[time-nuts] Sinlge ADC multi-band receiver

Bob kb8tq kb8tq at n1k.org
Mon Apr 10 09:00:17 EDT 2017


> On Apr 10, 2017, at 1:00 AM, 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.
> So, considering that you already have them, then what good do they do for the different scenarios.
>> 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.

The other point with E5 is the nature of the data on the various sub signals. Galileo has three 
classes of service and only one of them is free (open). As with traditional  L1 / L2 survey receivers, you
don’t *have* to recover full data from a signal for it to be useful. That said, the free (open) service
is only on one of the two sub signals. If you are building a L1 / L2 / L5 GNSS receiver, you might 
well opt to only grab the lower part of the E5 signal. 

You might also decide on a setup that only used two of the three bands. That would give you all 
the data and ionospheric correction. It is a bit unclear what the third band would add other than a “cool factor” 
if traditional criteria are used for the receiver design. There are various arguments for L1 / L5 and L1 / L2. One 
could even make a case for L2  /  L5. 

Even if ionospheric correction is not a bit issue in your design, jamming probably should be for a design
targeted to run for many years into the future.. A broadband jammer (intentional or accidental) can fairly 
easily take out one of the bands. It’s quite a bit harder to take out all of them at once. A lot would depend
on just how nasty an environment you intend to operate in, and how sensitive you are to occasional issues. 

Lots of choices ….


>> 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).
> Cheers,
> Magnus
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