[time-nuts] Sinlge ADC multi-band receiver

Attila Kinali attila at kinali.ch
Sun Apr 9 16:28:41 EDT 2017

On Tue, 4 Apr 2017 17:58:11 -0700
jimlux <jimlux at earthlink.net> wrote:

> > 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 
> topology.
> 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"

The beauty of the system would be that you don't need a SAW filter
at all. If the input stage (LNA + mixer) has a high enough dynamic
range, then the (first) IF filer alone can remove all those out of
band interference. And at the same time, because the IF frequency
being low, you don't need any specialized filter components that
might not be available in a couple of months.

Of course, this doesn't really work that way when significantly
wider signals (E5) have to be caught together with "narrow band"
signals (L1 C/A or L2C).

>   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.

Unfortunately, the AD9361 does not offer the IF bandwith necessary.
Even though it has a high sample rate and can offer high bandwidth
capture of signals, the zero-IF nature of its design doesn't work
for this design approach. The IF of the AD9361 has a low pass filter
of at most 56MHz, ie it offers to capture a bandwith of 56MHz of
frequency space (using both I and Q channels). But the above approach
would need an IF of >200MHz, but it would be enough to only have a
single channel. 

I looked up the prices for the components and figured that the prices for
mixer and IF amplifiers are actually quite low (a 2-4 USD per IC) so it
isn't that much more expensive to build such a system than using a 3 tuner
approach (eg using MAX2120 as Peter Monta did with the GNSS Firehose).

			Attila Kinali
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson

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