[time-nuts] Sinlge ADC multi-band receiver (was: GPS first LO need to be locked?)

[Attila Kinali]

On Tue, 4 Apr 2017 06:55:24 -0700
jimlux <jimlux at earthlink.net> wrote:

 
> So those folks were trying to use 1 ADC for all three bands, so they had 
> to choose a sampling rate that lets them separate the signals later in 
> software.
> 
> But that ADC is a MAX104 - a 1GSPS, 8 bit converter - that draws 5 Watts!!!
> 
> I'm not sure that's a good trade against a 1 or 2 bit converter for each 
> band, in terms of the downstream data rate and processing.

Honestly, I don't think the direct sampling approach is a good idea.
It folds a lot of noise into the signal band. I'd rather use a single
heterodyne with an LO frequncy of around 1000MHz, or something between
L2 and E5, such that the bands stay still seperated. Here I would add
a filterbank to get rid of as much noise as possible. And after that
use an ADC sampling frequency to fold the signals down again.
(Effectively forming a super-heterodyne receiver)

You don't need a 1Gsps ADC for that, but if you want to keep all
frequency bands completely seperate, even after sampling, a relatively
high sampling rate is necessary. L1C/E1OS needs at least 14MHz,
L2C needs 2MHz, E5 needs 50MHz. Ie to keep them separated, at least
66MHz of (un-aliased) bandwidth or a sample rate of 132Msps is needed
(alternatively, an I/Q ADC with 66Msps). There are plenty of ADCs
that go up to 120Msps with 10-14bits resolution available, and a couple
that go higher (up to 200MHz are easy to find). 8bit ADCs with >100Msps
are available, but not so many with >120Msps. So, it should be "easy"
to build such a system, if one can find a nice pair of LO frequency
and sampling rate. Alternatively, if one can accept a slightly decreased
SNR one can choose a pair of frequencies where all the signals fall ontop
of eachother, making the 50MHz of E5 the only real requriement. My guess
would be that the CDMA character of the codes would make them easily
seperatable, resulting in an (additional) SNR los of maybe 1-3dB, which
can be compensated by using an ADC with 8bit or even 12bit instead of
the 2-4bit that are now common for GPS receivers. The frequency requirement
can further reduced if one drops the E5b signal and just works with the E5a.
Then 21MHz of bandwidth would be enough.

Looking at the frequency band values, an LO frequency between
1367MHz (L1C touches E5) and 1405MHz (L1C touches L2) would be
the most sensible range. The IF would be below 240MHz and it
seems like the maximum needed bandwidth would be around 70MHz
(eyeballing the graph, no real calculation). I'd say the best
compromise would be using an LO of 1398MHz (IF=170-230MHz)
and using a sampling rate between 120Msps and 160Msps.

The advantage of such a system would be that there is only a single
path through the system for all signals, especially through the filters.
Thus the variability of the differential phase shift between the
frequency bands would be significantly reduced, which would result
in better stability. Of course, that's the theory. Whether things work
out this way in reality is a different question. What can be said for
sure is, because of the high IF frequency of >200MHz, the standard tuner
chips cannot be used anymore and the RX chain has to be build from
"discrete" components, which increases the BOM cost quite considerably.


			Attila Kinali

-- 
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor