L3Harris NARDA PMM ER8000 User Manual page 18

1.13 Principle of
operation
1-10 General Information
Differently from PMM 9010, which is a digital sweeping receiver based on
a zero-lock-time, step-by-step NCO (numeric controlled oscillator), fast
settling time FIR RBW filters followed by digital detectors - something
faster than conventional receivers anyway - the ER8000 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 ER8000 has no gaps and detects
any CISPR pulse even at the lowest repetition rates.
The ER8000 processes 6 detectors x 1024 frequencies x 16 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 hold time over the A band
(9 -150 kHz with a 200 Hz RBW filter), or with 1 second hold time over
the B band (150 kHz – 30 MHz with a 9 kHz RBW filter) in less than 3
seconds.