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

Attila Kinali attila at kinali.ch
Fri Mar 31 07:35:03 EDT 2017


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.

			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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use without that foundation.
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