[time-nuts] Sinlge ADC multi-band receiver

Magnus Danielson magnus at rubidium.dyndns.org
Wed Apr 5 04:37:07 EDT 2017

On 04/05/2017 01:21 AM, Attila Kinali wrote:
> 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)

Regardless you already have SAW filters on the LNA to provide selectivity.

Also, you don't really need to keep the bands fully separate in their 
mixed-down form, since they do not correlate except for the P(Y), but 
keeping enough frequency difference, such that doppler shift does not 
remove correlation margin, they remain uncorrelated. Some of the 
literature pay much attention to the band not wrapping around the 
band-edge, but I'm not convinced it is such a big issue.

A direct sampler of 100 MHz would work well for GPS for instance, but 
not for GLONASS, but 90 MHz would work there. The S/H would need to have 
the BW of the top frequency, but then the S/H action will act as the 
first mixer.

> 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.

Since you don't really need to keep signals very separated, you can pack 
them relatively tight. It's the E5 of GALILEO which is wide.

Using a 1,4 GHz range LO1 to pick L1 and L2 has been known to be used 
before. There is even existing chips which uses 1.4 GHz on LO1, which 
with a different set of filters could almost support L2, will have to 
check the details. While that front-end would be neat, I would not use 
that chip since it is no longer in production.

The fun thing about these types of receivers, is that there is so many 
ways to do it, that it allows for many different approaches to be tried 
as technology develops. There is no single one "right" way of doing it.


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