Hi Folks,

with this thread I would like to trigger a discussion on the obove topic. The

time domain output of OpenEMS can be lengthy and is therefore sometimes hard

to handle. One option to improve the situation could be the following:

The most common excitation is the modulated Gaussian pulse. The output of

probes or field dumps is stored with sampling rate of 1/(4(f_0+f_c)). f_0 is

the center frequency and f_c is the 20dB cutoff frequency. This can cover a

bandwidth of 4*(f_0+f_c). The smapling rate scales with f_0. However, the

whole information of a modulated signal is contained in the

baseband. Therefore it would be sufficient to "demodulate" the signal by

multiplying it with cos(2*pi*f_0) and sin(2*pi*f_0) and to store only these

two (low pass filtered) components. Any posprocessing must "re-modulate" the

data with f_0 in order to reconsruct the original signal.

The Gaussian excitation has a duration of 9/(pi*f_c), i.e. it starts and stops

at exp(-9)=-78dB. If we assume the same limits in the frequency domain we

arrive at a baseband bandwidth of 1/(4f_c). This approximation is good enough,

knowing that a signal cannot have a finite extent in frequency domain as well

as in time domain. As we have to store both in-phase an quadrature-phase

components, we will have a storage rate of 1/(8f_c).

The ratio of the original sampling rate and the baseband storage rate is

2f_c/(f_0+f_c). So for the simple patch tutorial with f_0=2GHz and f_c=1GHz we

obtain a ratio of 2/3 for the storage requirement. But the simple patch

tutorial would also work with f_0=2.5GHz and f_c=0.5GHz. This would result in

a ratio of 1/3.

It is clear that a lower bandwidth results in a longer excitation pulse. But

the system response is much longer than the excitation in most cases due to

resonant structures in the model. So this effect should be of minor

importance. Generally we have a time-bandwidth product of 4*9/pi for the

baseband pulse, i.e. we need 12 samples per excitation length to repesent the

signal.

Maybe there are additioinal options for lossy compression of the baseband

data, but I am no expert on this. We could also start the recording only when

a certain amplitude is reached. What are your ideas on this topic?

Cheers

Georg

## Efficient storage of narrowband time domain data

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