LMX2594: Phase Noise Performance and Drive Configurations for LMX2594

charan Raj

Part Number: LMX2594

Tool/software:

Hi,

I have a few questions regarding phase noise performance, temperature effects, and input clock drive configurations for the LMX2594.

1) Could you please provide Phase noise variation data, particularly at high temperatures? I'm concerned about how phase noise behaves across different temperature ranges.

2) What is the optimal slew rate value for achieving the best Phase noise & jitter performance for both single-ended and differential clock drive for OSCin inputs?

3) Will using a single-ended sine wave drive degrade Phase noise performance at high temperatures due to its low slew rate?

Thanks & Regards,

Charanraj

over 1 year ago

+1 vicente flores prado over 1 year ago

TI__Mastermind 29272 points

Hello Charanraj,

1. The VCO and PLL performance will definitely vary over over temperature - the reason being that as temperature decreases the loop bandwidth is also reduced thus affect overall phase noise.
2. Slew rate generally only becomes a cause of concern at lower frequencies and particularly for Sine waves. Any square wave will usually great skew rate (both SE & Diff). I couldn’t find an exact answer but I found a recommendation for a different device when utilizing a very low frequency (10MHz) sine wave which as mentioned - is the only time for concern. I can verify with a colleague if this also applies to LMX2594.

3. Yes - partially answered above but the impact is not just at higher temperature but in general overall due to slew rate. Phase noise also varies over temperature because at cold, the loop bandwidth is reduced impacting the overall phase noise.

Regards,

Vicente

0 charan Raj over 1 year ago in reply to vicente flores prado

Prodigy 40 points

Hi,

Thanks for the reply.

1) Could you please share the phase noise variation data in dBc/Hz, particularly at higher temperatures? If you have data for lower temperatures too, that would be great. Also, what steps can be taken to improve Phase noise performance at higher/lower temperatures?

2) I also need the recommended slew rate values in V/ns for both single-ended and differential clock drives to optimize Phase noise and jitter.

3) could you advise on how to improve Phase noise performance when using a single-ended sine wave drive, especially considering its lower slew rate

0 vicente flores prado over 1 year ago in reply to charan Raj

TI__Mastermind 29272 points

Hi Charan,
This is frequency dependent we do not have this.
For example in the DS:

There is not much you can do expect try recalibrating given the VCO parameters are temperature dependent.

2. For low frequency Sine Wave, 600V/us is acceptable.
To start off, what will be your reference clk format and what's the frequency?

3. There is no real workaround to this - you can attempt to use a Sine to square wave converter to improve the slew rate or increase the frequency if you're forced to use a Sine wave from a TCXO or an OCXO for example. A clipped Sine wave will have better slew rate than a regular unclipped sine wave.

Regards,

Vicente

0 charan Raj over 1 year ago in reply to vicente flores prado

Prodigy 40 points

Hi,

we are using 120MHz ,+10dBm sine wave from an OCXO as a reference clock to drive the LMX2594. However, at higher temperature the Phase noise degradation is significant. We did try running the VCO calibration at high temperature. But that did not improve the phase noise performance. Reference clock drive in the current configuration is single ended sine wave. The computed slew rate is >700V/us.

Please let us if the sine wave reference is converted to an LVDS drive, would the phase noise performance be better.

We would also want to know the degradation in the internal VCO phase noise would witness at higher temperatures.