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

Chris Albertson albertson.chris at gmail.com
Fri Apr 25 17:08:53 UTC 2014

On Fri, Apr 25, 2014 at 9:04 AM, Attila Kinali <attila at kinali.ch> 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)

I'n not a microwave expert at all.  But I took my Trimble amplifier apart
and looked.  They used a chip like you describe and some inductors to
bypass the DC.   The part that is impressive is that they maintained a 50
ohm transmission line all the way fem both N-connectors to the chip.  The
chip fits perfectly on the 50 ohm strip line trace.   The radius on the PCB
traces is just a about right too.

I suppose you could build the same circuit "dead bug" style by soldering
small rg178 coax directly to the IC pins.  I've tried.  I need a low power
microscope to get it right.  But those are available for about $100 on
eBay.  The circuit is more complex if you need to inject DC bias.
Truthfully I'd just buy one.  They make them for cable TV that work for GPS

But I've build tons of stuff, mainly for the education you get.  No hobby
is "practical".

> > 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)
>                         Attila Kinali
> [1] http://www.triquint.com/products/d/DOC-B-00000120
> --
> The trouble with you, Shev, is you don't say anything until you've saved
> up a whole truckload of damned heavy brick arguments and then you dump
> them all out and never look at the bleeding body mangled beneath the heap
>                 -- Tirin, The Dispossessed, U. Le Guin
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Chris Albertson
Redondo Beach, California

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