Re: [Discuss-gnuradio] Failure of sending square wave over USRPs (back-to-back)

[Martin Braun]

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