[time-nuts] Sinlge ADC multi-band receiver
kb8tq at n1k.org
Sun Apr 9 18:13:48 EDT 2017
> On Apr 9, 2017, at 4:28 PM, Attila Kinali <attila at kinali.ch> wrote:
> 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
>> 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.
Why would you substitute an expensive IF filter for a cheap front end filter?
> 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
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