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Re: [Discuss-gnuradio] no packet reception when using uhd_tune_request

From: Josh Blum
Subject: Re: [Discuss-gnuradio] no packet reception when using uhd_tune_request
Date: Wed, 31 Oct 2012 14:31:22 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:16.0) Gecko/20121011 Thunderbird/16.0.1

On 10/31/2012 01:10 PM, Zing Yu wrote:
> Hi All,
> I have a couple of questions this time. 1) I have a setup in which I
> transmit gmsk packets from a tx node which are then received
> successfuly at an rx node (both nodes are USRP N200 radios and are
> being operated by GRC). However, when I turn off the cordic at two
> nodes by putting this line in uhd usrp sink at tx node and in uhd
> usrp source at rx node, I do not receive any packets at all at rx
> node. uhd.tune_request(915e6,
> dsp_freq=0,dsp_freq_policy=uhd.tune_request.POLICY_MANUAL) I have
> verified (by means of get_center_frequency() method) that the two
> nodes in this case are set to the same center frequency which is
> 914.285 MHz, so, there is no center freq mismatch between tx and rx
> nodes. In this case, I see some underflows at tx node and if I put a
> scope at rx node to see the raw received samples, then I see that the
> samples are not being received continuously (i.e., I see received
> gmsk samples for a few seconds, then noise samples for next few
> seconds and so on) even though the tx node is sending packets
> back-to-back.

Every thing you are doing sounds correct.

Am am a bit confused about underflows. Are you saying that the transmit
underflows, but only with the special tune request? Normally it does not?

I suggest that you check that daguhterboard LO is locked after the tune.
print usrp_source.get_rx_sensor("lo_locked").to_bool()

What daughterboard are you using?

> 2) Previously, the frequency of DUC/DDC cordic, target frequency,
> actual frequency used to be printed out while running any flow-graph
> involving usrp sink/source but it is not the case anymore. What is
> the easiest way to print all this info while within GRC?

You can print the result of the set_center_frequency(), it returns a
tune_request struct with RF and DSP frequency fields.


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