LMX2594EVM: Programming 3GHz and 6.25 MHz using a 10 MHz Reference

Doug Bailey

Part Number: LMX2594EVM
Other Parts Discussed in Thread: LMX2594,

I am having a problem with generating frequencies out of a LMX2594 that I am unable to figure out.

I have a system where I have a 10 MHz reference clock being input to the OSCIn pins of a LMX2594. I am trying to set the RFOutA to 3 GHz and RFOutB to 6.25 MHz. TICsPro allows me to set these values. (Take the default, set the oscillator input to 10 MHz, set the outputs, enable the outputs and fix the PFD_DLY_SEL value) However, when I load the registers in my LMX2594EVM board, I get 15.625 MHz out of both outputs.

I have the MuxOut pins configured to show Lock Detect and it is active. The VTune TP is at ~1.2V.

I do not currently need to use the sync and ramp features of the part. I am simply trying to get the device to lock on to the 3GHz and 6.25 MHz and stay at those frequencies.

I am not very knowledgeable about the workings of PLL's. Any suggestions as to what I need to do to get my desired frequencies?

Regards,

Doug Bailey

over 5 years ago

0 Noel Fung over 5 years ago

TI__Guru** 109620 points

Hi Doug,

Could you provide a screenshot of your TICS Pro configuration?

0 Doug Bailey over 5 years ago in reply to Noel Fung

Intellectual 290 points

0 Doug Bailey over 5 years ago in reply to Doug Bailey

Intellectual 290 points

The more I look at this, the more it appears that the TICsPro is not handling this correctly. It appears that if I choose the channel divider for both outputs, they both should be the same frequency i.e. the channel divider only provides one frequency source.

It appears that I should be using the SYSREF block to divide down the VCO to produce the 6.25 MHz. What is required to output a clock via the SYSREF function?

Doug

0 Noel Fung over 5 years ago in reply to Doug Bailey

TI__Guru** 109620 points

Hi Doug,

the Channel divider is shared between RFoutA and RFoutB, if the output mux are "Channel Divider" for both outputs, they will output the same divided down frequency.

In order to get 6.25MHz from RFoutB, you have to use the SYSREF divider.

- change the output mux for RFoutB to SYSREF.

- check VCO_PHASE_SYNC

- check SYSREF_EN

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LMX2594EVM: Programming 3GHz and 6.25 MHz using a 10 MHz Reference