LMX2594: High temperature output anomaly

jiao he

Prodigy 40 points

Part Number: LMX2594

hi all，

1)My product normal output at normal temperature. See Fig.1 for spectrum diagram.

Fig.1

2)When the plate temperature reaches -70℃, the spectrum output is abnormal, as shown in FIG. 2.

Fig.2

3) Register configuration

Phase detection frequency 200MHz;
Charge pump current 15mA;

over 3 years ago

0 Noel Fung over 3 years ago

TI__Guru** 122030 points

Hi Jiao,

what is the temperature? 70C or -70C?

Is it the ambient temperature?

What is the reference clock frequency? Where is this clock came from?

0 jiao he over 3 years ago in reply to Noel Fung

Prodigy 40 points

sorry，temperature is 70℃，I write it wrongly;

It is ambient temperature；

The reference clock frequency is 100MHz;

The reference clock is rectified. It is not a clock problem.

Modify the phase detection frequency to 100MHz, and the PLL spectrum is normally at 70 ° C.

I want to ask what the reason?

0 Noel Fung over 3 years ago in reply to jiao he

TI__Guru** 122030 points

HI Jiao,

Did you use OSC_2x or MULT to get 200MHz fpd from the 100MHz input clock?

0 jiao he over 3 years ago in reply to Noel Fung

Prodigy 40 points

HI,Noel,

I configured register R43 to 0X2B0021, thus doubling the external 100MHz clock to 200MHz.

0 Noel Fung over 3 years ago in reply to jiao he

TI__Guru** 122030 points

Hi Jiao,

R43 is regarding PLL_NUM.

Are you using the Doubler or the Multiplier to get 200MHz fpd?

0 jiao he over 3 years ago in reply to Noel Fung

Prodigy 40 points

Hi,Noel,

I using the Doubler.

0 Noel Fung over 3 years ago in reply to jiao he

TI__Guru** 122030 points

Hi Jiao,

What is your loop filter design?

Looks to me that this issue is related to the input path. When you try 100MHz fpd, were you disable the Doubler or set the R-counter to 2? If you disabled the Doubler to get 100MHz fpd, would you also try keeping the Doubler enabled but make PreR = 2? I want to see if the problem is coming from the Doubler or from the fpd.