|
From: | mleech |
Subject: | Re: [Discuss-gnuradio] Sampling with TVRX daughterboard and Nyquist frequency |
Date: | Tue, 13 Dec 2011 11:52:28 -0500 |
User-agent: | Roundcube Webmail/0.5.1 |
On Tue, 13 Dec 2011 17:44:14 +0100, Cyril Cavel wrote:
Hi all,I use a TVRX daughterboard to acquire VHF signals. I believe (but I may be mistaken) that the TVRX daughterboard downconverts RF signals to a baseband of 6MHz bandwidth before going to the ADC. The function usrp_rx_cfile (which I use to acquire signals) has a minimum decimation factor of 8 for a sampling rate of 64Ms/s. So it gives a maximum sampling rate of 8MHz.Nyquist theorem tells us that to acquire properly a signal occupying a band between 0 and 6MHz, the sampling rate shall be at least 12MHz, which is then unreachable by the TVRX daughterboard.So, are my figures wrong, or am I missing something ?Thanks for your lights,CheersCyril
The underlying motherboard samples at a fixed rate--64Msps for USRP1/B100/E1XX, and the N2XX family samples at 100Msps, so Nyquist isn't violated at all. The FPGA downconverts that sampled 6Mhz to baseband, which is then run through CIC decimators and half-band filters. In general, a Gnu Radio flow-graph deals with complex-baseband signals, which "straddle" 0Hz (that is, for a signal of B bandwidth, it stretches from -B/2 Hz to B/2 Hz).
[Prev in Thread] | Current Thread | [Next in Thread] |