Re: [Discuss-gnuradio] strange values at the receiver

[Marcus D. Leech]

Hi, all, I am doing experiments on one USRP N210 with SBX. I connect
the RX/TX and RX2 by using a SMA cable and an attenuator directly. I
transmit 1 and 0 repeatedly (1010101010...). I also attached the GRC
file. However, I observed strange values received at the RX2. They are
pairs of (0.003, -0.003) which are not expected. I also attached the
figs of real and imaginary part. Can someone explain this weird
results? Thanks a lot.

Best,
Gang

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Hmm, where to start.

So, you're sending a 200kHz square-wave to the N210 DAC, and having that up-converted to 850MHz?  Is that what you intended?
  Keep in mind that square-waves are very rich in harmonics, so your spectral footprint at 850Mhz will be much wider than you'd probably
  like.

Then this wideband signal at 850Mhz is sent at some power level (not very much, since you have the gain set to zero) over a cable
  with attenuators, and then received by the other half of the N210/SBX combination, downconverted to baseband, and then sampled
  by your flow-graph at 200kHz.  The magnitude of the resulting signal will be related in some fashion to the magnitude of the transmitted
  signal, but it will only vaguely resemble the original square wave.  Remember the harmonics?

My suspicion is that you're trying to send data over the radio channel (the 1s and 0s) but you haven't yet learned about how data is
  represented on an analog channel like a radio link--we use *modulation techniques* that take "bits" and turn them into "symbols"--
  symbols in whatever the modulation system is.  There are a *lot* of different modulation systems for sending data over analog
  channels, with varying degrees of robustness and implementation complexity.  Things to look into include:

    o ASK      (Amplitude Shift Keying)
    o BPSK    (Binary Phase Shift Keying)
    o GMSK   (Gaussian Minimum Shift Keying)
    o nPSK    (PSK with multiple phase-shifts possible)
    o FSK      (Frequency Shift Keying)

That's just a brief sample.  Over the years, there have been *dozens and dozens* of different mechanisms invented for representing 1s and
  0s over an analog channel.   If you're hoping to send the 1s and 0s directly, without the assistance of any kind of "symbol system" (modulation),
  the closest you'll find is ASK, which uses carrier-level shifts to represent 1s and 0s.

-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org