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View all threadsRe: [USRP-users] [EXTERNAL] B205mini: Setting Q signal component to zero distorts the I component
Hi Nicholas,
Thanks a lot for your explanations! I tried to increase the sampling rate
at the receiver - the sine changed itself again and I still have the same
amplitude differences (attached below).
Another problem that I observed is that it works fine only at 433MHz. When
I tried to set 868MHz/915MHz it became even less readable. At 2.4GHz and
5GHz it was looked like noise...
So...
Try a few different scenarios, (locked clock, unlocked clock, under
sampled, oversampled, etc.) and observe how it changes.
---> May you please tell me a bit more about those locked and unlocked
clocks?
Thanks again,
Varban
On Mon, Mar 9, 2020 at 9:16 AM Linnenkamp, Nicholas <
nlinnenkamp@perspectalabs.com> wrote:
V.
I could be wrong…
It appears that you are under sampling on the receive USRP. This is
probably due to the transmitter and the receiver not sharing the same clock
and the receiver is drifting (slower). You can have the two USRPs share
the same 10Mhz clock to remove this problem but it is present in every QAM
communication system. Distributed systems don’t have the benefit of common
clock even with GPS so it is something that everyone deals with eventually.
This is a good picture of when you under sample and get aliasing.
https://dsp.stackexchange.com/questions/31899/how-do-you-simultaneously-undersample-and-oversample
This is one of the best treatments of QAM signals that I have ever read.
https://mriquestions.com/uploads/3/4/5/7/34572113/quad_signals_tutorial-lyons.pdf
The real part of the signal is getting shifted from the In Phase to the
Quadrature and back again over time. If you were to zero out the Q and
plot the I you would get this aliasing. If you were to zero out the I and
plot the Q, you would also get this aliasing. If you add them together,
the real parts make the whole sine wave. It just shifts, like water, back
and forth from the I and Q, periodically, directly proportional to the
under sampling.
Try a few different scenarios, (locked clock, unlocked clock, under
sampled, oversampled, etc.) and observe how it changes.
In QAM signals the first thing you need to do is estimate clock drift and
then de-rotate your signal. You do these things periodically so that you
can track and properly sample the incoming signal.
Best of luck!
Thanks,
Nicholas
From: USRP-users *On Behalf Of *Varban Metodiev via USRP-users
Sent: Monday, March 09, 2020 7:14 AM
To: usrp-users usrp-users@lists.ettus.com
Subject: [EXTERNAL] [USRP-users] B205mini: Setting Q signal component
to zero distorts the I component
Dear all,
I have two B205mini devices. One of those is my transmitting side and the
other one is the receiver. Both are connected via a cable (and an
attenuator, of course).
Here is what I execute on the receiver:
rx_samples_to_file --freq 433e6 --rate 2e6 --bw 1e6 --gain 0 --nsamps 0
--spb 200
And on the transmitter:
tx_waveforms --rate 1e6 --freq 433e6 --ampl 0.6 --gain 70 --bw 1e6
--wave-type SINE --wave-freq 10e3 --spb 100 --otw sc16
The results are being read in GNU Octave:
samples_file=fopen('/home/vmetodiev/Development/SDR/uhd/host/build/examples/usrp_samples.dat',
'rb');
data=fread(samples_file, 'int16');
So... when both I/Q channels carry the sines that are generated by the
wave_table function, the results looks fine - attachment "iq.png".
However, I need to work only with the I channel and force the Q to
disappear, that is why I added the following lines in the
tx_waveforms.cpp code to zero it:
// Zero the Q component
for (size_t n = 0; n < buff.size(); n++) {
buff[n] = std::complex<float>( buff[n].real(), *0 *);
}
And the result is a strangely distorted I-component sine - attachment
"i_only.png".
May you please advise if it is possible to avoid this, because I am doing
something a bit strange that is not related to the normal I/Q modulations?
Thanks in advance,
V.
