Really appreciate your reply. The carrier frequency that I use is 6MHz. I would like to try the preamplifier that you recommended. I got another problem which may clear the reason why I met this problem. I used a signal generator to produce a 6.01MHz sinusoid wave. After down conversion by 6MHz, it's supposed to show the 10kHz in the spectrum. However, the spectrum that I got gives a really big 0 frequency component beside the 10kHz component. It seems that a DC offset occurred during signal reception. Is it right? If so, what causes this problem? How to eliminate the DC offset?
Many appreciation! Yan On 03/28/11, "Marcus D. Leech" <mle...@ripnet.com> wrote: > On 03/28/2011 06:41 PM, Yan Nie wrote: > > Dear all, > > > > I'm using LFRX to detect a -120dBm signal. According to my measurement, the > > lowest power of signal that LFRX is able to detect is -90dBm. I tried > > deploy a preamplifier to amplify the received signal by certain dB in order > > to reach the -90dBm, but this turned out that noise is amplified as well > > and the signal still merges in noise, undetectable. I also tried to set > > proper bandwidth for the lowpass filter in order to eliminate noise and > > avoid the signal distortion to the largest extend, but this still cannot > > allow this weak signal be detected. > > > > > > The flow-graph of my receiver is: > > > > > > usrp_source_c --> Blackman lowpass filter --> > > file_sink(gr.sizeof_gr_complex) > > > > > > The signal that I wish to detect is a 13-bit Barker code with 40 us chip > > rate. The samples frequency at receiver side is 250kHz which gives the > > decimation rate 256. Regarding with the Blackman lowpass filter, the > > sampling frequency for this filter is 35kHz, and the cutoff frequency is > > 17kHz, transition_width is 0.1kHz. > > > > > > Really appreciate any of your help! > > > > > > > > > What's the carrier frequency? > > At -120dBm, you do need a low-noise amplifier with sufficient gain to > overcome the noise of the ADC. > Higher-noise amplifiers might indeed have the effect you're seeing. > I've used the ERA-3 series from > Mini-Circuits sucessfully for low-noise HF reception at around 38MHz > on a BASIC_RX. > > > > > > > -- > Principal Investigator > Shirleys Bay Radio Astronomy Consortium > http://www.sbrac.org > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >
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