I have a flowgraph that is supposed to be set up so that the tx frequency can be changed on the fly. It was originally changed by changing variables, but I've switched to publishing pmt_t destined for the usrp sink (it didn't seem to work on the rx side very well, something was buggy). Testing seems to indicate that the center frequency isn't always changed before the data is transmitted (in the test I should be sending 20 chunks of data, and I should be alternating between two frequencies). Is there a way to query the usrp sink to determine when it has changed it's center frequency? It doesn't publish a message, or it doesn't look that way after looking at source.