IQ changing with restart

GNU Radio
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Hi,

I’ve done my homework on this one, crawled through the web & talked to
colleagues. If I am missing something obvious please point it out - it’s not
for lack of effort on my part!

I’m not really sure what you’re expecting. Of course the phase
alignement between the Tx and Rx is going to be random depending on
restart.

Using the same clock will prevent it from drifting, but the initial
phase alignement is random. This is usually resolved by using training
sequence, headers, differential encoding, …

Cheers,

Sylvain
3

On 11.02.2014 06:19, Sylvain M. wrote:

Using the same clock will prevent it from drifting, but the initial
phase alignement is random. This is usually resolved by using training
sequence, headers, differential encoding, …

I had the same thought – it looks like a phase change. This is a PSK
signal, right? Have you looked at the constellation diagram?

MB

4

Is it an over-the-air transmission or TX and RX are connected with a
wire and attenuators?

-George

George S.
PhD Student and Research Assistant
Signals, Communications, and Networking Research Group
Department of Electrical Engineering
University at Buffalo, The State University of New York

Office: 238 Davis Hall, Buffalo, NY 14260

Web: http://www.buffalo.edu/~gsklivan

5

On 02/11/2014 11:57 AM, Martin B. wrote:

restart.

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Neither synthesizer will start with any particular phase offset, either.

–
Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

6

On 11.02.2014 09:14, SOUTHCOTT, MARK A CIV USAF AFMC AFRL/RITC wrote:

This is QPSK, and I have looked at the constellation diagram after carrier
recovery, matched filtering and timing recovery. The constellation is a
clean qpsk signal at that point. I’ve done radar work before and this
doesn’t “feel” like a phase shift from the LO (not a great reason to
discredit it). Also, possibly due to a difference in the physical
architecture, when I used a common clock between tx and rx in my radar work
the initial phase alignment was repeatable.

Your const plot was before the phase recovery? And if yes, was it
rotated?

MB