I have replaced the LFTX and the USRP motherboard with other boards and I get the same results, so I ruled out a hardware problem.
It's like when using the length_tag feature on the USRP block it resets the USRP after every packet.
I have created a simple experiment to reproduce this behavior. It consists of a simple signal generator, a tagger and two USRP blocks. When I enable the length tag in the USRP sink, I get a corrupted signal. When I don't use the length tag I get a nice signal. I attached a screen shot of GRC, the corrupted signal and the good signal.
So maybe burst mode should not be used for packets or there is a bug in uhd/gnuradio.
Roee
On 11/13/2015 02:38 AM, Marcus Müller
wrote:
Hi Roee,
are you changing the frequency on both the transmitter and receiver?
Because: mixing anything to a higher frequency is exactly that, a multiplication with a sinusoid.
Best regards,
Marcus
On 13.11.2015 07:01, Roee Bar wrote:
Hello,
I am experiencing some weird behaviour with USRP blocks.
I have a PHY block that generates packets with "packet_len" tag. This stream is the input to a USRP sink block (the transmitter). Between packets, my PHY block does not produce anything, so the USRP assumes zeros until a new packet arrives with “packet_len” tag (I guess this is what we call 'burst mode'?). The USRP source (the receiver) receives the signal from the USRP sink as expected (with zeros between packets).
The problem arises when I change the center frequency from zero to something higher (I use LFTX/LFRX boards). Then, the received signal envelop is corrupted like it's multiplied by some cosine (see screenshot attached, all frames are supposed to have the same height).
When I don't use the packet_len feature of the USRP block or when my PHY block produces packets continuously, the received signal is okay.
Why does this happen? Is my methodology of not producing anything between packets is the right approach?
Thanks in advance,
Roee
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