|
From: | Meelis Nõmm |
Subject: | Re: [Discuss-gnuradio] Timed USRP source commands and tags |
Date: | Thu, 24 Nov 2016 13:32:43 +0200 |
Dear Meelis,
On 23.11.2016 16:28, Meelis Nõmm wrote:
That's correct; since the GNU Radio blocks run on the PC, they simply *can't* know when something like changing the centerfrequency happens on the USRP – because that might happen with a command time, and the command time refers to the clock on the USRP, not to the PC's clock, and these two clocks are _completely_ independent.Now my questions are:HelloWe are building a frequency scanner. As suggested in many posts, like in [1], one should use timed command messages.
1. Do I understand it correctly that the gnuradio USRP Source block does not add a tag when the center frequency changes?
Yes. But that's UHD's job. You also have to trust your operating system to open a file and give you its content when you want to read a file...2. If that is correct then we just have to trust that the frequency changed at the requested time?
So, unless a "L" (for: "L"ate, ie. a timed command arrived at the USRP *after* its command time had already passed) happens, you should be fine.
There's no such information sent by the USRP.3. Does the USRP output the center frequency change information and it is lost along the line or is it never given out (just would imagine that the FPGA should send this vital information with the data packages)?
Yep – but that's the beauty of the timestamps in the first sample of a reception – because you know the sampling rate, and you know the first samples' exact time from that timestamp, you can infer any subsequent sample's (USRP-clock-relative) timestamp by a simple division of the number of samples processed since that timestamp and the sampling rate!4. This would mean that the data flow between work functions, knows nothing of the center frequency alteration mid block (unless we attach a tag to the "correct" sample)?
Best regards,
Marcus
Just trying to understand the situation and our options
Thank you
Meelis
_______________________________________________ Discuss-gnuradio mailing list address@hidden https://lists.gnu.org/mailman/ listinfo/discuss-gnuradio
_______________________________________________
Discuss-gnuradio mailing list
address@hidden
https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
[Prev in Thread] | Current Thread | [Next in Thread] |