On Mon, 10 Apr 2017 07:00:27 +0200 Magnus Danielson <mag...@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 <jim...@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 _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.