Re: [Discuss-gnuradio] bandwidth for USRP

[Marcus Leech]

Matt Ettus wrote:
> >
> Other than 802.11, there aren't many applications that require more 
> than 8 MHz of bandwidth.  Radio astronomy is one, but you can use 
> fewer bits per sample with RA.  It is not uncommon to use 1 or 2 bits 
> per sample, which would allow you to cover 128 and 64 MHz respectively.
> There is an 8-bit per sample mode in the USRP which will allow you to 
> get 16 MHz of BW across the bus.
>
> Matt
I tried my radio astronomy application on a 3.4Ghz system from gateway, 
and was unable to do more than 8Msps, despite
 the fact that the CPU was relatively lightly loaded (30%) at 8Msps.

In radio astronomy, the sensitivity goes up with 
SQRT(bandwidth*integration_time).  There are practical limits to
 the integration time you can use, but bandwidth well beyond 8Mhz is 
routinely used for continuum observations.
 At the higher radio astronomy frequencies, the "big boys" are now 
routinely running their 1 or 2-bit samplers
 at 1Gsps (which gives you 1Ghz complex bandwidth)!  There are, of 
course, practical limits in available bandwidth
 at lower frequencies.  For example, the area around 21cm only has 
27Mhz of dedicated spectrum, but in reality,
 that spectrum is being encroached upon both by deliberate 
transmissions, and spurious emissions.

That's why places like the Green Bank radio observatory (was there last 
week) are in the middle of
 100-mile-wide "radio quiet zones".  No deliberate radio transmissions 
allowed, and no microwave
 ovens allowed on-site, etc, etc.  There's no cellphone service in that 
part of West Virginia for a reason :-)