v2 Prototype issues and fixes

Sylvain Munaut 246tnt at gmail.com
Thu Jun 21 18:10:18 UTC 2012


The v2 prototype I still had a couple of issues so I'll describe them
here and the corresponding fixes:

- Bad TVS Diode: There is a diode that's supposed to protect against
ESD near the antenna connector. Turns out that the component supplier
swapped reels IIRC and so that's not a TVS diode that was mounted. My
board had the rework to fix this already done. I don't know if any
board shipped with the wrong part. It's the small ~ 0603 sized
component near the antenna connector and it's supposed to be greenish.
The result of this is a ~ 10-15 dB attenuation of the input signal.

- Missing LNA bias inductor: The v2 has a LNA at the input but it's
missing it's bias inductor which means it's powered off. So instead of
boosting the signal by 18 dB, it actually attenuates it quite a lot.
Solution is simple: Solder a 0603 bias inductor. Schematics call for a
470nH inductor and make sure to choose a good RF rated one and not
random junk.

- FPGA 1.2V LDO Oscillation: The output capacitor of the LP3965
regulator generating the 1.2V for the FPGA core has too low an ESR (a
ceramic cap is mounted). This regulator needs a capacitor with an ESR
greater than 0.5 ohm and lower than 5 ohm for it to be stable. Without
this, the LDO is unstable and has high spectral content at 55 kHz and
harmonics. This noise is present at the output but also leaks to the
input voltage of the LDO which is the 3.3V rail that powers the LNA
... This is causing unwanted images of the signal at   f +- 55kHz  f+-
110 kHz ... The solution is simple: replace the cap by a tantalum one
within the right ESR range. ( I used a TR3A106K016C1700 from Farnell
). This is before the fix: http://i.imgur.com/rRDDQ.png   and this is
after : http://i.imgur.com/VlOW7.png

- Impedance mismatch between E4K output and ADC input: Ideally we'd
like to imagine that the E4K IQ output have very low impedance and
that the ADC input have very high impedance. Turns out that neither is
true: The E4K has a ~ 500 ohm output impedance and the ADC is a track
and hold type and the capacitance of the input changes when you start
sampling, causing a small current flow. The combination of the 2
creates a noise at the sampling frequency on the IQ lines ... The
effect of this is currently unknown and is being investigated ... it
might be possible to lower the noise floor a bit and get less DC
offset with this. But it certainly doesn't have as much impact as the
3 erratas above for sure.

See for components location. http://i.imgur.com/M4Lvt.jpg



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