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[Paparazzi-devel] Home made GPS filters

From: Helmut Wabnig
Subject: [Paparazzi-devel] Home made GPS filters
Date: Sat, 07 Jun 2008 13:27:30 +0200

On Sat, 07 Jun 2008 06:16:25 -0400, you wrote:

>Message: 2
>Date: Fri, 6 Jun 2008 10:48:13 -0600
>From: "Daniel Morgan" <address@hidden>
>Subject: [Paparazzi-devel] Hardware filtering quesiton
>To: address@hidden
>       <address@hidden>
>Content-Type: text/plain; charset="iso-8859-1"
>SO i have been looking at this website trying to figure out how to make a
>filter for the gps so it will reject outside frequencies but have been
>unable to figure out what a 1206 coil is specifically.. I think it is a
>inductor but i thought those were measured in Henry's.. Here is the website
> . The filter i am looking
>to make is the: Homemade low-pass filter for GPS and block for 2.4GHz
>using 1206 coils (20% tolerance) and cheap 0805 1pF (10%) 100V capacitors.

You cannot make such a filter without the adequate measuring
equipment. Even the tiniest mechanical tolerance will shift the 
resonant frequencies by many Megahertz.

But there is no need to make filters for your GPS, they are readily
available in a good active GPS mouse antenna.
Not every GPS Mouse has such a filter, there are EL CHEAPOS
around with only contain a broadband amplifier.
But a reasonable product will contain a narrowband filter.
I put an example here:
Scroll down to image nr. 4, the reverse side of the print.
There is a component with an "L" structure on it.
This is a parallel  GPS filter, connected from the active signal
line to ground. Actually it is a parallel resonant circuit.

Other products may contain a series filter, typically
a small chip with even smaller holes at the sides.
You will recognize when you see it.
Usually the frequency is labelled on the serial chips.

You may either use the complete active mouse 
or you may remove the chip for your own design.
Filter chips are also available from distributors.

I use to make simple parallel bandpass filters with a lamda/4
short circuited quarterwave coax stub. See above link.
A coax stub is of intermediate bandwith, not too narrow to make
reproduction impossible, but good enough for many purposes.

GPS wavelength is 300/157,5 = 190,5 mm
lamda/4 = 47,6mm.
A coax with a shortening factor of 0,66 will have to be 31,4mm long.
Short circuit one end with a solder ball and connect the other
end parallel to your GPS antenna input.
This will bring down unwanted interferences.

I proceed the following way: Make the stub longer, then pinch the
end with pincers until I get maximum reception.
Requires 2 tries normally, because when the first stub gets too short,
you will have to make another one.


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