On 14.03.2014 09:50, Activecat wrote:
In spite of "calibrating" things, you have only made the transmitter and
receiver local oscillator frequencies "close". All real-world receivers
must implement a correction loop to estimate this frequency and
compensate for it, which eliminates the rotation. This correction loop
often takes the form of a phase-locked loop, of which there are several
in GNU Radio. For the type of waveform you are transmitting, the "PLL
Carrier Tracking" loop should work, as your baseband waveform results in
significant carrier energy when upconverted to passband.
I was told that the Ettus SBX daughtercard has built-in PLL capability.
In this case is the flowgraph-based PLL still necessary ..?
Refer below message from Ettus support engineer.
Hi Activecat,
our synthesizers use PLLs, but this won't solve frequency offset issues
with your transceiver. This is not a limitation of USRPs, but
fundamentally necessary in every receiver.
Here's a very brief explanation: The PLL for the synthesizer makes sure
the locally generated frequency is stable (per-device). It's physically
impossible to make perfectly aligned oscillators. By throwing money at
the problem, you can reduce the potential offset. But since you can
never get rid of it entirely, you also need a mechanism (e.g. a PLL) to
lock on the incoming signal.
Phase is another story. Even in a world with perfect oscillators, your
phase can be rotated. That's something else you have to account for.
Have a look at the examples in gr-digital/examples/demod. You'll see
many mechanisms to correct offsets between tx and rx (time, phase and
frequency). These are all standard components for digital transceivers.
Martin
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