Frequency-sweep with grc

GNU Radio
1

Hi

I want to perform a frequency sweep with grc by sweeping the
center-frequency argument of the usrp-block and generate and display a
single complex-number for each value (this is for a vector-network
analyzer).

What I have come up with so far, is a block that processes a fixed
number of samples at a single frequency (using the head-block) and
outputs these to a file.

My problems:

  1. Even though I can output these samples to a file using the file-sink,
    I only get a blank display on the scope-sink whenever I use the
    head-block in the signal flow. I do get “live-data” though when I remove
    the head-block. How can I display the results of a single pass
    generating say 1000-samples within grc?

2)Is there a way to change a parameter (e.g. the center frequency) after
a number of samples have been processed?

Thanks

Wolfgang