My take is the TS wants to change pulse-width and repetition rate (i.e. duty-cycle and frequency).

 

want to use a signal of 7 ms...how can this be done if I can only change frequency on generator?

 

All the generators I have seen are perfectly capable of setting a square wave with period such as 7ms and whatever pulse length or dutycycle you desire.

 

Some function generators have a duty-cycle control.
Or you can do what @Aleph(0) suggested. Use the one-shot feature and adjust the pulse-width and repetition rate independently.

 

7ms is roughly 143Hz. 1 / .007 = 142.86