Usrp_sink tags confusion

Dear all,

I would like to do some simple frequency hopping over my USRP (x300 with
UBX-160 daugherboard). To that end I wish to use the following tags:

  • tx_len (defined in the length-tag field of the USRP sink). This
    (naturally) gives the length of a burst

-tx_freq -> used to change the centre frequency of the USRP before the
first bit of every packet

-tx_time -> used to have a fixed time interval between 2 consecutive
bursts

Here is my confusion:

When I add the tx_len tag to specific items, I get NO “tG”-output from
gnuradio: this means that I am adding the tx_len tag correctly at the
first
item of every burst according to the specified burst length, correct?
However, when I add the tx_freq tag to the same item that I add the
tx_len
tag to, I get these messages from gnuradio:

DEBUG: Received tx_freq mid-burst.
DEBUG: Received tx_freq on start of burst.
DEBUG: Received command: freq
– Tune Request: 5793.000000 MHz
– The RF LO does not support the requested frequency:
– Requested LO Frequency: 5793.000000 MHz
– RF LO Result: 5793.003663 MHz
– Attempted to use the DSP to reach the requested frequency:
– Desired DSP Frequency: -0.003663 MHz
– DSP Result: -0.003663 MHz
– Successfully tuned to 5793.000000 MHz

How can the tx_freq be received mid_burst if I place it in the same item
as
the tx_len tag, i.e. on the first item of a new burst? And how can it be
received mid-burst AND on start of burst?

Any help is greatly appreciated!

Kind regards,
Matija

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