Quick Installation Guide
Document
WERA4-Quick-US-200917
Page 52 of 73
7.1.18
Chirp Length
The chirp length value defines the duration in seconds of each linear frequency sweep.
The Chirp Length field has got the choice as listed in the pull-down menu. We
recommend to use one of the chirp rates listed below:
Chirp length
Doppler spectra
width
Optimum operating
frequency
Comment
0.216667
±2.308 Hz
43.2 MHz
Best for 60 Hz line freq
0.260000
±1.923 Hz
30.0 MHz
Best for 50 Hz line freq
0.433333
±1.154 Hz
10.8 MHz
Best for 60 Hz line freq
0.520000
±0.962 Hz
7.5 MHz
Best for 50 Hz line freq
1.876875
This is the duration of the chirp during frequency scans
Remarks
There are some restrictions regarding the choice of the chirp length:
1.) The chirp length must be kept synchronous with the sample rate to maintain phase continuity. So
only certain chirp length values are allowed for the fixed sample rate.
2.) It is recommended to keep the chirp length synchronous with the period of the line frequency (0.02
sec for 50Hz or 0.0167 sec. for 60 Hz). If this is not the case, the net frequency and its harmonics
can produce artifacts that might disturb the Bragg lines.
Chirp length values of 0.2600 and 0.5200 are synchronized to 50 Hz net frequency, values of 0.2167
and 0.4333 to 60 Hz net frequency.
Different unsynchronized values should be tested for each system and the resulting Bragg spectra
examined, before activating automatic cycles.
Note that an unstable line frequency will smear out the line artifacts into the Bragg spectrum. So a
minimum offset to the Bragg lines should be kept.
If a chirp length is selected, which is not synchronized to line frequency, then select a shorter chirp
length for your operating frequency regarding the row „Optimum operating frequency“.
7.1.19
Range Offset
This parameter is used to correct the absolute range measured by the WERA by the
offset introduced by cable delays. Both Rx and Tx cable lengths must be taken into
account.
This value is not directly the cable length. It needs to be calculated for free space, so
the velocity factors of the cables need to be taken into account. A radio wave travelling
in a coaxial cable will have a shorter wavelength than a radio wave of the same
frequency travelling in free space. The signal is slower inside the cable. The velocity
factor, which can be different for different types of cable, is a factor expressing this
difference relative to the radio wave in free space.
The chosen range cell depth value is the value in meters for one complete range cell.
Note that in the header of the resulting data files this value will be saved as multiple of
one range cell.
