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Re: [Discuss-gnuradio] 64MHz USRP Oscillator


From: kaleem ahmad
Subject: Re: [Discuss-gnuradio] 64MHz USRP Oscillator
Date: Thu, 24 Jul 2008 06:42:17 -0700 (PDT)

Hi,

Can some one explain in a little bit detail how this software PLL can be
implemented and how this synchronization can be acheived. I am using RFX2400
and facing same problem.

First I tried to solve the problem by calculating the frequency shift at
receiver end by using usrp_fft.py and then by adding/subtracting that
frequency from RX USRP frequency. But it dont solve the problem.

and secondally I am confused why we dont discuss TX clock which is 128MHz
(true...?????), I mean what is the role of that.

Thanks


Johnathan Corgan-2 wrote:
> 
> On Wed, Jul 23, 2008 at 3:57 PM, Chris Stankevitz <address@hidden>
> wrote:
> 
>>> I am having lots of issues with the USRP  64MHz (20ppm) on board
>>> oscillator which does not allow me to get exact and constant RF
>>> frequencies
>>> out of the RFX900 board. I can not really fix that in SW so I was
>>> thinking
>>> about replacing the 64MHz crystal with a more precise one. Has anybody a
>>> suggestion of which part to use?
>>
>> I have replaced the crystal with the 20ppm crystal, but I was unable to
>> get
>> an "exact and constant" frequency.  I ultimately added a software PLL to
>> track the clock errors.  Before I upgraded my software PLL, I used a
>> signal
>> generator as an external clock source which worked very well.
> 
> Just to reiterate, it's not physically possible to get "exact and
> constant" frequencies in a receiver.  This is not a bug.  All
> realistic radio receivers will have to deal with frequency and phase
> offsets.  Sometimes you can reduce them to the point where you can
> ignore them.  Other times, such as in mobile wireless or satellite
> communications, Doppler effects will make any existing frequency
> stability issues worse.
> 
> Decades of research have gone into algorithms one can use on a
> receiver to recover the exact carrier phase and frequency of a
> received modulated waveform, resulting in many tried and true
> engineering solutions.  Some these, such as software PLLs as Chris
> mentions, are available in GNU Radio for your use.
> 
> I believe (ISTR) that there are 5 PPM versions of the crystal on the
> USRP that fit the same solder pattern.  But that still leaves you with
> many KHz of potential offset and drift at 900 MHz, so unless your
> chosen modulation can withstand that, you'll still need to solve the
> receiver synchronization problem.
> 
> -- 
> Johnathan Corgan
> Corgan Enterprises LLC
> http://corganenterprises.com/
> 
> 
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> 
> 

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