For a long time, spectrum analyzers and scanning receivers have been widely used in EMI laboratories. The technical capabilities of these instruments are similar to those of a classic stepped EMI receiver, with the primary exception being the dynamic range for quasipeak measurements.
At present, EMI receivers must meet all specifications in CISPR 16-1-1 if they are to be used for most commercial EMI compliance measurements. For that reason, spectrum analyzers without preselection currently are not permitted to be used for such measurements. This is due to the inability to correctly measure pulses with low repetition rates using the quasipeak detector.
However, a project addressing this issue within CISPR/A, the technical committee responsible for the maintenance of CISPR 16-1-1, is nearing completion. This change to the standard will allow the conditional use of spectrum analyzers without preselection for compliance testing in accordance with CISPR and CISPR-based standards such as European Norms (ENs). It is estimated that this revision of CISPR 16-1-1 will be published in 2010.
There are some requirements that spectrum analyzers will have to meet to be used for compliance testing in the future. In addition, a measurement process will be introduced that users must perform to verify the suitability of a spectrum analyzer without preselection for compliance measurement of a specific EUT.
Rationale for Proposed Change
CISPR 16-1-1 (2007) clause 4.4.2 specifies the pulse response of EMI receivers for quasipeak detection (Table 1). Currently, the standard states that any receiver for which compliance with CISPR 16-1-1 is claimed must measure the specified pulses with the stated pulse repetition frequencies (PRFs) in accordance with Table 1. This includes PRFs of 20 Hz and lower for the frequency range up to 1 GHz.
Despite this existing requirement, which applies when performing measurements in accordance with CISPR-based standards such as EN 55011 or EN 55022, many EMI test laboratories currently disregard this requirement and use spectrum analyzers without preselection. Most of these instruments meet the specification as defined in CISPR 16-1-1. For that reason, measurement results obtained with these instruments are comparable within the established accuracy specifications to those achieved with fully compliant receivers. This similarity of test results can be observed for many types of products under test.
However, in accordance with ENs, the use of these more cost-effective instruments is not permissible since CISPR 16-1-1 is referenced as the mandatory standard describing the characteristics of EMI receivers. Again, this standard calls for full compliance with all stated requirements and disallows the use of spectrum analyzers without preselection, despite the fact that comparable measurement results can be achieved.
The most noticeable difference between a fully complaint EMI receiver and a spectrum analyzer without preselection is the limitation of the dynamic range when measuring pulses below 20 Hz with the quasipeak detector. In most cases, the noise floor of spectrum analyzers in the required bandwidth will limit the required dynamic range for correct quasipeak measurements of these low PRF pulses. Consequently, measured amplitudes are too high for low PRFs, resulting in measurement errors.
In 2004 at the CISPR/A meeting in Shanghai, China, a proposal was made to amend CISPR 16-1-1 to formally allow the use of spectrum analyzers for compliance measurements. Subsequently, an inquiry was submitted the following year to all CISPR product committees to obtain feedback regarding this proposal. Since the feedback was overwhelmingly positive, CISPR/A began the process of formally introducing this change.
At present, the project is at the final draft international standard stage, and formal completion is targeted for 2010. After publication of the revised version of CISPR 16-1-1, which will be Edition 3, spectrum analyzers will be permitted for use when conducting compliance measurements under the condition that the EUT tested does not emit broadband signals with a PRF of less than 20 Hz.
Additional Requirements
Spectrum analyzers without preselection must meet all of the currently defined requirements for quasipeak measuring receivers with the following exceptions:
•?In clause 4.3.2, the relationship between amplitude and repetition frequency is defined in Figures 1a, 1b, or 1c but limited to repetition frequencies equal to or greater than 20 Hz.
•?Clause 4.3.2 states that the requirements in Table 2 of CISPR 16-1-1 Edition 3 for pulse repetition frequencies less than 20 Hz are not applicable to spectrum analyzers without preselection. It also states that the use of such instruments for compliance testing is conditional.
In cases where such spectrum analyzers are used for compliance measurements, the user must verify and document that the EUT does not emit broadband signals of pulse repetition frequencies of 20 Hz or lower. A determination of the suitability of a spectrum analyzer for testing shall be made by performing the procedure documented in CISPR 16-2-1 Annex E, CISPR 16-2-2 Annex D, or CISPR 16-2-3 Annex F.
Required Suitability Determination for Spectrum Analyzers
The user of a spectrum analyzer without preselection must demonstrate, either through specifications from the manufacturer or by measurement, that the analyzer meets the quasipeak detection requirements for pulse-repetition frequencies greater than 20 Hz in the frequency range of use.
Since the measurement of the pulse repetition frequency of an emission may not always be possible, a simple method to verify the validity of the quasipeak measurement shall be applied when a spectrum analyzer is used. This method is based on a comparison of measurement results with the peak and quasipeak detectors. From the quasipeak weighting functions, the maximum amplitude difference between peak and quasipeak amplitude for a signal with a pulse repetition frequency of 20 Hz is 9 dB for Band A, 12.5 dB for Band B, and 21 dB for Band C/D.
The comparison measurement is to be made at signal frequencies that show amplitudes close to the applicable limit in quasipeak detection. If the difference between the peak and quasipeak detected amplitude is less than the value stated, the quasipeak measurement is valid, and the result obtained with a spectrum analyzer can be used to demonstrate compliance.
If the amplitude difference is greater than the stated values, a measuring receiver which fully complies with the low PRF requirements of CISPR 16-1-1 clause 4.3.2 is to be used for the quasipeak measurement instead of a spectrum analyzer. This comparison measurement requires an adequate signal-to-noise ratio to ensure proper results.
The results of the verification measurements must be included in the test report.
Summary
With the publication of CISPR 16-1-1 Edition 3, the conditional use of spectrum analyzers without preselection will be permitted. This will allow the use of much more economical and general-purpose test instruments for EMI compliance measurements.
However, the user must demonstrate the suitability of such spectrum analyzers by performing a brief measurement as part of each EMI compliance test to verify that the EUT does not emit broadband signals of 20 Hz or less. If the presence of such signals is determined, an EMI receiver fully compliant with the pulse weighting requirements of CISPR 16-1-1 is mandatory for the measurement.
About the Author
Werner Schaefer is a technical leader and quality manager with Cisco Systems. e-mail: [email protected]
December 2009