# Switching and high spike in spectrum

**URL:** <https://www.ruby-forum.com/t/switching-and-high-spike-in-spectrum/232819>\
**Category:** GNU Radio\
**Created:** [October 29, 2014, 6:55pm UTC](https://www.ruby-forum.com/t/switching-and-high-spike-in-spectrum/232819 "2014-10-29T18:55:54Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\
**Post date:** [October 29, 2014, 6:55pm UTC](https://www.ruby-forum.com/t/switching-and-high-spike-in-spectrum/232819/1 "2014-10-29T18:55:54Z")

</div>

USRPN210r4 with SBX

I am observing a strong spike at the center of the receive spectrum when  
I  
start burst transmission.

My top flowgraph contains following two hierarchical blocks

1. A transmitter flow graph with (tx\_time, tx\_sob, tx\_eob)
2. A receiver flow graph

When I run top flowgrpah (without transmitting anything) and observe the  
FFT of the received signal the spectrum does not contain high spike in  
the  
center.

But as soon as I start transmitting in burst mode I see a very high  
spike  
in center of the received signal FFT spectrum. It looks like LO  
(transmitter or receiver ) is being received? Which one is it ? And why  
is  
it happening? How can I avoid it because it is affecting my packets.

When I apply the offset in digital using DDC/DUC, the spike moves out of  
the band.