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

[Activecat]

Dear Johnathan,

On Fri, Mar 14, 2014 at 2:21 PM, Johnathan Corgan
<address@hidden> wrote:
> On 03/13/2014 09:32 PM, Activecat wrote:
>
>> I try to send square wave from one USRP to another.
>> The received signal at the receiver USRP is very different from what
>> was being sent.
>> This is just a very simple setup. What could be wrong ..?
>
> What you are seeing is a classic case of frequency/phase offset between
> the transmitter and receiver, introducing a "rotation" in the I/Q domain
> at baseband equal to the difference frequency.
>
> 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.


> ---------- Forwarded message ----------
> From: Marcus D. Leech <address@hidden>
> Date: Sun, Feb 2, 2014 at 8:50 PM
> Subject: Re: [Discuss-gnuradio] undocumented SBX behavior
> To: Activecat <address@hidden>
>
>> On 02/02/2014 07:03 AM, Activecat wrote:
>> Dear Marcus,
>> At the receiver USRP, does the SBX daughtercard have any mechanism of 
>> phase-lock-loop (PLL) ?
>> I guess the daughtercard should have it because PLL is essential for 
>> quadrature downconversion.
>> Please advise, thanks.
>> Regards,
>> activecat
>
>Yes, the synthesizers are PLL based.