Down Conversion on USRP


#1

Hi all
Newbie here. I am using FLEX 2400 daughterboard with USRP.
On
the exploring GNUradio web page it is mentioned under the title “what
goes
in FPGA” that there is a NCO and Decimation filter in FPGA. I did a
quick
scan of the FLEX 2400 receive schematic and it appears to me that it is
a
direct conversion receiver. So if it is a direct conversion receiver
with
the demodulation and filtering bringing the signal to baseband, why do
we
need the NCO and Decimation filter in the FPGA? Please correct me if I
have
interpreted this incorrectly.

Thank you
SQ


#2

science quest wrote:

Hi all
Newbie here. I am using FLEX 2400 daughterboard with USRP.
On the exploring GNUradio web page it is mentioned under the title
“what goes in FPGA” that there is a NCO and Decimation filter in FPGA.
I did a quick scan of the FLEX 2400 receive schematic and it appears
to me that it is a direct conversion receiver. So if it is a direct
conversion receiver with the demodulation and filtering bringing the
signal to baseband, why do we need the NCO and Decimation filter in
the FPGA? Please correct me if I have interpreted this incorrectly.

The RFX2400 is direct conversion, but the VCO/PLL only tunes in 4 MHz
steps (this is changeable, but leaving it this way gives the best phase
noise). The DDC downconverts the rest of the way if necessary.

Some other daughterboards, like the TVRX, are not direct conversion.

Matt


#3

Hey Matt:

I am trying to implement USRP as a mote ( zigbee - 802.11.25.4 )
receiver. I need to extract a 5MHz channel at 2.4 GHz. Should I just use
decimation of 6 for USRP source or should I use the freq translation
filter ? Also does frequency translation filter remove other frequencies
in the spectrum which will superimpose on 0 Hz because of sampling ?

Gautam

Matt E. wrote:

science quest wrote:

Hi all
Newbie here. I am using FLEX 2400 daughterboard with USRP.
On the exploring GNUradio web page it is mentioned under the title
“what goes in FPGA” that there is a NCO and Decimation filter in FPGA.
I did a quick scan of the FLEX 2400 receive schematic and it appears
to me that it is a direct conversion receiver. So if it is a direct
conversion receiver with the demodulation and filtering bringing the
signal to baseband, why do we need the NCO and Decimation filter in
the FPGA? Please correct me if I have interpreted this incorrectly.

The RFX2400 is direct conversion, but the VCO/PLL only tunes in 4 MHz
steps (this is changeable, but leaving it this way gives the best phase
noise). The DDC downconverts the rest of the way if necessary.

Some other daughterboards, like the TVRX, are not direct conversion.

Matt