On 3/31/17 4:35 AM, Attila Kinali wrote:
On Thu, 30 Mar 2017 20:00:31 -0700
Peter Monta <[email protected]> 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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