[time-nuts] how good an oscillator do you need for a GPS simulator

[Don Latham]

Slightly peripheral: Just got a bit of TI advertising with a couple of
chips of TN interest:
http://www.ti.com/product/lmk03806
http://www.ti.com/product/lmk00301
These llok very interesting. 3.3 v i think in cmos mode is enough for
most of the instrument external ref inputs.
Don

Jim Lux
> On 12/16/11 7:46 AM, Azelio Boriani wrote:
>> A used Spirent is only 26K to 37K. Interesting: playing back bits from
>> RAM... can it be that simple? Obvious: a DAC is required.
>>
>
> No.. you don't even need a DAC.  The underlying waveform is a binary
> code that is BPSK modulated.
>
> there are a variety of commercial GPS record/playback units that
> basically are single bit digitizers with no mixer (other than the
> sampler with a well chosen sample rate to put the GPS signal at a good
> place in the passband)
>
> The GPS receiver we're flying for the CoNNeCT project samples the three
> bands with a single bit at about 38.6 MHz, which aliases the L1 of 1575
> down to about 9 MHz: close to fs/4, which is a convenient place.  The
> other bands alias to convenient places as well.
>
> There is some art in picking a good sample rate... you want something
> that aliases to somewhere convenient, and you want to not have the
> Doppler push you past a folding point.   Doppler from GPS satellite
> motion to a stationary platform is on order of 5kHz max.  For a receiver
> LEO satellite (probably a worst case) you'd need to add a platform
> motion of 7 km/s, which is about 10 kHz.  Real worst case would be
> something that's reached escape velocity, which I think is 40-50 kHz
> Doppler.
>
> So don't go picking sample rates that are exact fractions of L1.  You'll
> wind up in alias unwrapping hell.
>
> For generating, though, sample rates that are a multiple of the chiprate
> (1.023 MHz) might be the best strategy. Say you clock the bits out at
> 10.23 MHz, and you could use the 154th harmonic of the same oscillator
> as your L1 carrier and all that.
>
>
>
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-- 
"Neither the voice of authority nor the weight of reason and argument
are as significant as experiment, for thence comes quiet to the mind."
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"If you don't know what it is, don't poke it."
Ghost in the Shell


Dr. Don Latham AJ7LL
Six Mile Systems LLP
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