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.