Calibration of EMI Pulse Generators

Calibration Pulse Generators for EMI applications generate extremely short pulses with rise- and decay times in the PICO-SECOND range and pulse length in the order of tenths of a nanosecond (model IGU 0.3 ns) to cover frequency ranges up to 1 GHz without more than 1 dB of amplitude variation (flatness). Considering construction, tests and calibration microwave principles must be observed. Test equipment must be intermodulation resistant and free of pulse overload ("IGU sub-nanosecond pulses reach 100 Volt across 50 ohm when representing a quasi peak CISPR indication of 1 millivolt [60 dBµV] in the vhf-uhf range Usually only input-selective equipment should be used, preferably CISPR quasi-peak EMI receivers. Laboratory test receivers and spectrum analysers might suffer from pulse overload, their dynamic range for such pulses is often reduced to a fraction of the normal range or even become "negative" (overload before leaving the noise floor). Oscilloscopes normally do not provide sufficient bandwidth to show the true shape of sub nanosecond pulses. It should be > 5 GHz and no overshoot should occur. Input VSWR must be near 1 at input of 50 ohm up to 5 GHz. Slower scopes may be used for basic checks, the rise time indicated will be near that of the scope. These instruments will not be sufficiently accurate to verify the 0.5 times 0.044 µVs pulse area across 50 ohm of the "60 dBµV" pulses at l00 Hz prf. Proper test equipment for dependable calibrations will usually not be available in test laboratories, but basic checks may be performed with top-performing CISPR EMI Receivers and 500 MHz digital oscilloscopes. But it should be kept in mind that EMI receivers need pulse checks ("amplitude relationship" at 100 Hz prf to accurate 1 mV [60 dBµV] rms sine wave calibration signals, pulse weighting tests at other prf's, overload checks) and are not in any case dependable calibration instruments for EMI pulse generators. The most accurate calibration instruments are available at the manufacturer's laboratory. Standard calibration pulse generators traceable to basic units of voltage and time consist of precision 50 ohm solid coaxial lines of a well-defined electrical length (including the input of a mercury switch) and precise dc standard voltage sources to charge this line. The discharge follows to a 50 ohm load with perfect vswr 1.00 up to 5 GHz. The output pulse shows one half of the dc charging voltage and twice the length of the coaxial precision air line. The required pulse areas (at no load) for CISPR applications are 13.5 µVs for the vlf range (Band A) at 25 Hz, 0.316 µVs for the hf range (Band B) and 0.044 µVs at 100 Hz prf for vhf/uhf (Band C and D) for a 60 dBµV indication of a quasi peak EMI receiver. Such a basic calibration generator requires continuous comparison checks with several alternative calibration sources: Electronic laboratory pulse generators with synthesizer drive and rise/decay times of less than 1 ns are used to generate precise pulses of 30 ns to 100 ns duration that may only be used in the flat part of the sin (x) / x - spectrum up to receive frequencies of 1 MHz or less (100 ns pulses show their first spectrum null at 10 MHz, this is the most accurate method to check the true pulse duration). Pulse voltages are checked with a comparator uhf oscilloscope against dc voltage standards with fractional promille accuracies. The comparison to the precision line discharge basic calibration generator shows differences of no more than 0.1 dB. Both methods are directly traceable to basic units of time and DC Voltage. Further checks are performed using a Bessel function crystal filter of defined impulse bandwidth and checking the true-peak pulse voltage in comparison to rf voltage with thermal converter checks). Calibration and traceability continuously checked to standards.