If you are measuring data, several of the Structures Acquisition products can measre FRF and Coherence functions. If you are processng data, then the Signature Throughput processing add-in can calculate FRF / Coherence functions if you specify the reference channel. Note that tracking mode should be set to stationary and more than one average should be used. If you want a waterfall / colormap of Coherence then you can set tracking to Run up / Run Down / Time, etc. and use semi-stationary averaging with more than one average. Cohence cannot be calculated if just one average is used.
how can I define a reffrence chennel? I want to measure a presure to pressure transfer function, I have a source signal and a response signal. I don't know how to declare the source signal as a refrence channel.
If you are trying to calculate an FRF using two pressure channels I would call that a transmissibility function since both are responses. An acoustic source would normally be measured as volume acceleration or volume velocity for example. The calculation will still work with two pressures though.
It's hard to say without a better screen shot, but is your data in two measurements or runs? If so, you cannot turn on the reference channel because it is not in every run. On the Time Data Selection worksheet, use the Blue Arrow to move one of the channels to the other run so that you have one run. You can verify this by looking at the Measurements view or the Channels Pivot view rather than Channel list view to verify you have just one Measurement \ Run. If you have multiple runs, the reference channel must be in every run before you can select it.
You can probably do a Skype meeting with your local support team and they can assist if this is not clear.