Reducing the settling time for LMX2595

Vikas Kumar40

Part Number: LMX2595

Good Morning !!

I am using the LMX2595 chip for generation of S-Band frequency.

Currently , I am trying to sweep S-Band from 2.7 GHz to 2.9 GHz, because of which the VCO switches between VCO3 and VCO4.

I using the Mux out status to see the lock time. What I am observing is, when the VCO switches from VCO3 to VCO4, settling time is greater than 250 us and vice versa is around 120 us.

How to reduce it further. I have attached the plots as well

Any help !!

Regards

Vikas

over 2 years ago

0 Noel Fung over 2 years ago

TI__Guru** 108550 points

Hi Vikas,

I assume you are not using any kind of VCO calibration assist. You can try enable the QUICK_RECAL_EN bit so that the calibration will start from the last VCO status.

0 Vikas Kumar40 over 2 years ago in reply to Noel Fung

Intellectual 660 points

Hi Noel

I tried using the Partial assist and it worked. From the 260 us, settling time got reduced to 120 us. Now I am trying with full assist mode. I am facing some issues.

As per the datasheet , VCO SEL,VCO DACISET and VCO_CAPCTRL is mentioned to write along with other setting.

VCO DACISET and VCO_CAPCTRL , I am reading through Mux OUT and then writing it, but it unlocks.

Am I missing something.

Regards

Vikas

0 Noel Fung over 2 years ago in reply to Vikas Kumar40

TI__Guru** 108550 points

Hi Vikas,

There is an appnote put this in details.

https://www.ti.com/lit/pdf/snaa336

0 Vikas Kumar40 over 2 years ago in reply to Noel Fung

Intellectual 660 points

Hi Noel

Thanks for the application note. With the help of this, I was able to do the Full assist mode . I have some observations.

I have the reference of 50 MHz. In PLL, I use doubler to make PFD 100 MHz.

Initial frequency is 2770 MHz.

I keep the toggling frequency 2670 MHz and 2870 MHz ie switching between VCO3 and VCO4. Considering the SPI write time 14.72 us

1: Lock detect time for 2670 MHz is 39.72 us

2: Lock detect time for 2870 MHz is 67.6 us.

Can we reduce it further for 2870 MHz.

Regards

Vikas Kumar

0 Noel Fung over 2 years ago in reply to Vikas Kumar40

TI__Guru** 108550 points

Hi Vikas,

You can try write the DACISET value first followed by other registers that are required to program.

Clock & timing

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Reducing the settling time for LMX2595