LMX2820EVM: LMX2820EVM

Denny Yuen

Part Number: LMX2820EVM
Other Parts Discussed in Thread: LMX2820

Tool/software:

Base on the LMX2820EVM user guide (page 7), the default 6GHz output should be around 35.965fsec RMS jitter.

I have been using a few difference input clock sources, such as: Crystek CPROBS5 100MHz, Mixed-Signal Devices MS1310A TCXO, etc. All of them have less 30fsec PN.

The following shows the PN 100MHz reference:

The best is around 58fsec. I use the EVM default mode. I have tried both single ended and differential inputs. There is no difference.

The following picture shows my setup:

Please educate me your setup to generate 6GHz output with PN around 35fsec.

A million thanks for your support and patience in advance.

over 1 year ago

0 Noel Fung over 1 year ago

TI__Guru** 107320 points

Hi Denny,

Suggest change C31 to 10µF. This will reduce the bump in the 10kHz offset range.

We could also increase C29 to 1µF to further reduce in-band phase noise.

0 Denny Yuen over 1 year ago in reply to Noel Fung

Prodigy 10 points

Hello Noel,

I have tried your above suggestion and a few different capacitors on both C29 and C31:

(1) The following picture shows the Original one: (C29=0.47uF)

(2) The following picture shows the recommended change in LMX2820 page 4 Table 5-1 Pin Functions: BIASVCO: (C29=4.7uF)

(3) The following picture shows your recommended change C31 from 1uF to 10uF:

The RMS changes from 60fsec to 56fsec. This is the biggest improvement, but the RMS jitter is still short of 20fsec to be 35.965 as shown on LMX2820EVM Evaluation Module page 7 Figure 4-2. Default Output. Do you have any further suggestion?

But it didn't run long (about 3 minutes), the board went dead:

Do you have any idea why? The board should be under warranty.

Please educate me how to get the replacement.

Thanks.

0 Noel Fung over 1 year ago in reply to Denny Yuen

TI__Guru** 107320 points

Hi Denny,

Please contact customer support for replacement. https://www.ti.com/info/customer-support.html

FYI, below is the phase noise of the EVM after changing those capacitors. Reference clock is 100MHz Wenzel.

0 Denny Yuen over 1 year ago in reply to Noel Fung

Prodigy 10 points

Hello Noel,

We use the the Crystek CPROBS5 100MHz which has the same and better performance as yours (100MHz Wenzel):

The following is copied from TI Application Note: Page 3 of LMX2820 RF Synthesizer Phase Noise Improvement with Alternative Topologies:

https://www.ti.com/lit/an/snaa361a/snaa361a.pdf?ts=1725938936078&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLMX2820%253FkeyMatch%253DLMX2820%2526tisearch%253Duniversal_search%2526usecase%253DGPN-ALT

So, we should have the same result, but mine is off by 20fsec RMS. Is it possible our board problem?

Please advise.

Thanks.

0 Noel Fung over 1 year ago in reply to Denny Yuen

TI__Guru** 107320 points

Hi Denny,

Maybe, since your board is not working at all now, we cannot verify.

I just noticed that the GUI default configuration has CPG = 14mA, it should be 15.4mA. This should give use a dB or more in-band phase noise improvement.

In addition, your clock source is approx. 6dBm. Wenzel has higher output power, this may benefit to the synthesizer.

Anyway your clock source is pretty good, good to know there is more good clock sources available in the market.

Clock & timing

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LMX2820EVM: LMX2820EVM