LMX2595: lmx2595RHAT

Hi Carmen, 
Refer to this past thread: LMX2595: Variation in power level with Temperature - Clock & timing forum - Clock & timing - TI E2E support forums

Regards, 

Vicente 

Hello Vicente,

We have been using the LMX2595 in about 30 units of our product and we know output power goes down when temperature is high.

We use it with a RF chain with variable atenuators. The problem is that between 13.5 and 14 GHz in this unit we can not compensate it

With a switch we choose between an external signal or the lmx2595 output (CAL1 or PLL)

With PLL (LMX2595) at 14.5-15 GHz the the output power is a bit higher than at 13.75 GHz

In other units both grafics are flat at 70ºC. Example of other unit at 70ºC and PLL

Here is the schematic we are using with LMx2595

Could be one of the R or C be the origin of the problem at 13.75 GHz?

Best regards

Carmen

Hi Carmen,

Did you mean some, but not all units have lower power issue at 13.5GHz - 14GHz? This problem only happen at 70C?

This is the only unit that between 13.25 and 13.9GHz has less power. At 14 GHz it goes up. See picture with the red circle.

We have used the lmx2595 in more than 30 units and at hot temperature this is the only one with that type of valley.

Here is the schematic in more detail for the output..

Perhaps a component or some components are not working well at 13.25-13.75 GHz.

Which R or C are the ones that determine the output power?

Hi Carmen,

Both R and C will affect the output power. We use "broadband" resistors and capacitors in our EVM.

In your schematic, you have one C to support two R. In your layout, it should be similar to below, R and C are connected together on the same pad. If the R are not connected on the same pad, we need two C. 

I try to improve my unit output power and I replaced 10nF (AT Ceramics 520L103KT..) and power at 70ºC is still low.

In other units I do not have this problem.

With this one power at 70 ºC and 13.75 GHz is 4.5 dB lower.

At T 25ºC and T -40ºC it works well

What can I do to improve power at 13.75 GHz?

Hello Noel,

I have checked LMX2595 tags and I have seen "LMX2595: Regarding LMX2595 O/P POWER with inductive pull up as in datasheet given 7dB." 

This tag is about less output power at 15 GHz. You gives two possible solutions:

- Cs change from 0.3pF to higher value

- Leaving RFoutB floating.

In my board Cs are 10 nF.

Could it help is I do not connect RFoutB to voltage supply. Desconnect R2 and R5?

Hello Noel,

I tried to romeve R2 and R5  and output power is still low.

With the other 30 units we leave OUTA_PWR = 31 at all frequencies.
The datasheet says for high temperatures between 13.3 and 14.3 GHz OUTA_PWR = 15.

If I change OUTA_PWR = 15 can the output power at 13.75 GHz increase?

Hi Carmen,

I tested two boards, one of them can bring the power up with OUTA_PWR = 15 in 13.3 - 13.775GHz output. Worth to try.