NARDA PMM ER9000, User Manual

Page: 18 / 106

1-10 General Information
1.13 Principle of
operation
The ER9000 is a real-time gapless receiver based on FFT (Fast Fourier
Transform) which evaluates N frequencies in a single shot.
Real time means that the FFT calculation must be as fast as the incoming
data are sampled to achieve the result that no data are missed.
FFT itself would be not adequate for full compliant measurements without
preventing:
•
Aliasing effect, by adopting an appropriate input filter which
suppresses the frequencies beyond the Nyquist limit.
•
Spectrum leakage, due to the fact that observation of the input
signal must be limited to a finite interval (an infinite series of
subsequent finite intervals in our method). An appropriate time-
windowing function is applied so that the spreading or leakage of
the spectral components away from the correct frequencies is
negligible.
•
Picket fence effect, related to the resolution bias error that may
cause in an FFT spectrum the peaks to be measured too low and
the valleys too high in level. This phenomenon is avoided by adding
a certain number of overlapped FFTs, calculated in parallel over
almost the same input samples.
Thanks to these techniques, the PMM ER9000 has no gaps and detects
any CISPR pulse even at the lowest repetition rates.
The ER9000 processes 6 detectors x 8192 frequencies x 10 FFTs at once.
In this way it is possible, for example, to carry out a complete fully CISPR
16.1.1 compliant measurement with 2 seconds observation time, over the A
band (9 -150 kHz with a 200 Hz RBW filter), or with 1 second over the B
band (150 kHz – 30 MHz with a 9 kHz RBW filter) in less than 3 seconds.