[Gta04-owner] GPS fix?

[Dr. H. Nikolaus Schaller]

Hi Neil, Boudewijn, et al.,

over the weekend, we have done some more experiments
and now it is clear where the bug is sitting. It is indeed an IC
with 6 feet (like bugs) IC: the antenna switch (U1201).

Although we have copied the circuit 1:1 from the Freerunner
we did make one mistake. We have swapped the two antenna
inputs. But only for the antenna switch chip and not the control
signal which should have been inverted.

So, if the current sensor U1202 detects an external antenna,
this tries to switch off the power for the internal antenna (T1201)
and moves the RF switch - to "internal".

And if there is no external antenna, it moves the switch to the
external input.

Additionaly, the Freerunner circuit design is quite critical since
it does operate the antenna switch in some linear mode.
We have never seen a clear control signal that is within
the specification - so I wonder why it ever worked in the
Freerunner...

We could also not see that it switches off the power of the
internal antenna completely. Therefore, the receiver always
sees a mix of internal and external antenna signals.

This was proven by temporarily connecting and removing
the internal antenna, while monitoring the satellites (GPGSV
record).

Adding the internal antenna reduces the SNR by 10dB.
Since the maximum SNR I have seen is 35dB these
10dB reducting is enough to make all satellites invisible.
Even with external antenna.

Therefore, the only mode where you currently can get a
GPS fix with the GTA04A3 boards is after removing the
internal antenna and using an external antenna outdoors.
Indoors, signals are too weak.

Now the other experiment was to solder some wires to
manually control the RF antenna switch. 

When forcing the RF-switch to "external", it is really like
removing "ear-protectors" and the SNR raises by 15 dB:

red color: no signal
brown: weak signal
green: sufficient signal
light green: very strong

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Now as we have identified the reason, what is a cure?

At the moment we do not know yet, but are looking into some
options. One is to really cross over the input signals by two tiny
wires. We have to discuss that with the SMD and RF specialists.

Maybe of interest for Open Hardware development lab stories:

Why didn't we notice earlier? The GTA04A2 board didn't
work well enough to test any GPS. And the first GTA04A3 boards
arrived in July. Back then, we did some tests with an external
antenna and got a location fix. So we thought it works and missed
the detailled analysis of the SNR values etc. And we had to solve
other even more important issues first (you may find a collection
in the list archives).

Conclusions:
1. testing can only prove the presence of bugs, not their absence.
2. there is always one more bug.

Nikolaus