LMX2820EVM: LMX2820EVM 10MHz external reference source

Chad Labarre

Prodigy 10 points

Part Number: LMX2820EVM

Tool/software:

I would like to take phase noise data of the LMX2820EVM using a 10MHz external reference instead of a 100MHz reference.

How do I do that?

Thanks,

Chad

7 months ago

0 Noel Fung 7 months ago

TI__Guru** 111540 points

Hi Chad,

I don't quite understand what support is needed here, could you be more specific?

0 Chad Labarre 7 months ago in reply to Noel Fung

Prodigy 10 points

The default external reference in the eval firmware is 100MHz. I want to use a 10MHz external reference instead. Is there a way to use a 10MHz external reference w/ this eval board?

0 Noel Fung 7 months ago in reply to Chad Labarre

TI__Guru** 111540 points

Hi Chad,

Yes, you can use 10MHz reference clock.

0 Chad Labarre 7 months ago in reply to Noel Fung

Prodigy 10 points

OK, now, do I need to download firmware to use a 10MHz clock? I tried using a 10MHz clock w/ the default program, but the PN looked pretty bad.

0 Noel Fung 7 months ago in reply to Chad Labarre

TI__Guru** 111540 points

Hi Chad,

There is no firmware. All you need to do is to configurate TICS Pro properly and then program all the registers. For example,

Phase noise becomes bad is expected. The EVM is design with fpd=200MHz, loop filter bandwidth is approx 440kHz.When you use 10MHz fosc, as shown in above diagram, fpd = 20MHz, so the loop bandwidth will decrease to approx. 58kHz. Inband phase noise is going to be increased. Furthermore, if your 10MHz input clock is a sine wave, PLL noise will get hurt because of poor slew rate.

If you have to use 10MHz sine wave clock, suggest convert it to square wave with our clock buffer. See the details in https://www.ti.com/lit/pdf/snaa411

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LMX2820EVM: LMX2820EVM 10MHz external reference source