LMX2594: input waveform

Noriyuki Takahashi

Part Number: LMX2594
Other Parts Discussed in Thread: LMX2820

Hi team

1. Is it OK to use the clipped waveform as the input signal of this device. For instance, I assuem the TCXO as below. Could you tell how the device performance e.g. jitter, spurious and so on can be degrated by the clipped waveform?

https://urldefense.com/v3/__https://www.sitime.com/support/resource-library/freq-test-reports/sit5356-3072mhz-clipped-sinewave__;!!G3vK!Vn2e6h648VjvO3FBqUMSbHvxkW28nbiIuaJzaVdYG3COk9Zskllr1Em8xVlUdnyYFmi9eOIM4Z6OTfujP0GZqAE52buY29s5EWjYyjg$

2. The datasheet recommends the higer slew rate is better as the input signal. So could you confirm the following understanding is correct?

Clipped sinewave is better than sinewave.

Rectangular waveform is better than clipped sine waveform.

Regards,

Noriyuki Takahashi

5 months ago

0 Noel Fung 5 months ago

TI__Guru* 88130 points

Hi Takahashi-san,

Correct, square wave clock is the preference clock for a synthesizer as it has the highest slew rate, especially when the frequency is low.

At 30.72MHz, I recommend use square wave TCXO.

0 Noriyuki Takahashi 5 months ago in reply to Noel Fung

TI__Genius 9850 points

Noel

How about the Clipped sinewave like link below? The clipped sinewave should be better than sinewave, but worse than square wave?

And are there any app notes explaining that a square wave clock is the preference clock for a synthesizer?

Regards,

Noriyuki Takahashi

+1 Noel Fung 5 months ago in reply to Noriyuki Takahashi

TI__Guru* 88130 points

Hi Takahashi-san,

That is correct, slew rate of square wave clock > clipped sine wave clock > sine wave clock.

LMX2820 datasheet has a plot of slew rate vs PLL noise metric.

As shown below, PLL FOM (noise floor) is very sensitivity to input clock slew rate.

Clock & timing

Clock & timing forum

LMX2594: input waveform