Strange values at the receiver

GNU Radio
1

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

2

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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.