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Re: [Discuss-gnuradio] USRP N210 Clock generation

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] USRP N210 Clock generation
Date: Wed, 23 Jul 2014 20:06:28 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.5.0

HI Anusha,

this is the wrong mailing list for this -- for your replies, I think
it's best to join address@hidden [1] and continue this
discussion there; I will crosspost my answer.

On 23.07.2014 18:56, Yarlagadda, Anusha (337G) wrote:
> Hi,
> I am using URSPN210 series, have few questions regarding the  clocking 
> schemes for this USRP N210. In the website, its mentioned USRP2 and N200/N210 
> have fixed 100MHz clock which can't be tuned and do not have option to use 
> external clock.
You *can* use an external clock, but it should be a 10MHz one.
> We would like to use a different clock rate (49.2MHz) instead of the fixed 
> 100MHz that is generated  from the Reference and system clock generation 
> circuit( I think all these clocks i.e fpga clock, TX/RX clock for the RF 
> boaord ADC/DAC clock are generated by the AD9510 that has on chip PLL core 
> and multi ouput clock distribution function).
I'm afraid that doesn't fit the N210 design. You can't use an arbitrary
clock rate -- the idea is to decimate your signal to a integer fraction
of the master clock rate, and transport the signal at that sampling rate
to the host pc for further processing, e.g. resampling.

Furthermore, I'm a little curious what kind of system would preferably
have a sampling rate of $49.2MHz/n, n \in [2,3,4,...]$ -- please be
aware that you can't fit 49.2 MS/s of 16bit I+Q data through 1 Gigabit
of ethernet!
>  An external oscillator (U27 or external reference clock) is phase locked to 
> a reference input reference frequency  clock of the AD9510.
> In the USRP N210 FPGA Code can we program the divider values of AD9510  over 
> the SPI interface so that the output clock are not fixed?
As being said, the N210 design doesn't really leave much room for this.
> We would like to be compatible with our internal Radio waveform rates and 
> frequencies.
In this case, I recommend using a rational resampler on your host. GNU
Radio comes with some -- you can try if you can match your desired
sampling rate with that.

With best regards,
Marcus Müller

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