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

[Marcus D. Leech]

On 03/14/2014 01:05 PM, Johnathan Corgan wrote:
> 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.
I would emphasize that this requirement isn't a "quirk" of SDR 
hardware--every digital radio system ever has needed some kind of
  offset error estimator to track differences between TX and RX 
phase/frequency.

In analog systems (like NBFM voice, or FM radio, or AM audio), it's 
generally not as necessary, because it's hard for humans to hear
  minor frequency offsets.  But for data systems, an error estimator is 
vital.    Even in WBFM, I've implemented an AFC block to compensate
  for the crappy (RTLSDR) master clock on the receiver.  But each 
modulation type and coding system will have its own way of estimating
  phase/frequency error which you'll have to implement, or just "live 
with" bad BER.



--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org