# Re: tx\_time tag accuraccy

**URL:** <https://www.ruby-forum.com/t/re-tx-time-tag-accuraccy/230210>\
**Category:** GNU Radio\
**Created:** [June 4, 2014, 8:47pm UTC](https://www.ruby-forum.com/t/re-tx-time-tag-accuraccy/230210 "2014-06-04T20:47:18Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![jason\_sam](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/7993a0/32.png) [@jason\_sam](https://www.ruby-forum.com/u/jason_sam)\
**Post date:** [June 4, 2014, 8:47pm UTC](https://www.ruby-forum.com/t/re-tx-time-tag-accuraccy/230210/1 "2014-06-04T20:47:18Z")

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Marcus! Thanks for you comment.

I think that USRP transmit FIFO is at the start of the DSP chain in FPGA  
i.e it is prior to both of the interpolation filters? right? I am not  
talking about when the burst will be over the air, I want to know when  
the  
first sample of the burst will leave the transmit FIFO if it has been  
tagged as tx\_time=X.

–  
Bob

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**Author:** ![bob\_wole](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/b4bc9f/32.png) [@bob\_wole](https://www.ruby-forum.com/u/bob_wole)\
**Post date:** [June 4, 2014, 9:09pm UTC](https://www.ruby-forum.com/t/re-tx-time-tag-accuraccy/230210/2 "2014-06-04T21:09:42Z")

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Discuss-gnuradio mailing list  
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[https://lists.gnu.org/mailman/listinfo/discuss-gnuradio](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio)

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**Author:** ![bob\_wole](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/b4bc9f/32.png) [@bob\_wole](https://www.ruby-forum.com/u/bob_wole)\
**Post date:** [June 5, 2014, 8:03am UTC](https://www.ruby-forum.com/t/re-tx-time-tag-accuraccy/230210/3 "2014-06-05T08:03:56Z")

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Sorry for not stating the hardware earlier. I am using USRP N210, with  
RFX2400 and WBX boards.

–  
Bob

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**Author:** ![bob\_wole](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/b4bc9f/32.png) [@bob\_wole](https://www.ruby-forum.com/u/bob_wole)\
**Post date:** [June 9, 2014, 7:07am UTC](https://www.ruby-forum.com/t/re-tx-time-tag-accuraccy/230210/4 "2014-06-09T07:07:12Z")

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On Wed, Jun 4, 2014 at 2:01 PM, bob wole [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> I think that USRP transmit FIFO is at the start of the DSP chain in FPGA i.e  
> it is prior to both of the interpolation filters? right? I am not talking  
> about when the burst will be over the air, I want to know when the first  
> sample of the burst will leave the transmit FIFO if it has been tagged as  
> tx\_time=X.

If you tag a burst with time X, that burst will be released from the  
device buffer at time X - assuming the burst arrives at the device  
before time X. After time X, there will be a sample rate dependent  
delay and a fixed analog group delay before the burst can be observed  
at the antenna output.

In other words, a burst tagged to transmit at time X will leave the  
buffer at time X.

-TT

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**Author:** ![bob\_wole](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/b4bc9f/32.png) [@bob\_wole](https://www.ruby-forum.com/u/bob_wole)\
**Post date:** [June 9, 2014, 8:16am UTC](https://www.ruby-forum.com/t/re-tx-time-tag-accuraccy/230210/5 "2014-06-09T08:16:51Z")

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Hi Tom,

Thanks for your comment. I do not think if I tag a burst with time X,  
it’ll  
go out of USRP transmit FIFO at exactly X, there would be some small  
delta  
involved depending on the clock resolution, and I want to know that  
delta.  
I know that there would be some DSP delays, depending on sample rate,  
and  
analog delays _AFTER_ the burst leave transmit buffer.

–  
Bob

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<div class="post-metadata">

**Author:** ![bob\_wole](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/b/b4bc9f/32.png) [@bob\_wole](https://www.ruby-forum.com/u/bob_wole)\
**Post date:** [June 9, 2014, 4:23pm UTC](https://www.ruby-forum.com/t/re-tx-time-tag-accuraccy/230210/6 "2014-06-09T16:23:27Z")

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On Mon, Jun 9, 2014 at 2:15 AM, bob wole [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> Thanks for your comment. I do not think if I tag a burst with time X, it’ll  
> go out of USRP transmit FIFO at exactly X, there would be some small delta  
> involved depending on the clock resolution, and I want to know that delta. I  
> know that there would be some DSP delays, depending on sample rate, and  
> analog delays _AFTER_ the burst leave transmit buffer.

The N210 clocking rate is 100 MHz, which gives timing resolution of  
10ns.

-TT