LMK03318: Jitter on PLL Clock on status and shape of 1GHz output

Prasoon Joshi

Part Number: LMK03318
Other Parts Discussed in Thread: LMK00304, TRF1305B1

Tool/software:

Hi,

I am evaluating the LMK03318 using TI's EVM board. I measured the output waveforms on a Scope. I have a couple of questions:

The 1 GHz output looks more like a sinewave. Is this the best that can be expected? If so, can you recommend a device that can clean this up downstream?
When using the PLL clock from the status pin as a scope trigger, the waveforms are more jittery, indicating that the 3.3V output itself is jittery. I was thinking of using this output in my end application as a time base sync. But, looks like, I shouldn't use this. Would you concur?
Why does the EVM have capacitors in-line on the clock outputs? For AC coupling? Could they be degrading the 1GHz waveform?

EVM CharacterizationForTI.pptx

Thanks,

Prasoon

9 months ago

0 Cris Kobierowski 9 months ago

TI__Expert 6435 points

Prasoon,

1. Higher square wave frequencies will inherently become more similar to sine waves due to the driving speed. For redriving the signal downstream, I recommend using the LMK00304.

2. Since the signal you are triggering off of is a significantly lower frequency, it will cause the scope effects you are seeing. Try using a higher frequency for your base LVCMOS signal.

3. Yes, the capacitors on the clock outputs are for AC-coupling. They're likely not degrading the waveform as many applications require these capacitors.

Best,

Cris

0 Prasoon Joshi 9 months ago in reply to Cris Kobierowski

Prodigy 240 points

Thanks Cris.

Is there a comparator that I can use for converting the 1GHz output to a more squarish wave?

Thanks,

Prasoon

0 Cris Kobierowski 9 months ago in reply to Prasoon Joshi

TI__Expert 6435 points

Hello,

Today is a US Holiday. Please expect a delay in response.

Thank you for your patience.

Cris

0 Cris Kobierowski 9 months ago in reply to Cris Kobierowski

TI__Expert 6435 points

Prasoon,

One of the RF amplifiers may be of use here. Perhaps the TRF1305B1? I am not fully familiar with this portfolio so I am including someone for the RF team for further comments.

Best,

Cris

0 Cris Kobierowski 9 months ago in reply to Cris Kobierowski

TI__Expert 6435 points

Prasoon,

Could you also take a capture of the 1GHz waveform using CML instead of LV-PECL? It may help the waveform shape.

Best,

Cris

0 Prasoon Joshi 9 months ago in reply to Cris Kobierowski

Prodigy 240 points

Hi Cris,

Thanks for the suggestion. I did try that, but, that does not help:

Using CML/2.5V does seem to clean-up the jitter on the 3.3V LVCMOS output on the status pin and I am able to trigger the scope of the 10MHz output on the status pin. What I am seeing, is that the best the PLL chip can deliver, or, is it getting degraded on the EVM? The datasheet mentions ~200ps for rise/fall time?

Prasoon

0 Jennifer Bernal 9 months ago in reply to Prasoon Joshi

TI__Genius 17035 points

Hi Prasoon,

The clock generator team is currently out of office and will be back towards the end of next week. Can you please specify your timelines?

Regards,

Jennifer

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LMK03318: Jitter on PLL Clock on status and shape of 1GHz output