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Re: [Discuss-gnuradio] WBX daughterboard frequencies

From: Marcus D. Leech
Subject: Re: [Discuss-gnuradio] WBX daughterboard frequencies
Date: Thu, 04 Nov 2010 01:44:05 -0400
User-agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv: Gecko/20100907 Fedora/3.0.7-1.fc12 Thunderbird/3.0.7

On 11/04/2010 01:09 AM, Steve Mcmahon wrote:
> Hello:
> I am running GNU Radio 3.3.0 under openSUSE 11.2, and I have a USRP2 board 
> with a WBX daughterboard. I'm still somewhat new to GNU Radio, so please bear 
> with me.
> What is the minimum and maximum frequency at which I can transmit and 
> receive? I thought it was between 50 MHz and 2.2 GHz, but I thought I have 
> seen other ranges specified such as 50 MHz and 2.3 GHz.
Any frequency between 50Mhz and 2.2GHz.
> Also, can I select any frequency in that range to transmit or to receive? Can 
> I set the "frequency" parameter of a USRP2 sink block to any value between 50 
> MHz and 2.2 GHz, such 51.4 MHz, or 773.66 MHz, or 1855.2 MHz, etc.? Must it 
> be an integer number of MHz? Can I specify the value down to a single Hz? Are 
> there limitations/rules as to what frequencies I can set a USRP2 sink block 
> to be?
> Thanks a lot for your help.
> Steve McMahon
The ultimate frequency resolution is under 1Hz, thanks to the
digital-upconverter function in
  the USRP2 FPGA (and similar in the USRP1).

It's often the case that synthesized LOs (particularly for UHF and
higher frequencies) have
  rather-coarse frequency steps (several 10s to several 100s of KHz is
not uncommon), so the
  FPGAs can do digital up-conversion to make sure that your signal
appears exactly
  where you want it (modulo any uncertainty in the reference clock).

The frequency specified in a USRP2 sink block has to match the range
provided by the
  daughter-card you're using, otherwise an error is provoked.  That's it.

Now, understand that if you specify the center frequency down to 1Hz
levels, there's non-zero
  uncertainty in the reference clock, usually.  On the order of
5-10PPM.  So when that
  error is effectively multiplied by the synthesizer, the error in
actual frequency can be
  somewhat significant. Let's take a +/-5PPM error as an example, and
you've set your
  center frequency to 900.000MHz.  That nominal 900MHz signal could
actually have a center
  frequency that's +/- 4.5Khz from the nominal 900.000Mhz you've asked for.

If absolute frequency precision is important, you'll need to use an
external reference clock for
  the USRP2.  People generally use 10MHz GPS timing receivers for this
purpose, but a "lab standard"
  precision 10Mhz reference source (perhaps based on a DOCXO, for
example, and calibrated) could
  also be used.

It's pretty-normal for commercial and consumer equipment to have small
frequency errors, it's
  a natural consequence, ultimately, of the physics of crystal
oscillators, and the manufacturing
  techniques that are used to make them.  Crystal oscillator precision,
stability, and phase-noise,
  generally scales non-linearly with price ;-)

Here, for example, is a pretty-good 10MHz OCXO you can buy at Digikey:


With a claimed +/-5ppb frequency stability.

Compared with a garden-variety TXCO:


With a claimed +/-2.5ppm frequency stability.

That's a frequency stability ratio of roughly 500:1

You're much more likely to find that $150.00 OCXO inside a $20K lab
instrument than in a piece
  of commercial or retail RF gear.

Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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