[time-nuts] Sinlge ADC multi-band receiver

Magnus Danielson magnus at rubidium.dyndns.org
Mon Apr 10 01:00:27 EDT 2017


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.

> 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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