SparkFun Forums

[maokh]

The setup would consist of a surface mount GPS antenna (from sparkfun), an in-line LNA, and GPS receiver.

I would like the surface mount GPS antenna and LNA to be an "active antenna", as the GPS receiver injects 3.3VDC on the coax.

The reason why I can't just power up the LNA with the traditional power supply is because the GPS has great power management using push-to-fix, and I do not want to run the amplifier when it wont be used.

I have had a very difficult time trying to track down an amplifier (and more importantly) the app notes related to the amplifier to operate it off DC power supplied by coax.

Does anyone know of any components that would be of use for the LNA? Im certain there has to be some low cost, cheapo LNA IC that these gps antennas are using...

[henryhallam]

Most modern GPS receiver modules include a built-in LNA for use with a passive antenna. If the antenna and receiver are on the same PCB, there is no advantage in putting another LNA between them. In fact, it may be a bad idea because if it has more than about 10dB gain it could saturate the receiver and give poorer performance.

An active antenna can't intrinsically give better signal-to-noise ratio than a passive antenna; it can't get rid of noise that is already present. What it does is prevent signal loss and extra noise being picked up in the cable between the antenna and the receiver.

If you are planning to have the antenna more than a few inches away from the receiver, go ahead and use an LNA - but I might ask why you don't just buy an active antenna in that situation.

[maokh]

Sure, i'll throw in more detail if it helps.

I am using the GM8630-GPS, which apparently requires the use of an LNA. Sadly, using the surface mount antenna on stripline or coax without amplifier provides time only.

The surface mount component seems to be a logical choice in reducing size/weight/complexity.

Also, in push to fix mode, the telit's sirf chip turns off LNA power unless its refreshing its position information. Saves power compared to running the LNA constantly.

[henryhallam]

Oh, sorry - I should have figured you'd thought it through

I can't help you in choosing an LNA, but separating the power shouldn't be too hard. Inductor, diode, capacitor. I'm pretty sure I've seen a suitable circuit complete with part list out there, but a couple minutes googling doesn't bring it up.

[henryhallam]

This is the circuit I think you should use:

Bear in mind, I haven't built it and I'm pretty new to RF.

[maokh]

That makes sense -- thanks for the jump start on this. Are you under the impression that the value of L could impact the 50 ohm match of the microstrip line?

My RF is a bit weak too, so im a slave to app notes.

If you ever find the document, let me know.

[henryhallam]

I have a hunch that it shouldn't, because it should only pass DC, but I'm really not sure. Sorry!