Re: [Discuss-gnuradio] 64MHz USRP Oscillator

[Johnathan Corgan]

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/