[Discuss-gnuradio] gr-uhd: rx_freq tag and lo_locked

[Anderson, Douglas J.]

Hi all,

I've been working on a flowgraph that controls sweeping a USRP by retuning and then dumping samples until catching the sample tagged with rx_freq of the correct value.

I was confused as to why I was still getting mountains of garbage samples (several hundred thousand at 10MS/s) after catching the rx_freq tag, until today I put "assert(usrp->get_sensor("lo_locked").to_bool())" after the block that identifies the correct rx_freq tag, and it popped immediately.

I'm wondering about the prudence of saying a sample is "at a certain frequency" before the LO has had a chance to settle.

I also realize that gr-uhd folks are trying to make the message interface more robust, and the rx_freq tag is an awesome idea, but what I really want to know is what is the first USABLE sample at my requested freq. If I still have to carry around a usrp_source pointer so that I can check the LO state, then the tags output by the usrp_source lose a lot of appeal.

Any chance of making rx_freq tag "the first usable sample", or is there a good reason for the way it works that I haven't considered?

-Doug