Re: [Discuss-gnuradio] Large FFTs

[Martin DvH]

Dear Carles,

On Mon, 2010-08-23 at 21:37 +0200, Carles Fernandez wrote:
> Dear Thomas,
> 
> we have found some problems when using USRP2+DBSRX for GPS due to
> phase noise. See details in http://www.ruby-forum.com/topic/213845
I have experimented with using the clock from the usrp to a modified
TVRX (disabling the internal TVRX clock, using a 4 Mhz clock from the
usrp).

One of my findings was that if you use a low resistor from the usrp
clock output to a daughterboard, followed by a relatively large
capacitor to ground (for lowpass filtering) you can get a lot of
spurious.
The usrp can generate quite some power on this clockline.
When you load the clockline to heavy, it wil start generating RF power.
The load being a low resistor and big capacitor. 
The antenna being the track from the usrp to the lowpass filter.

In the worst case you can be generating several milliwatts.

This can definitely have an effect on the performance.
Especially if you happen to be on a multiple of the daughterboard clock.

My advise would be to experiment with a higher impedance lowpass filter
which does not put a heavy load on the clockline.
You could also experiment with a buffer amplifier.

Good luck,

Martin Dudok van Heel

Olifantasia.com


> 
> If someone have succeded on this, we would appreciate some hints to
> make it work. Our software GPS receiver was not able to track signals,
> while it run smoothly when using USRP1 + DBSRX without modification.
> 
> Best regards,
> Carles Fernandez
> 
> 
> 
> 
> On Mon, Aug 23, 2010 at 10:00 AM, Thomas Hobiger <address@hidden> wrote:
> > We are considering to purchase a USRP2 + a DBSRX board in order to utilize
> > it for some GPS stuff. Thus it would be interesting to know what's the
> > maximum supported (implemented) FFTs size. I have checked the old
> > discussions, but there's nothing really conclusive. What we are looking for
> > is something larger than 16K FFT points.
> > Maybe someone has experiences with such large FFTs and how they perform
> > (Flops or FFTs per second)?
> >
> > Best regards,
> >  Thomas Hobiger
> >
> > --
> > ******************************************************************
> > Dr. Thomas Hobiger
> > Space-Time Measurement Project
> > Space-Time Standards Group
> > New Generation Network Research Center
> > National Institute of Information and Communications Technology
> > ------------------------------------------------------------------
> > 4-2-1 Nukui-Kitamachi, Koganei
> > 184-8795 Tokyo
> > Japan
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> >
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