Re: [Discuss-gnuradio] Several basic questions

[Trond Danielsen]

2007/6/11, Teun <address@hidden>:
> Hi Guys,
>
> I tried searching the forum, but I still got some problems which I don't
> fully understand.
>
> To my knowledge, the receiving ADC Rate is 64 MSamples/second, at a
> granuality of 14 bits/sample. This would make it possible to have a total
> system bandwith of 32 Mhz (Nyquist), which is enough for example 802.11
> standards. However, samples are transfered to the PC, using the USB bus. The
> usual I/O handling of the USB bus is 16 bits I and 16 bits Q signals, which
> means that you require 4 bytes per complex sample. The raw data rate for the
> USB is 480Mbit/s, but due to overhead it is possible to get only about
> 320Mbit/s, which is about 40Mbyte/s. Summing everything up, it is possible
> to transfer 40M/4 = 10Msamples/second over the USB bus, which means that you
> can only check a system with a bandwith of approximately 10Mhz. Am I right
> about this? If this is true, the limiting factor is the USB bus, not the
> ADC's on the USRP. Is it possible to use some sort of 1Gbit ethernet
> connection for the USRP? I know this wouldn't help that much, but It would
> make it possible to achieve the 802.11 standards (Channels of 20Mhz). Is
> this possible or are other things like CPU speed/PC memory becoming an
> issue?

Correct me if I am wrong, but in the usrp_rx_cfile.py script there is
an option to transfer 8-bit samples across the USB. With this option
enable, it should be possible to increase the BW at the expense of the
resolution.

--
Trond Danielsen