Hi Marcus, > >> > Not at all! > Sorry, I was taking pictures of hardware and hence just took one for > the GRC too. As I showed in previous picture, I am only using RX-B. I > am not sure how to connections will be for dual mode, but currently I > am just using one. I get complex values in the GRC and I can plot > > complex FFT of them > > http://postimg.org/image/a69avcfrl/
> This is my setup where i am extracting signals from Powerline using RX-B pin of the daugterboard There are two *different* projects : a) I want to understand the powerline channel b) I want to transmit and receive signals on the channel > I have a better picture[2] now : <http://postimg.org/image/hptdrl2vl/> > http://postimg.org/image/hptdrl2vl/ > > Interesting! Why is there a time axis? I must admit those graphs look like > they took some effort to create; really, a clean drawing on paper would > have been as good for me, but thanks. > I am showing the spectrogram when I set center freq at 125 kHz 0 kHz. I am trying to show what I am getting from the channel > > >> And, as I mentioned in IRC when I asked for them, it is important to >> define wheter you're doing I&Q or real sampling. >> > > I am dong I&Q sampling as shown in [1]. By real sampling, I suppose > you meant that I am taking magnitude before feeding to the Spectrogram. > > No, I didn't mean that. > I mean: Is the signal you *want* to observe a single voltage signal > varying over time, or is it an I & Q signal pair? > > > The signal in (a) is the EMI on the channel by various devices on the channel. I want to observe the I, Q samples on the channel. The GRC pic showed the complex datatype of USRP source and I assume that it is giving me IQ samples from channel on RX-B pin. Please let me know if I am incorrect ? If I had set real data type of USRP source block or complext to mag in front of complex datatype of USRP source block, I would have got real signals ... according to my current understanding. > It's no use discussing any of your question until we know whether we're >> looking at a complex equivalent baseband or at a real signal. >> >> > I am doing complex baseband as in the picture. > > I'm not convinced you actually are in both cases! That's why I'm asking :) > Okay, I assumed I am doing since I am using complex datatype of USRP source and just changing the center frequency and sample rate. Either both case i am doing complex baseband or I am doing it in neither, because changing center frequency shouldn't make a difference. > My question was related > to the sampling. I thought doing sampling (complex baseband as shown > in GRC) > > OK, we need to talk about sampling: > Sampling takes place where your analog signal becomes a series of digital > numbers, in the ADC of the N210. That is before you digitally tune to e.g. > 125 kHz. The LF and Basic daughterboards don't have tuners! > > in 250kHz at center frequency 125kHz is equavelent to doing > 500kHz, with center frequency 0Hz using LFRX. > > That really really depends! It depends on whether you want to observe a > single real signal or an I&Q signal pair. So: what do you want to observe? > In a) I want to observe the channel EMI In b) I am transmitting real signal on one side and able to decode a signal on the other side. The TX setup is exactly the same as above, just with a different daugtercard which transmits, receives at center freq 1 MHz. > So here's the problem I'm having with your original question: > You said > > I want to sample first 250KHz on the channel. > > But: What *are* the first 250 kHz? > If you're just sampling a real signal with a single ADC, then obviously, > you get a real-valued stream of samples, and to cover the bandwidth of 0 Hz > – 250 kHz you will need to sample at 500 kHz. > I don't get " a real signal" ? There is one single ADC per LFRX card's RX pin that will give complex values with a Complex USRP source right from the each RX pins ? I am using RX-B, hence I should be receiving complex samples from channel with complex USRP source. Please correct me, because that is what i think. > If we're talking about complex baseband, I'd define (and that's purely > something that I like, not something that is uniformly defined somewhere) > the "first 250 kHz" to be the spectrum from -125 kHz – +125 kHz, because > those 250 kHz correspond to the 250 kHz bandpass signal "closest" to the > center frequency of your down-mixer. To get those 250 kHz, your ADC will > need to run at 250 kHz, but it will need to produce one I and one Q sample > simultaneously. > > I would definitely want to understand what am I getting with the current GRC pic. I think it is complex baseband. Please correct me. > You can configure the N210 carrying a Basic or LF board into either mode – > real sampling of up to two channels, and complex sampling of an I&Q pair > that you feed into the two SMA connectors on the daughterboard. > Does this mean, I need to use both SMA pins to get the complex samples and I am currently getting real samples ? > We really need to understand what you *want* to be doing! > As I explained above, I want quadrature samples from the channel, which I think I am doing in my GRC and the setup pic. It might be best to know how to get both - real or complex setup. I would like to do complex baseband always and do processing on the received data file as part of analysis, and not restrict to real sampling. > So, I really don't know what literature you've got access to – if you have > access to the scriptum for a communication theory introduction, that might > be a good start. > > I have read some chapters from Oppenheim but I am still not clear what is practically going on in my system. I really appreciate your time for helping me, Abhinav
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