GPS receivers Front-End Design Problems

Thread Starter

HAMADA2YOU

Joined Mar 11, 2022
2
I am designing a GPS L1(PWR=-160dBW) band Direct RF sampling receiver and the front-end:
(passive antenna + LNA(21dB) + BGA1(30dB) + jumper + BPF(L1(fc=1575.42MHz,BW=15MHZ) + BGA2 (30dB) + BGA3(30dB) + VGA(-13.5 to +18 Gain Control in 0.5 dB Steps) ).
all amplifiers are supplied with a +5 V power with decoupling capacitors and a test point after each component to view the signal.

The problems of the printed circuit:

  • noise peaks through the RF line that gets amplified even though we have no input signal after each amplifier seen through the test points (+5V power is ON), no noise when +5V is OFF even when there is an input noise.
  • The BPF filters the noise peaks generated by BGA1, but the Nosie peaks still get regenerated by BGA2 .
  • One issue is that each amplifier amplifies with different GAIN values even though they are the same BGA, also I am sure I did not reach 1dB compression point(10dB).
The test point output using Network analyzer to see S-parameters ( the port1= input , port2= test points) S21 outputs are :

  • Port2= test point after LNA:
1647008167648.png
  • Port2= test point after BGA1:
1647008186435.png
  • Port2= test point after BPF:
1647008207757.png
  1. Port2= test point after BGA2:
1647008227773.png
any suggestions on the matter would really be helpful.
 

drjohsmith

Joined Dec 13, 2021
394
RF is real difficult
its more of an art than digital
even the way a track is laid out, or how close other parts are can affect the results
You have a high gain front end
perfect to oscillate,

Way forward,

Start at the front,
build up only the first amp circuit
do not fit any of the rest
once that works as you expect,

build up only the second stage, nothing else
once that works as you expect,
join 1 and 2 together, and check that works as you expect,

repeat
 

Thread Starter

HAMADA2YOU

Joined Mar 11, 2022
2
thank you for replying,
I realized that the amount of amplification is a lot therefore I removed the VGA and BGA3 to just amplify with only LNA+BGA1+BGA2 which is actually the output of the screen shots I provided in thread,

when it comes to the PCB I designed 4-layer PCB (1stlayer=microstrip (50ohm matching),2ndlayer= GND,3rd layer= PWR(+5V,+3V),4thlayer=GND).

I think the circuit contains enough decoupling caps between VCC and GND.

The second test:

The input is an antenna for GPS L1 signal(L1=-160dBW) received with modified PCB(noVGA and BGA3) and these are the outputs I see in the signal using spectrum analyzer:

  • test point after LNA:
1647085214496.png

  • test point after BGA1:
1647085224099.png

  • test point after BPF:
1647085240276.png

  • test point after BGA2:
1647085251455.png
one issue I realized that each amplifier does not provide the same amount of gain even though they are the same.
 
Top