[USRP-users] N210 WBX annoying spurs while switching

Andy Walls andy at silverblocksystems.net
Thu Oct 23 07:49:58 EDT 2014


FWIW, this paper investigates using a Tukey window around an AIS GMSK
TDMA burst to ramp up and down, optionally followed by a LPF:

http://www.univ-brest.fr/lest/tst/publications/pdf/0506itstAis.pdf

Figures 5 and 8 show the spectral difference between Rectangular (R),
Exponentional (E), and Tukey Window (W) ramps.

Regards,
Andy

On Thu, 2014-10-23 at 11:06 +0500, bob wole via USRP-users wrote:
> Okay, I'll make a custom block in gnruadio to achieve ramp up/down for
> FSK waveform to avoid "keyclicks" . 
> 
> On Thu, Oct 23, 2014 at 10:52 AM, Matt Ettus <matt at ettus.com> wrote:
>         
>         No problem.  The RF filter at the output really isn't the
>         proper way to solve this problem, it will just keep the noise
>         within a band.  What you really want is to avoid putting out
>         energy outside of your channel, and that can only be done with
>         ramping.
>         
>         
>         Matt
>         
>         On Wed, Oct 22, 2014 at 10:46 PM, bob wole <bnwole at gmail.com>
>         wrote:
>                 Thanks Matt for your response, that is the exact
>                 answer I was looking for. Actually I am using USRPs
>                 for TDMA and I do not want my time slot to increase
>                 very much. I wanted to know the ramping duration
>                 because this will affect the time slot duration.
>                 Second option could be that I can add an RF filter at
>                 the output of USRPs. 
>                 
>                 On Thu, Oct 23, 2014 at 10:25 AM, Matt Ettus
>                 <matt at ettus.com> wrote:
>                         
>                         Yes, but it is even better to ramp up at the
>                         beginning and down at the end, and then pad a
>                         little bit with zeros.   If you ramp up and
>                         down smoothly for 1us, you'll confine the
>                         emissions to about 1 MHz on either side.  All
>                         bursty communication systems will do this,
>                         like Bluetooth, Wifi, even morse code, and
>                         most newer standards even specify the ramping
>                         rate. 
>                         
>                         
>                         Matt
>                         
>                         
>                         
>                         On Wed, Oct 22, 2014 at 10:08 PM, bob wole
>                         <bnwole at gmail.com> wrote:
>                                 So you are suggesting that I should
>                                 zero pad samples at start and end of
>                                 my burst to avoid this ? 
>                                 
>                                 
>                                 On Wed, Oct 22, 2014 at 9:16 PM, Matt
>                                 Ettus <matt at ettus.com> wrote:
>                                         
>                                         When you start and stop
>                                         transmission without smoothly
>                                         ramping the signal to zero at
>                                         the beginning and end, you
>                                         will create wideband spurs.
>                                         Just think about it as
>                                         amplitude modulation.  Even if
>                                         you are transmitting a pure
>                                         sine wave, if you turn it on
>                                         and off quickly, it will
>                                         create out of band spurs.
>                                         Hams call this keyclicks.
>                                         
>                                         
>                                         Matt
>                                         
>                                         
>                                         
>                                         On Wed, Oct 22, 2014 at 1:25
>                                         AM, bob wole via USRP-users
>                                         <usrp-users at lists.ettus.com>
>                                         wrote:
>                                         
>                                                 I noticed that there
>                                                 are a lot of
>                                                 significant spurs
>                                                 generated in the
>                                                 output of WBX, on
>                                                 different frequencies
>                                                 randomly, when I use
>                                                 tx_time, tx_sob_tx_eob
>                                                 tags. I do not see
>                                                 these spurs if I
>                                                 transmit continuously.
>                                                 These spurs appears
>                                                 only when I start and
>                                                 stop transmission
>                                                 using stream tags. 
>                                                 
>                                                 How can I get rid of
>                                                 these spurs and what
>                                                 is the cause?
>                                                 
>                                                 -
>                                                 
>                                                 Bob
>                                                 
>                                                 
>                                                 
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>                                                 
>                                         
>                                         
>                                 
>                                 
>                         
>                         
>                 
>                 
>         
>         
> 
> 
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