[time-nuts] Low SNR GPS reception and cheap LNAs

Magnus Danielson magnus at rubidium.dyndns.org
Sat Apr 26 06:25:11 EDT 2014



On 04/25/2014 06:04 PM, Attila Kinali wrote:
> On Fri, 25 Apr 2014 08:42:16 -0700
> Chris Albertson <albertson.chris at gmail.com> wrote:
>
>> You best bet is to change out the antenna.  You can buy them with a higher
>> built-in gain up to about 40dB.
>
> Buying a better antenna is also on the list. But i would still like to
> have an LNA, even if it's just to see that it doesn't help :-)
>
>> My understanding is that designing a GOOD
>> LNA is not so easy as little things like the exact layout of the PCB and
>> how the PCB transitions to connectors matters a lot.
>
> Well, the idea is to use one of the monolitic types like the TQP3M9036 [1]
> which basicall only need power on the output. These should work quite
> well without a PCB. Though i'm really thinking about getting a small
> PCB run, both for my bias tee and the LNA, but that will incure a minimum
> cost of around 100EUR (same price whether i buy 1 or 10, though)
>
>> But you can buy these
>> ready made for cheap.  I've seen complete LNAs in an enclosure with
>> connectors at good prices on eBay.    The user manuals I have say using 75R
>> cables with compression type F connects is OK.    I doubt the cheaper type
>> f-connectors would work well.
>
> I looked at the ones available on ebay, but they were either made for
> sat solutions and require 12V, which would complicate the whole power
> supply system. Or are >100USD. Given that i can get a cheap LNA chip for
> 1USD or an expensive one like the TQP3M9036 for 4, then i can build one
> myself for less than 20USD that should do the job just as well.
>
>
>> I have a good high quality Tremble in-line amplifier with N-connector and
>> the ability to pass DC.   In my experiment I place the antenna indoor and
>> use amplifier and then outdoors with no amplifier.  I get MUCH better
>> results with my 26dB gain antenna on the roof and 25 feet of cable than
>> with indoor amplified antenna with short cable.      My un-scientific
>> conclusion was that amplified noise is still noise.
>
> Well, GPS signal is mostly noise anyways ;-)
> The idea would be to place the LNA close to the antenna, in order to
> need less amplification in the bladeRF. And also to compensate for
> the longer cable i plan to use (getting the antenna to a better location)

The indoor antenna would see the noise of the house 290-300 K rather 
than the background noise of the sky 3 K. The signal will also be 
attenuated when indoor.

If you have a passive antenna, put a LNA right at the antenna, since any 
cable damping will cause the S/N to go down. Also, if you put an 
aditional amplifier in line, your want that too up at the antenna.
Then, low-loss cable should be natural.

Cheers,
Magnus



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