismatch (RF input port not exactly 50 ohms) Compression due to overload (high-level input signal) Distortion products Amplitudes below the log amplifier range Signals near noise Noise causing amplitude variations Two signals incompletely resolved 24 Copyright ANRITSU 24

24 S P A H a r d w a r e a n d i t s i n f l u e n c e on l e v e l a c c u r a c y 25 Copyright ANRITSU

25 Block Diagram of a typical Spectrum Analyzer Block Diagram of a one stage Heterodyne Spectrum Analyzer Almost all blocks can have a significant contribution to level accuracy Copyright ANRITSU

26 How hardware implementation affects total level accuracy The hardware implementation has a major effect on the total level accuracy of the analyser. Defines which circuits are included into the compensation system, and what parameters are included into the compensation. Traditional designs of high performance bench top analyser use compensation circuits that operate at only a single fixed frequency (e.g. 50MHz) and level setting (e.g. -10dBm). 27 Copyright ANRITSU

27 How hardware implementation affects total level accuracy Anritsu has implemented a unique compensation technique in the MS8230A/MS2840A and MS2690A series of spectrum analysers. Uses a frequency swept reference source to calibrate the whole receiver chain across a wide frequency band, which minimises all systematic errors. Alternative solutions are only making such a calibration at single frequency (e.g. 50 MHz) When the measurement frequency is changed from this value, an additional uncompensated uncertainty is introduced. A frequency response uncertainty must be added to the absolute accuracy. 28 Copyright ANRITSU

Wideband compensation circuit The calibration signal is swept across the frequency range 50 Hz to 6 GHz in the MS2690A/MS2830A, validating the compensation across this whole range, This compensation

28 Wideband compensation circuit The calibration signal is swept across the frequency range 50 Hz to 6 GHz in the MS2690A/MS2830A, validating the compensation across this whole range, This compensation circuit includes the input level attenuator as well as the whole receiver chain. Level accuracy is maintained across the whole 6 GHz frequency range and with the full range of input attenuator settings. This technique reduces the systematic errors by having a much more comprehensive compensation circuit. The actual measurement uncertainty the user has when making measurements is significantly reduced. Dependency on frequency, input level, and input attenuator setting are all compensated automatically in the measurement. 29 Copyright ANRITSU

Improvement with wideband compensation The result of the wideband compensation techniques provides a significant improvement in the useable accuracy of the spectrum

This is based on the individual errors being statistically independent and hence unlikely to all occur in the worst case simultaneously.

The charts show the specification of a standard spectrum analyser, where each uncertainty error is specified separately as they are independent sources of error, compared

29 Improvement with wideband compensation The result of the wideband compensation techniques provides a significant improvement in the useable accuracy of the spectrum analyser. Better Accuracy? and -10dBm ) Note: the Root Sum Squares (RSS) method may be used to combine the individual errors into a total uncertainty. This is based on the individual errors being statistically independent and hence unlikely to all occur in the worst case simultaneously. Hence RSS method can be applied to calculate a typical uncertainty error. The charts show the specification of a standard spectrum analyser, where each uncertainty error is specified separately as they are independent sources of error, compared to the Anritsu MS2830A where the broadband compensation method and high stability oscillator / filter technology is used. In this case, the amplitude level accuracy is specified as the combined RSS value, as the compensation circuit removes the independence of the individual error sources. 30 Copyright ANRITSU

30 Frequency reference (LO) related level accuracy errors. Level uncertainties are introduced by the type of component technology used in the spectrum analyser. Traditional spectrum analysers use a YIG type RF local oscillator (YIG Tuned Oscillator, or YTO), providing the reference frequency (LO) for the RF down-conversion mixer. The YIG oscillator is based on Yttrium Iron Garnet as a ferro-electric material that changes resonance frequency according to strength of a magnetic field applied. This allows for a high quality (low noise) reference oscillator with a wide tuning bandwidth. YIG technology is susceptible to performance variations over temperature as the device normally warms up and temperature cycles during operation. These provide an additional level accuracy that drifts slowly over time as the device warms. New generation spectrum analysers, such as the MS2830A, have moved to a VCO type local oscillator, and this has much lower temperature fluctuation and performance degradation. New generation spectrum analysers have used the very latest low noise VCO component technology to now produce a Local Oscillator that matches the required specifications for a high performance spectrum analyser. 32 Copyright ANRITSU

YFT filters suffer from self-heating Become warmer due to the interaction of the electro-magnetic fields with the ferrous materials

31 YIG Tuned Filter (YTF). High band pre-selector filter (YTF) is implemented using YIG technology YIG elements is used as frequency selective tuneable devices. YFT filters suffer from self-heating Become warmer due to the interaction of the electro-magnetic fields with the ferrous materials used in the YIG crystal sphere, Heating causes a change in the frequency characteristics of the YIG sphere, Hence the amplitude response is changed because frequency versus amplitude is not precisely linear in the YTF. 33 Copyright ANRITSU

Switched Filter Bank architecture as an alternative to YTF YTF filters can be replaced with the filter bank architecture for the high band pre-selector filter.

32 Switched Filter Bank architecture as an alternative to YTF YTF filters can be replaced with the filter bank architecture for the high band pre-selector filter. Replaces a single tuneable YIG filter with a series of switchable filters configured to the desired selectivity. Used to select the frequency band in the range higher frequency ranges (normally 6GHz to 13,5 GHz). Above 13GHz then a YTF is still used as the wider bandwidths mean the filter bank is not an effective solution. When operating a spectrum analyser in the range from 6GHz to 13.5GHz then the choice of high band pre-selector filter type will have an effect measurement performance. The filter bank technique uses a set of Band Pass Filters (BPF), typically a bandwidth of 400MHz. Implemented using printed circuit techniques, to give low cost, low power consumption, and high stability. The stability comes from there being no tuneable or variable circuits involved. Does not have the same level of selectivity (the filter resonator has a lower Q value). The analyser may not have such strong rejection of out of band signals. 34 Copyright ANRITSU

33 Total Level Accuracy (including all systematic errors). Wide band calibration oscillator Total Level Accuracy Traditional YTF and single frequency calibrator 36 Copyright ANRITSU

34 Unique Calibration Circuit in MS269xA & MS2830A The built-in tracking generator calibrates the RF circuit over the entire frequency span up to 6GHz and achieves an absolute level accuracy of ±0.5dB 37 Copyright ANRITSU

35 Conclusion. The total level accuracy is defined as the addition of the absolute level accuracy and the un-corrected system errors that are not compensated by the analyser s own correction circuits. The new generation Anritsu MS2830A series significantly minimise the uncorrected systematic errors. Key improvements are in the area of the wide bandwidth compensation circuits, and the switch away from YIG based frequency components (YIG reference Oscillator and YIG Tuned Filter) to technologies that offer more stable performance (VCO and Switched Filter Bank) for operation up to 13.5 GHz. These changes significantly reduce the number of uncompensated systematic errors. The effects of these changes are seen when looking at the total level accuracy of different spectrum analysers. An standard type must be specified for level accuracy with additional terms of measurement frequency and input attenuator setting in addition to the absolute level accuracy. An advanced architecture (that has compensation circuits across wide frequency band and includes input attenuator) is only specified in the single total level accuracy that includes all of the above level accuracies. Additional level accuracy errors due to frequency and input signal level / attenuator setting can be eliminated. 38 Copyright ANRITSU

White Paper. Understanding amplitude level accuracy in new generation Spectrum Analyzers. Since 1895

White Paper. Understanding amplitude level accuracy in new generation Spectrum Analyzers. Since 1895 White Paper Understanding amplitude level accuracy in new generation Spectrum Analyzers Since 1895 Introduction When specifying the amplitude level performance of a spectrum analyzer, there are many factors