wiki/ news/ 2005-05-26 - GPS redux 2: trying a LNA with two added saw filters


  1. Test the LNA hacked with SAW filters.
  2. See if that solves the ATV interference problem.


Tim managed to fit two SAW filters in the GPS LNA, which is astounding. One (the closest to the SMA connector is between the antenna and the LNA. The other is between the LNA and the receiver. Here is one pre-SAW filter picture, and two post-SAW filter pictures (the SAW filters are the silver squares):

pre_saw_filter.thumb.jpg saw_filter_1.thumb.jpg saw_filter_2.thumb.jpg

We set up in front of Andrew's house (not the cabin, which has big trees all around it). We set up the Jupiter board and let it acquire 5-6/10 satellites using the 2 x 2 in. active patch antenna. Then we quickly switched it over to the GPS cylindrical patch antenna (CPA) + the unmodified low noise amplifier (LNA). That dropped us down to 4-5/10 satellites, but it did work. So then we reattached the active patch to keep the receiver locked, replaced the LNA on the GPS CPA with the LNA with SAW filters, and tried it again. Nadda, nothing, zippo satellites.

There are two SAW filters in the LNA: GPS CPA-> SMA connector -> SAW filter -> LNA -> SAW filter -> SMA -> Receiver. So we soldered on a jumper wire to short out the one closed to the antenna, which was the hard one to solder in anyway. We then got 1 - 3 satellites, which means that the 10 ohm impedance of the jumper wire (roughly 2 * pi * f * L = 2 * pi * 1.5E9 * 1E-9) was better than the first SAW filter. We put another wire across the second SAW filter, but that didn't help much... 3 - 4 satellites now, not the 5+ we got with the regular LNA (and we think the constellation may have changed a bit since the first one wire test).

So: something is wrong with the first SAW filter between the LNA and the antenna.

Injecting a signal into the input (antenna side)

The 2m LO broadcasts at 1.569 GHz right now (gack), so we're using that as a GPS signal generator.

The mod wire is: 0.25 inch long, 0.1 inch high, 0.0225 in in diameter

We cut the mod wire.

Injecting a signal into the output (receiver side)

Just to try the other SAW filter, we put the injected signal into the output SMA connector. Note the wire is in there.

NOTE Now all measurements are from the top graticule, reference level is 50 dBuV, attenuation is 0dB.

Now we snip the wire.

So: the SAW filters are attenutating the signals in a big way, and the only reason we maybe got a signal after the wire across the input SAW filter is because of the massive gain of the LNA.

HEY. WAIT A MINUTE. DIGIKEY SENT US THE WRONG @(#*@($(# SAW FILTERS: we have the 1842, not the 1575, filter!!

OK, that explains a lot. Time to yell at Digikey.