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LNA connected in Series and position of blocking DC capacitor in such design

yeocker

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Hi all, I am looking for some answers on getting my current GPS to work on my setup below with an external LNA.
The module that I am using (Quectel BG-95) has an internal LNA, nonetheless I have connected an external LNA in series in hope of improving the gain further. The spec sheets of this module specifies that gain of the external LNA should be < 17dB to prevent signal saturation and I have picked an LNA with gain less than 17dB. However I wasn't able to get GPS to work with this LNA.
I researched and saw that some design placed the 100pF DC blocking capacity after the LNA right before the ANT_GNSS input on the module. I wonder if this would fix my issue here.
By the way, I have tried bypass the LNA and was able to get GPS fix on the module as long as I have clear skies but when the GPS signal gets weaker, the GPS fix is less reliable, hence would really like to try having an external LNA to see if I can get reliable GPS fix when the signal is weaker.
Appreciate any help offered here. I am pretty much out of ideas now!
 

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BigBoss

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But Skyworks said that a Matching Circuit is necessary between Antenna and Input of the LNA.. didn't you see that ?/
Also, some tweaks will always be needed around the matching circuit so they are not computer plug-in-play boards.
There might be some mismatches between external LNA and GPS Module, consider that too..
 

biastee

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However I wasn't able to get GPS to work with this LNA.
I researched and saw that some design placed the 100pF DC blocking capacity after the LNA right before the ANT_GNSS input on the module. I wonder if this would fix my issue here.
How do you know that the LNA is working? Did you assemble it yourself?

I don't think a DC blocking cap is necessary in your case because the SAW filter at the module input is already blocking DC (please confirm this by measuring the DC resistance between the SAW filter's input & output).
 

albbg

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It depends. I'm not sure the LNA has or not an internal dc-block capacitor. The data sheet doesn't mention it, then probably is AC cupled.
In any case the SAW filter can stop the DC, but can shunt the LNA bias (especially if there is a matching network having an inductor towards ground).
The matching network is required for best performances, so you could loose same sensitivity, but the system have to work.
Do you correctly drive the enable signal ?
 

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