Signal Sourcing Requirements for Spectrum Analyzer Calibration an Introduction and Overview

Simposio de Metrología 25 al 27 de Octubre de 2006

SIGNAL SOURCING REQUIREMENTS FOR SPECTRUM ANALYZER CALIBRATION AN INTRODUCTION AND OVERVIEW

Paul Roberts Fluke Precision Measurement Ltd Norwich, UK. +44 (0)1603 256781 [email protected]

ABSTRACT: Calibrating a Spectrum Analyzer usually involves a variety of measurements, with each requiring a suitable input signal. A typical procedure generally utilizes a number of signal generators together with other pieces of equipment to generate the signals and ensure their accuracy. This paper provides an overview of spectrum analyzer calibration with discussion of the most common calibration tests, their signal requirements, and approaches for meeting those requirements.

1. INTRODUCTION Harmonic Distortion Analyses of the calibration procedures for 15 models 3rd order Intercept (TOI) from 5 different manufacturers found approximately Tracking generator tests 80 different tests were described. Many of the tests were identical, except for their names – for example As it is beyond the scope of this paper to consider all Display Linearity, Scale Fidelity and Log of the listed tests in great detail, an overview is Conformance all test the ability of the analyzer to presented for each test, with additional detail for the accurately measure relative signal levels at a single some of the more critical or difficult tests. Further frequency, and are tested using the same details, including setup diagrams, etc, may be found measurement technique. Allowing for this in the application note and other documents listed as duplication, the majority of calibration procedures references in section 5. included a core of around 20 tests, listed below. It is generally considered that these are the tests that 2. SPECTRUM ANALYZER ARCHITECTURE should be performed for a spectrum analyzer The tests discussed in this paper apply to the typical calibration to be adequate and sufficient swept tuned superhetrodyne spectrum analyzer. The figure below shows the simplified block diagram for Frequency Accuracy this type of spectrum analyzer. Its architecture Level Accuracy resembles that of an AM superhetrodyne receiver, in Frequency Response which a mixer is used to down-convert the input signal Attenuator Response to a lower, intermediate frequency (IF) for processing. Display Linearity Most spectrum analyzers use two or three stages of Displayed Average Noise Level down-conversion, but a single conversion is shown Resolution Bandwidth Accuracy here for simplicity. Recently, real-time spectrum Resolution Bandwidth Selectivity analyzer instruments have become available, Resolution Bandwidth Switching Accuracy employing a different architecture, where the input Sweep Time Accuracy signal is digitized over a wide bandwidth and digital IF Image Response signal processing techniques are used to obtain Noise Sidebands frequency domain information. The calibration Residual FM requirements of these real-time spectrum analyzers Residual & Spurious Responses will not be discussed here.

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Input Video Attenuator Mixer IF Gain IF Filter Detector/Log Amp Filter

(RBW) (VBW)

Local Display Sweep Oscillator

Figure 1. – Typical block diagram of a swept-tuned superhetrodyne spectrum analyzer

As figure 1 illustrates, a swept-tuned, • A local oscillator, which can be swept to superhetrodyne spectrum analyzer typically generate the normal display or held consists of the following components: constant in zero-span mode. With modern analyzers, which use frequency • An RF input attenuator, which reduces synthesizers as the local oscillator, the the amplitude of high-level input signals to resolution of the synthesizer setting will prevent the mixer from being overloaded. influence the accuracy of both the display and the cursor frequency. • A mixer, which combines the input and local oscillator frequencies to frequency- • A sweep generator, which controls the shift the input signal as the local oscillator frequency of the local oscillator, and for sweeps, allowing a narrow band of input older analog analyzers, also the refresh frequencies to pass through the IF gain rate on the analyzer display. Modern amplifier and filter for measurement. analyzers generate the sweep by control of the frequency synthesizer used to • A variable IF gain circuit, which amplifies generate the local oscillator frequency. the mixer output before passing it to the IF filter, filtering out the signals of interest. • A display, which shows the spectrum of This gain is changed with reference level the measured input signal. As the local setting to allow the reference level at the oscillator sweeps, the spectrum analyzer top of the display to correspond to the digitizes the measured signal levels, required input signal level. storing them for subsequent display as a complete spectrum. (Older analyzers • An IF filter, which is a bandpass filter without digital storage used long- whose bandwidth is adjustable from the persistence CRT displays that displayed spectrum analyzer’s front panel. This the spectrum trace as the sweep bandwidth, referred to as the resolution progressed.) Modern analyzers usually bandwidth, determines how well input also offer a variety of marker, readout, and signals with small frequency differences can measurement functions which utilize the be distinguished from each other. measurement data obtained during the sweep, often employing digital signal • A detector/log amplifier, which responds processing. to the IF signal level, performing a logarithmic conversion to obtain a display scaled in dB per division. 3. TEST DESCRIPTIONS AND SIGNAL • A video filter which uses low-pass filtering REQUIREMENTS OVERVIEW to average and smooth the displayed trace. Frequency Accuracy For a modern analyzer employing a frequency synthesiser for the local oscillator, the basic

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frequency accuracy of the analyzer depends on its Display Linearity frequency reference. This can be measured This test verifies the analyzer amplitude linearity directly by testing the analyzer 10 MHz frequency over a wide dynamic range. Performance is a reference output. For analyzers without a function of the analyzer IF circuits, which operate reference frequency output, frequency accuracy is at a fixed frequency, and therefore only requires tested via the analyzer frequency display and testing at a single frequency with fixed input readout. Frequency display and frequency span attenuator and reference level settings. Testing accuracy are influenced by other factors, such as requires a signal source with high-precision synthesizer resolution and divider performance, attenuation linearity as a reference standard. and are typically tested using signals obtained Alternatively, a general purpose signal generator from calibrated frequency synthesiser instruments. can be operated at constant output level setting with external precision step attenuators as a Level Accuracy reference standard. This test measures the absolute accuracy with which the analyzer will display a signal level. Displayed Average Noise Level Analyzer specifications often quote absolute This test measures the analyzer noise floor without accuracy at a single, relatively low frequency, and an input signal and is performed with a 50 Ω at specific level or range of levels. The test is terminator at the input. To reduce test time, the typically performed with a signal of known level, test is usually performed at a relatively high obtained by measuring the output of a signal resolution bandwidth setting, with the result generator with a power sensor immediately before reported normalised to either 1 Hz or 10 Hz applying the signal to the analyzer, or by bandwidth, as most manufacturers usually specify connecting the signal source via a power splitter to analyzer noise floor at these narrow bandwidths. allow simultaneous level measurement with a power sensor. Other tests described later also Resolution Bandwidth Accuracy determine the analyzers ability to accurately This test measures the accuracy of the 3 dB display signals throughout is specified operating bandwidth of the IF (resolution) bandwidth filters, envelope. and is repeated for each resolution bandwidth setting. The test is usually performed with an input Frequency Response signal of constant amplitude and frequency This test determines the frequency response or (typically 50 MHz or 100 MHz), varying the flatness of the analyzer, relative to its response at analyzer center frequency to determine the filter a reference frequency. The test is typically response. performed with a signal of known level at the frequencies required, obtained by measuring the Resolution Bandwidth Selectivity output of a signal generator with a power sensor This test measures the characteristic of the IF immediately before applying the signal to the (resolution) bandwidth filters to ensure the cut-off analyzer, or by connecting the signal source via a is sufficiently steep to allow the analyzer to resolve power splitter to allow simultaneous level any low level signals close in frequency to a high measurement with a power sensor. level signal. The test determines the 60 dB bandwidth, the result expressed as a the ratio Attenuator Response between 3 dB and 60 dB bandwidths, known as This test checks the accuracy of the input the shape factor. Test method is identical to the attenuator steps. Analyzer performance is typically resolution bandwidth accuracy test. specified at the reference frequency used to specify frequency response accuracy, so this test Resolution Bandwidth Switching Accuracy is also performed at that frequency. Testing This test evaluates any residual gain changes that requires a signal source with known attenuation may occur as the resolution bandwidth filter performance of sufficient accuracy at the reference settings are changed. The test is usually frequency. Alternatively, a general purpose signal performed with an input signal of constant generator can be operated at a constant output amplitude and frequency, changing the analyzer level setting with external calibrated step resolution bandwidth setting with constant center attenuators. frequency (typically 50 MHz or 100 MHz) and span settings to determine any variation in the displayed signal level.

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Sweep Time Accuracy • Correction for Non Rectangular BW filter, In the zero span mode, the analyzer remains subtract 10 log 1.2 dB. tuned to a fixed frequency and displays the time • Correction for log amplifier compression of domain characteristics of the signal level much like noise peaks and measurement of noise an oscilloscope. This test determines the accuracy average rather than RMS, add 2.5 dB. of the horizontal time axis, typically using an • Correction for Analyzer Noise Floor amplitude modulated signal such that a specific contribution at low Signal to Noise Ratio, 10 number of modulation rate cycles are displayed dB SNR gives 0.4 dB error, 3 dB SNR gives and the time/division can be determined. 1.8 dB error (alternatively, treat as uncertainty contribution). IF Image Response • Reference Source Phase Noise contribution, When the local oscillator and input signals are 10 dB margin gives 0.4 dB error, 0 dB margin heterodyned (mixed) in the mixer, signals are (source noise comparable to analyzer) gives 3 generated at the sum and difference frequencies dB error so treat as establishing upper limit for of the two signals and the IF filters select the analyzer noise. desired combination (usually the sum frequency). Alternatively, most modern analyzers include However, if another signal were also present, at a phase noise measurement capability, allowing a much higher frequency such that it mixed with the direct readout of phase noise in dBc/Hz, where the local oscillator to produce a difference frequency analyzer itself applies all the required equal to the IF center frequency, the analyzer normalizations and corrections. would also respond to this unwanted signal, known as the IF image frequency. In practice, use of Residual FM double or triple conversion techniques, harmonic This characteristic of the analyzer is related to its mixers and pre-selectors complicate the situation, local oscillator phase noise at offsets close to the but IF image frequencies exist. The analyzer carrier, but is often considered (and measured) in response at these frequencies is usually tested by terms of residual frequency modulation. To applying signals at the IF image frequencies. This perform a test, a low residual FM signal source is test requires signals of known amplitude over a required. The measurement technique typically wide range of frequencies. uses the analyzer IF filter response as and FM slope demodulator. By arranging for the signal Noise Sidebands frequency to coincide with a liner portion of the This test evaluates the ability of the analyzer to filter response, the filter slope in dB/Hz is first display a signal without adding its own close-in determined by introducing a small frequency noise which would tend to broaden the skirts of the change in the input signal and measuring the displayed response. Performance is determined by displayed amplitude change. Next, the analyzer is the phase noise of the analyzer local oscillator, placed in zero span mode, operating at the same and the test is often called phase noise. It requires point of the analyzer IF filter so that any signal a signal source of sufficiently low phase noise level changes observed are due to residual FM. such that it does not significantly influence the Hence, the residual FM can be calculated by measurement result. The test is usually performed multiplying the peak to peak amplitude change by at a single input frequency, and the noise the dB/Hz slope value. sideband level is measured at a number of offset frequencies from the input frequency (usually Residual & Spurious Responses referred to as the carrier frequency). Analyzer These tests are somewhat dependent on the specifications may list performance in terms of dBc specific analyzer model, based on its design (level of the noise below the carrier), for specific architecture and manufacturer’s resolution bandwidth settings, or in terms of recommendations. Some tests require an input dBc/Hz (level of the noise below the carrier, signal to explore responses close-in to the normalised to a 1 Hz bandwidth). Results are displayed signal, requiring an input signal which generally obtained in dBc directly from the itself has no close-in residuals and adequately low analyzer display, and a number of correction noise sidebands and phase noise. Other tests may factors must be applied to obtain a value in explore low-level residual responses that occur dBc/Hz. These are: under no-signal conditions, requiring only a 50 Ω • 1 Hz BW Normalisation, subtract 10 log (BW in terminator to be applied to the analyzer input. Hz) dB.

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Harmonic Distortion setup, which can be particularly advantageous This test checks for any harmonics of the input when measurements are automated. However, it signal generated internally by the analyzer. is often necessary to use different signal Unwanted harmonic generation occurs within the generators to cover the required frequency range analyzer mixer, and depends on the level of the or use sources with signal characteristics input signal at the mixer. Analyzer performance appropriate for the particular tests being may be specified in terms of harmonic response performed. for a given input signal level and attenuator setting, or more commonly, a mixer level Tests can also be somewhat interdependent, with independent specification is given in terms of the the results of one test relying on successful second harmonic intercept (SHI). It is important to completion of one or more previous tests. For ensure the input signal used for testing is example, the resolution bandwidth accuracy test sufficiently free from harmonics, and often general relies on the analyzer having an accuracy centre purpose signal generators do not provide low frequency and an accurate level linearity, tested in enough harmonic content such that use of external the frequency accuracy and display linearity tests low-pass filters is required to correctly determine respectively. analyzer performance. Some care is required interpreting results, such as 3rd Order Intercept (TOI) the need to apply corrections in the noise The analyzer mixer can also generate unwanted sidebands/phase noise test which may cause intermodulation products if the input signal confusion with inexperienced operators. contains two or more input frequencies. If two input signals at slightly different frequencies are Spectrum analyzer calibration can be a complex present, second and third intermodulation products process, but an understanding of the signal are generated. The generation of second order sourcing requirements will ensure that valid results products is related to unwanted harmonic will be obtained. generation, and is effectively tested by the harmonic distortion test so it is only the third order REFERENCES intermodulation products that are measured. Like harmonic performance, third order intermodulation Fluke application note: ‘A Guide to Calibrating performance is usually specified in terms of an Your Spectrum Analyzer’, document ref 2519570. intercept value. To make the measurement, two input signals of equal level and slightly different VDI/VDE/DGQ/DKD 2622 ‘Calibration of frequency are required. The signals are obtained Measuring Equipment for Electrical Quantities: from two signal generators connected Spectrum Analyzer’. simultaneously to the analyzer via a power splitter. As the third order products depend on the level of Agilent Technologies application note: AN150 the input signals, it is often necessary to have ‘Spectrum Analysis Basics’. knowledge of the input level to an accuracy better than that which the specifications of typical signal Rhode & Schwarz publication: ‘Fundamentals of generators and power splitters will allow, so Spectrum Analysis’, ref PW0002.6635. measurement of the input presented to the analyzer is also made with a power sensor.

Tracking generator tests If a tracking generator (usually optional) is fitted to the analyzer, some testing of its output signal frequency and amplitude is performed.

4. CONCLUSIONS Many of the tests share similar test setups, such as a signal generator connected to the analyzer input, but explore different analyzer performance characteristics. This allows for many tests to be performed without the need to reconfigure the