[time-nuts] Sinlge ADC multi-band receiver

Magnus Danielson magnus at rubidium.se
Mon Apr 10 09:13:00 EDT 2017


Hi,

On 04/10/2017 03:00 PM, Bob kb8tq wrote:
> Hi
>
>> 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.

L2 and L5 is so close, that bringing them down together is relatively 
simple.

If you make a receiver today, it should be able to use any set of bands, 
including L2 only or L5 only. Even more refined than that, any set of 
signals. One should attempt to get any of the transmitted signal, so 
there might be L1 from one, L2 and L5 from one, L1 and L5 from one etc. 
Each signal contributes. Signal pairs and tripples allow for ionospheric 
estimation, with the added benefit.

> 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 ….

Indeed. I have been advocating for use of multiple bands, besides L1, 
and for multiple GNSS. This helps to build signal redundancy. Such 
receivers should not be too expensive eventually.

Cheers,
Magnus


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