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

[Johnathan Corgan]

On 03/14/2014 01:50 AM, 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.

The hardware PLL in the receive section of the daughterboard serves an
entirely different purpose; it is there to create the local oscillator
signal at the frequency requested when tuning.  However, that frequency
is ultimately derived from a hardware oscillator that is subject to
manufacturing tolerances, drift, thermal effects, phase noise, etc.  So
even when both transmitter and receiver are physically tuned to what you
set as the same frequency, in reality there is an offset between them,
and even the amount of that offset changes on a moment-by-moment basis.

It is an unavoidable reality of designing radio receivers that one must
compensate for this offset in transmitter and receiver local oscillator
frequencies.  In software radio systems, this is most often done by
estimating the frequency/phase error and performing de-rotation on the
resulting waveform.

-- 
Johnathan Corgan, Corgan Labs
SDR Training and Development Services
http://corganlabs.com

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