Re: [Discuss-gnuradio] Using external clock references in GRC environment

[Nazmul Islam]

Dear Josh,

Thanks for the reply. The USRP is locked to the function generator. The E LED light is on. It is not as bright as LED D & F light, though.

I am sorry for not clarifying on the "strange output" last time. In my experiment, I am transmitting an LFSR PN sequence. Thereafter, I correlate the transmitted PN sequence with the received baseband complex I & Q. If there were no timing error between the two USRP's, then I would get high correlation after every chip sequence period.

I get this high correlation for small chip sequence lengths, e.g. 31, 63, etc. However, when I use high chip sequence lengths, e.g., 1023 or 4095, the output correlation is smeared. I don't see any high peaks. According to Jonathan, the timing slips by enough for longer chip sequences and therefore it leads to reduced correlation. 

I thought that an external function generator, connected to the reference clocks of Tx an Rx USRP's, would solve this problem. Unfortunately, the attached figure will tell you that it did not. I just changed the 'Clock Source' option to 'External' in the Source & Sink GRC blocks. I don't know if I need to make any other changes.

Thanks,

Nazmul

On Tue, Jun 5, 2012 at 9:45 PM, Josh Blum <address@hidden> wrote:

On 06/05/2012 04:53 PM, Nazmul Islam wrote:
> I have attached a screenshot that shows that way I am using the USRP:UHD
> Sink block. I am running both of my USRP's with the same 10 MHz clock
> signal. The clock signal is coming from a function generator. Therefore, I
> have changed the clock source option in the USRP Sink block to be external.
>
> Unfortunately, I am getting wrong/strange outputs due to this change. Do I
> need to make any other change in the flowgraph to make it work with
> external clock source?
>

Please define "wrong/strange outputs".

Is the device locked to the reference? ie, is the ref lock light on?
http://files.ettus.com/uhd_docs/manual/html/usrp2.html#front-panel-leds

> Thanks,
>
> Nazmul
>
> On Tue, Jun 5, 2012 at 4:31 PM, Nazmul Islam <address@hidden>wrote:
>
>> Hello,
>>
>> I am running two USRP2's with a 10 MHz external clock. I am using
>> gnuradio-companion to transmit data between the USRP's. I have changed the

>> *clock source* option as 'external' in the UHD:USRP Source & Sinc

>> blocks. I kept the time source and clock rate block as default since I am
>> not providing any external pps.
>>
>> Unfortunately, I am getting unexpected and wrong outputs. Do I need to
>> make any other changes in the USRP Source & Sink options?
>>
>>
>> Thanks,
>>
>> Nazmul
>>
>>
>>
>>
>> --
>> Muhammad Nazmul Islam
>>
>> Graduate Student
>> Electrical & Computer Engineering
>> Wireless Information & Networking Laboratory
>> Rutgers, USA.
>>
>>
>
>
>
>

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-- 
Muhammad Nazmul Islam

Graduate Student
Electrical & Computer Engineering
Wireless Information & Networking Laboratory
Rutgers, USA.

output_correlation_63_chips.fig
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PN_Correlation_1023_chips.fig
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