[time-nuts] Sinlge ADC multi-band receiver

jimlux jimlux at earthlink.net
Tue Apr 4 20:58:11 EDT 2017

On 4/4/17 4:21 PM, 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.

in most of the designs, the noise is from the LNA, and is band limited, 
so the additional noise from the amplifier chain is less. COnsider if 
the LNA has 40dB gain and a 2 dB NF. Let's say all the other amps in the 
chain have 5 dB NF.

The thermal noise into the next amp is -132 dBm/Hz.  In order for the 
5dB NF noise (-169 dBm/Hz) to get up high enough to be noticeable, say, 
30dB, you'd have to fold 1000 times the sampling bandwidth. if the 
sampling bandwidth is 40 MHz, to get the noise up high enough it would 
have to extend to 40 GHz... I'll bet it doesn't<grin>

> 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.

I don't think keeping the bands together buys you much - you don't need 
a multibit ADC for a signal that is below the noise floor. (unless 
you're trying to reject strong interference signals, but that's a 
different kind of receiver).

> 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.

Oh, I'm not sure about that. It would depend on the filter kind and 

If it's a SAW or BAW filter, it's all one "brick", but I think you'd 
still need to calibrate the differential phase shift vs temp.  And it 
might be very predictable in a "measure 10 of them, and now you know the 
characteristics of the next 1000"

  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,

There's a ton of integrated demodulator/ADC parts out there these days 
that go up to 6GHz.
AD9361 for example

it will do 56 MHz BW through the IF, with 12 bit ADC feeding a 128 tap 
FIR filter, etc.

  which increases the BOM cost quite considerably.
> 			Attila Kinali

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