LMX2592EVM: LMX2592EVM will not operate with input higher than 600 MHz

Brian Walker

Part Number: LMX2592EVM
Other Parts Discussed in Thread: LMX2592, LMX2594

I am evaluating the LMX2592-EVM. It operates well with input frequencies up to about 600 MHz. Beyond that, I am unable to get the board to function. Interestingly, the board will not function with an 800 MHz reference input but if I change the input to 400 MHz and engage the doubler such that the chip sees 800 MHz internally, I get good results. The instructions for the EVM board say to apply 100 MHz and doesn't mention going higher than that. The data sheet specifies operation from 5 MHz to 1400 MHz for the chip itself. Is there a problem with the board that is crippling high frequency inputs?

over 2 years ago

0 Derek Payne over 2 years ago

TI__Mastermind 28960 points

Hi Brian,

While the LMX2592 can operate with up to 1400MHz reference input, subsequent blocks may not work correctly unless the reference frequency is divided down in the Pre-R stage. I believe the maximum supported frequency into the PLL_R divider is 250MHz, based on similar restrictions in LMX2594 (with a similar input stage). Try using the Pre-R divider to reduce the input frequency to <250MHz before it hits the R divider.

I believe we did not capture this limitation anywhere in the datasheet or the GUI. I'll make a note to clarify this limit (whatever it may be) in a future release of both.

Regards,

Derek Payne

0 Brian Walker over 2 years ago in reply to Derek Payne

Prodigy 10 points

Hi Derek,

Thanks for the very logical reply. I did already try this though. With an 800 MHz input I tried setting the first R divider to "8" and the second to "1" with no results. Also tried "4" and "2" with equally disappointing lack of operation. Strangely, the board works with a 400 MHz input and the doubler turned on followed by a divide by 8 then divide by 1 on the R dividers. The internal frequency would be 800 MHz in this case of course. I am driving the reference port single ended with the other port terminated to 50 ohms. Increasing the reference drive level doesn't seem to help. I have it set to +10 dBm at the moment which works fine at lower frequencies.

Best,

Brian Walker

0 Noel Fung over 2 years ago in reply to Brian Walker

TI__Guru* 81400 points

Hi Brian,

What were the setting for FCAL_HPFD_ADJ and CAL_CLK_DIV? These registers should be set according to the fpd and OSCin frequency in order to produce a robust VCO calibration.

0 Brian Walker over 2 years ago in reply to Noel Fung

Prodigy 10 points

Hi Noel,

Thanks for your suggestions. I had ACAL_CMP_DLY set to 100, FCAL_LPFD_ADJ set to Fpd >=10, FCAL_HPFD_ADJ set to Fpd <= 100, and the Fpd is set to 100 MHz. The VCO frequency is 4600 with Prescaler of 2 and N of 22 with Fnum/FDen equal to 1/1. Final output is /3, /8, /8 for a total of 192 and 23.958 MHz output.