# Sampling with TVRX daughterboard and Nyquist frequency

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 ?

Cheers
Cyril

On Tue, 13 Dec 2011 17:44:14 +0100, Cyril C. 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
?

Cheers
Cyril

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).

On Tue, Dec 13, 2011 at 8:44 AM, Cyril C. [email protected]
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 ?

Nyquist says that you need 2 samples per second for every 1 Hz of
bandwidth
you want to sample. However, we use complex (I and Q) samples, which
count
double. So if you want to sample 6 MHz of BW, you would need 6 MS/s
complex or 12 MS/s real. You are best off oversampling by about 25%, so
the 8 MS/s complex of a USRP1 or B100 device can give you the full
bandwidth.

Matt