The following was written by Adrian Lincoln in response to a customer asking about averaging Frequency Response Functions (FRFs)
Further to your reply it should be noted that there is no such mathematical or physical quantity corresponding to an averaged structural FRF except in those special cases where you are testing a symmetric structure comprised of nominally identical components (such as turbine blades). For symmetric structures, where you can impact at a similar position on each component and measure the corresponding acceleration at a similar response position, then the individual FRF’s should have the same characteristics and therefore can be averaged.
For structures that don’t have multiple symmetric components then you should not perform any sort of averaging because the FRF’s will not have the same characteristics. For example, if you have beam-like structure and are measuring the FRF’s between different locations along the beam, then these FRF’s should only be averaged as moduli and not with respect to phase and coherence.
A question was recently posed regarding the grounding conditions of a BNC type input. The shield or outer of a BNC cable or connector is not connected to the chassis or ground. What would happen if it was connected to ground?
When using an analogue channel input channel of a P8000, with a tachometer sensor, is it possible to utilise the tachometer processing functions as you would if you were using a tachometer input of the P8000?
For example could you display an RPM on an analogue channel?
When using an analogue channel it is possible to select, from the signal setup matrix, ‘Trigger Type’, with this option set to ‘Tacho’ and the ‘Trigger Level’ set to the voltage crossing threshold several features become available in terms of tachometer processing.
By selecting the above options the user will be able to use the ‘Digital Panel’ display type from the signal display and thus by selecting the ‘Speed’ option in the ‘Display Type’ drop down list an RPM will be available.