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GNSS Receiver designed IC circuit noise issues

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Ahmedalboshra

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Hello everyone
I wanted to share with you the design of a 2-layer full GNSS board that I have recently developed using Direct RF sampling architecture. The board consists of several blocks, each equipped with SMA input/output connectors, as well as an SMA connection between VCC and GND to check the noise. The board is designed to enable efficient processing of GNSS signals. The blocks are : 1. LNA , 2. 3xBGA amplifier blocks , 3. Splitter/3xBPF/Combiner, 4. Variable Gain Amplifiers (VGA). which are shown in Figures below:
1685775994459.png
1685776029039.png
1685776049321.png


One of the initial tests conducted involved checking each amplification stage using a single-tone signal at the GPS L1 center frequency of 1575.42MHz. The signal was intentionally set to a very low amplitude for testing purposes.

During the test, one notable issue arose when connecting the VCC using shielded cables to the BGA block. Even before turning on the VCC and RF , a signal was observed at the VCC SMA connection, as depicted in the following figure:

1685776135785-jpeg.141680


when VCC in ON / RF OFF:

1685776178604.jpeg



Next test is in the output SMA VCC OFF/RF OFF :

1685776325943.jpeg


RF OFF/ VCC ON:

1685776374661.jpeg


This output makes no sense, its too much for amplifier no input its just amplified the noise, I still don't get it.

I would really like some suggestion regarding this issue.
 

Are your VGAs part of an AGC loop which decides to
go to max gain when you give no signal, so all that
local radio is brought up out of the noise?

Maybe sample more so you get a look at what's in
the noise, for steady tones, and check out screened
test cages and other incoming-EMI reduction measures.
GNSS/GPS are low level signal at best, and test needs
to be accordingly clean. I'd bet there's articles on that
subject out in test & measurement land.
 

Are your VGAs part of an AGC loop which decides to
go to max gain when you give no signal, so all that
local radio is brought up out of the noise?

Maybe sample more so you get a look at what's in
the noise, for steady tones, and check out screened
test cages and other incoming-EMI reduction measures.
GNSS/GPS are low level signal at best, and test needs
to be accordingly clean. I'd bet there's articles on that
subject out in test & measurement land.
the screen shorts I provided are just based on the BGA circuit, no input was provided, just vcc which in turn gave us that amplified noise, the main issue I thought is that the vcc cable works as a antenna so we shielded them, but still nothing changed.
 

I think the RF circuit operates in a unstable state,resulting in self-excitation phenomenon.
 

As I have observed, there are 2 issues.
-Shielding is very poor
-Amplifiers/LNA oscillate.
** Layouts are not so appropriate. Don't use thermal relief for the connectors in RF
** Shielding is a must
** Low Level RF measurements must be done in a shielded chamber. Otherwise there are many interferer signals around
 

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