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RE: [Discuss-gnuradio] GPS for USRP ideas

From: Whitaker, Brian
Subject: RE: [Discuss-gnuradio] GPS for USRP ideas
Date: Fri, 16 Apr 2004 09:03:10 -0700


I think maybe the answer to this is that 100kHz is outside of the loop BW of
both of these applications; this means the the phase noise is really just a
function of the VCO itself. I would have expected that the inherent phase
noise of the 2118's VCO would have been worse than the 2740 (for the Kvco
alone)... so I'm guessing that the 2740's VCO just plain stinks. In Maxim's
defense, it's a really old part (pre SiGe, even!)

So remember that the close-in phase noise is mostly dependent on the
reference and the noise generated from the PLL itself (FM modulates VCO,
lots of noise + high tuning gain Kvco => higher noise in PLL BW). It's
pretty tough to compare phase noise unless you know about the references
used and the loop BW and such.

I had always thought that a USRP board from one of these sat tuners would be
a good general-purpose receiver... but Krys has the Microtune TV tuner
boards done already -- I'd suggest working with those and just see what
issues people run into in trying to use this for GPS. 

Also -- I'd suggest not designing a USRP board around the 2740. There are
better front-ends out there.

One last thing -- Krys, you said something about the 2116/8's not needing an
external reference?  Pins 21/22 are for the XTAL -- the part has an internal
reference oscillator, but that just means that we provide the positive-fb
amplifier that rings against the (passive) crystal. Tuners that don't have
this require a 'reference oscillator' (often 'TCXO') module that includes
the passive resonator along with the active oscillator circuit.  IC mfg's
are busy trying to design out everyone in the app ckt they can, but no one
has built an on-chip crystal reference :)


-----Original Message-----
From: Krzysztof Kamieniecki [mailto:address@hidden 
Sent: Thursday, April 15, 2004 7:21 PM
To: address@hidden
Subject: Re: [Discuss-gnuradio] GPS for USRP ideas

Bare with me everyone, I am just beginning to learn about RF/Analog

Looking at the Specs
Phase Noise (dBc/Hz)
          1KHz   10KHz   100KHz
MAX2740   ?      ?      -91.5    (GPS front-end)
MAX2118   ?     -75     -99      (L-Band Tuner: fLO = 2175MHz)
GP2010   -68    -75     -88      (GPS front-end)

So what does this mean?
Is there other information needed? (Phase Noise @ 50Hz?)

Brian Whitaker wrote:

> Krys, Saber,
> GPS receivers are a little strange in that the received signal is often
> 20-40dB below the thermal noise floor (yes, below -174dB/Hz). SNR in the
> traditional sense is always negative. Like CDMA, what makes this work is
> spreading gain of the DSSS signal -- the brains of the baseband is able to
> reconstruct the signal out from the noise.
> Because  of this, GPS front-ends need to offer a couple of key things to
> work well:
> -> 2-5dB NF from the antenna to the low-IF (usually about 4MHz),
> -> ultra-low phase noise, especially at 50-100Hz (as I understand, there
> a lot of information at about 50Hz in the signal -- I need to get a book,
> though!)
> The 2116/8's are satellite (DBS) receivers, and are pretty nifty in that
> they have their own synthesizers and on-chip VCO. An obvious sacrifice in
> the VCO being able to tune like crazy is that the tuning gain (Kvco) is
> high -- this means that the phase noise is high. In sat applications, the
> BWs are so big that the extra phase noise isn't really an issue... But my
> hunch is that it would corrupt the GPS signal to the point that you
> be able to use it.
> Also, every GPS receiver I've seen uses a low-IF -- the receiver brings
> signal down to about 4MHz, where the BB ADC samples it from a very
> 'GPS clock' (this clock is the low-IF center freq). Not that a GnuRadio
> implementation would have to do this, but we'd need to figure out why the
> rest of the world does this so understand what we'd be giving up in going
> a direct conversion system.
> For more general-purpose applications, a sat tuner like the 211x family,
> an IC TV tuner (Microtune has theirs out already), offers a lot of
> flexibility. As soon as I think I can put together a USRP radio board and
> use it for work, I'll do it... Until then, work and family will keep me
> committing to a venture like this. I'll try to serve a support role here
> the time being.
> Brian. 
> On 4/14/04 6:05 PM, "Krzysztof Kamieniecki" <address@hidden> wrote:
>>There is also the max2116/max2118 which covers 925 to 2175 MHz. There
>>are eval. kits for these parts
>>which do not seem to need an external frequency reference.
>>Brian Whitaker, an apps. engineer at MAXIM, was looking into using these
>>parts on a USRP daughter
>>card but I think he ran low on free time.
>>Any comments Brian?
>>tebinuma wrote:
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> Discuss-gnuradio mailing list
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Krzysztof Kamieniecki

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