Power Control loop in Transmitter

[Chest]

Hi all experts,

I have encounter problem in the power control loop in my project. My Tx power is lower than half when I tested at -30 degC compare with room temp. It is also become unstable and need some time (> 1 mins) to lock the output power to a specific value.

I had checking on the Pre-driver and PA, all is working fine. Based on your experience, what experiments or actions need to do to solve this issue? Anything that I miss up?

Between, May I know where can I getting much information regarding power control loop?

Thanks in advance!

 

[FvM]

Interrogate the reason for temperature dependency of output power first. I guess, the problem is rather in the PA power gain
than in the control loop. If the gain drops seriously, it may be difficult to keep it constant by the control loop.

 

[Chest]

Hi FvM,

Thanks for promptly reply. This failure is occured when supply 6V , 7.5V supply given normal.

I had given external Vcntrl to the Pre-driver and PA, it do not seem any gain drops (Open loop). But when i close loop it, the output power is worst. Any ideas?

Thank for your helps.

 

[FvM]

O.K. in this case, it seem to be actually a control loop problem. May be controller output voltage exceeds the specified? Did you
verify the power sensor operation at low temperatures in open loop?

 

[vfone]

Feedback loops could be unstable, and this is compensated with a dominating pole RC filter. This pole decreases gain of the loop on higher frequencies to get phase margins high enough.

I don’t know which variant you use but there are more options for PCL.
-Varying the DC bias of the PA transistors,
-varying the power supply voltage of the PA,
-using a controlled PIN diode attenuator in the front of the PA.
The last option is the most stable because the gain of the PA remains constant.

All the power detectors part of a PCL requires a temperature compensation, to maintain the stability of the loop. If the detector is an IC this already has it inside, but if is a simple Schottky diode you may need to use another diode for temperature compensation.