LNA self excitate and receiver sensitivity poor

[helen.liu]

lna de-sensitivity

Hi all,

i want to know how to judge whether the LNA is self excitated?
I design the CDMA mobile phone, the sensitivity is very low in some channel, only -100dBm, and some channel is normal, about -108dBm, this is in B2, before B2 the sensitivity is very good, and there is no change from B1 to B2, except add the ground vias around the Receiver chipset. (LNA is integrated in the receiver chip)
I think this maybe caused by LNA self excitated, but I don't know how to judge? and what will cause the LNA self excitated?

I think the receiver debug is very difficult, because the signal is very small, anyone please give me some advice to debug the receiver?
now, the phone is in production. and the senstivity of few phones also have problem, then I don't know how to find the problem, and I have no idea about this.

 

[khouly]

u may check if there is any harmonics of fall into ur receive band ,

the new chipsets contain all the receiver blocks , so u need to check the interface , like matching network between the LNA and SAW components.

also check about the sheilding

Khouly

 

[vfone]

Is unusual the Noise Figure of the same LNA to be different in different channels. NF could go up and down in the same RX band, but not in different channels.
For those channels most probably you get RX de-sensitivity from somewhere else (BB, digital processing, spurious, etc)

 

[helen.liu]

Khouly and vfone, thank you for your advice.

now, i do some experiments, and i found the problem was caused by the RX chipset, i changed the RX chipset, the sensitivity is ok, and i put the poor sensitivy rx chipset on a good board, the good board sensitivity is poor.

i don't RX chipset how to cause the sensitivity poor, and i don't know how to find the root cause, and how to test it?

 

[khouly]

mmm , this means that this is a damaged chip. if the number of damaged chips is high u need to check the handling procedures of these chips.


khouly

 

[kspalla]

I have a question.
If the chip is damaged hhow can it work? i mean it should not respond to the incoming signal right?
I suppose the chip has wide range of tolerance.

Hi helen.liu,
Do you have the charectozation treport of RF chipset?
mean over temp, voltage, frequency etc.

 

[stober]

Khouly is right. There's some harmonics of spurs fall into the bad bands. This is very usual in RF chips but very hard to control.
I think you can do nothing about it now except re-design the chip under the condition that the spurs are considered tenderly.