Re: [Discuss-gnuradio] Problem of using UHD blocks as Tx & Rx

[Hongliang Zhang]

Dear Jason and Josh,

Thanks very much for your reply. I followed your instructions and use the
UHD MIMO  Sink and Source again. They should be synchronised now. 

First, I transmitted a constant and observe the received signal. The
following figure  is my flow graph of GRC:
http://old.nabble.com/file/p30086842/FlowGraph.jpg  

But I got the received signal with large fluctuation (I observe the received
signal in a long time period window):
http://old.nabble.com/file/p30086842/Result1.jpg 
The fluctuation increased when I increased the amplitude of the transmitted
signal.

Then I made a test for transmitting a 10K Hz sine wave

http://old.nabble.com/file/p30086842/FlowGraph2.jpg 

The result seems better :
http://old.nabble.com/file/p30086842/Result2.jpg 
However, if I watch the received signal in real time, I can still find the
slight vibration of the period of the signal wave. 

I used a signal generator and a spliter to provide two identical 100MHz ref.
clocks for the boards.
In my opinion, the results may imply that there are still frequency
difference between transmitter and receiver. And this frequency difference
is a random value which may goes from -100 to 100 Hz.

May I have your understandings of the results? Would you mind to provide me
with some ideas or suggestions about them?

Best wishes,
Hongliang



Jason Abele wrote:
> 
>> Why do I receive a sine wave when I transmit a constant?
> 
> Probably because the 100MHz reference clocks of your two USRP2s are
> slightly different
> 
> Here are a few suggestions for being sure that you are locking to your
> 10MHz reference clock:
> 
> If you have not modified the generated python code from your GRC
> flowgraph, then you are definitely not locked to refclock.  The
> easiest way to fix this would be:
>     Use a multi-usrp sink and source from the UHD categorey
>     Set the number of motherboards and channels to 1 each
>     Use the Sync to Unknow PPS option
> 
> I know that is a strange interface, we will put an option to use
> external ref clock in an update coming soon to git repositories near
> you.
> 
> Finally, be sure your refclock meets the power/amplitude requirements
> shown here:
> http://www.ettus.com/uhd_docs/manual/html/usrp2.html#ref-clock-10mhz
>    +5dBm to +20dBm (roughly 1Vpp to 5Vpp)
> 
> Jason
> 
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> 
> 

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