[time-nuts] GPS first LO need to be locked?

jimlux jimlux at earthlink.net
Fri Mar 31 12:26:28 EDT 2017


On 3/31/17 4:35 AM, Attila Kinali wrote:
> On Thu, 30 Mar 2017 20:00:31 -0700
> Peter Monta <pmonta at gmail.com> wrote:
>
>>> BTW a lot of GPS receivers don't have a "first LO".. they are more like a
>>>> Tuned RF receiver - an input BPF for L1, L2, or L5, then direct sampling at
>>>> around 30-40 MHz -  something that makes the GPS signals alias down
>>>> somewhere convenient (and always have positive frequency offset from zero,
>>>> even at max negative Doppler)
>>>
>>>
>> True.  I've been wanting to try this with an FPGA transceiver; even the
>> cheap ones go to 6 Gb/s now, but binary only.  The newest transceivers
>> support PAM-4, which would be great, but they're not affordable yet.  Also
>> that's a lot of gain at one frequency.
>
> I guess you know of [1] already?
>
> Alternatively, instead of using a MAX2021, you can use a discrete
> mixer and use the high analog bandwidth of todays ADCs to use them
> as downmixers. [2] and [3] describe how to do this in detail.
>
> The epitome of this is using direct sampling of the signals without
> previous downmixing (e.g. [4]). Though I have no idea how easy or hard that
> is with todays electronics. It will definitely need good preselection
> filters to keep SNR high.
>

Our space GPS receiver flying on SCaN Testbed/ISS is a direct sampling 
at 38 MHz. Filters are nothing special as I recall.  A chain of 
amplifiers with filters - Lots of gain into the thresholders, so good 
layout is important.

https://trs.jpl.nasa.gov/handle/2014/41781

https://trs.jpl.nasa.gov/bitstream/handle/2014/41781/11-0046.pdf?sequence=1&isAllowed=y



> 			Attila Kinali
>
> [1] http://www.aholme.co.uk/GPS/Main.htm
>
> [2] "A Prototyping Platform for Multi-Frequency GNSS Receivers",
> by Akos, Ene and Thor, 2003
> http://waas.stanford.edu/~wwu/papers/gps/PDF/AkosIONGPS033FreqRX.pdf
>
> [3] "Design of a GPS and Galileo Multi-Frequency Front-End",
> by Parada, Chastellain, Botteron, Tawk, Farine, 2009
> http://202.194.20.8/proc/VTC09Spring/DATA/04-04-01.PDF
>
> [4] "Design and Implementation of a Direct Digitization GPS Receiver Front End",
> by Akos, and Tsui, 1996
>




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