[time-nuts] Sinlge ADC multi-band receiver

Attila Kinali attila at kinali.ch
Mon Apr 10 13:12:48 EDT 2017

On Mon, 10 Apr 2017 07:00:27 +0200
Magnus Danielson <magnus at rubidium.dyndns.org> wrote:

> God Morgon Attila,
> On 04/09/2017 10:28 PM, Attila Kinali wrote:
> > On Tue, 4 Apr 2017 17:58:11 -0700
> > jimlux <jimlux at earthlink.net> wrote:
> >
> > 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).
> You got it backwards.
> You need to protect your LNA and mixer from other signals, not to be 
> blocked out by out of band signals which is strong. That's why you have 
> SAW filters to start with. This has become a larger issue these days.

I don't think they are necessary anymore. Todays LNAs have a very high
IP3 (in the order of 10-30dBm) and even IP1dB is usually around 0-10dBm.
Ie unless there is a very strong, narrow band interference, the LNA will
not cause any problems. Same goes for modern mixer.

Or, to make it a bit more practical: if you take an RTL-SDR dongle,
then you have a cheap, zero-IF system that has no frontend filter
and relies solely on selectivity of the antenna (which is often
a cheap puck without any filter) and its IF low-pass filters.
I have used this a few times and have not seen it fail. I took my
bladerf a few times and looked at the spectrum around 1575GHz and
haven't seen any strong interferer yet.

> > 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.
> Only if you *need* the Galileo E5.

With only L1 and L2 it is still ~180MHz. The two bands are ~350MHz appart.
You cannot get around that without introducing a second down-mix step.

				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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