LMX2594: PLL Question

ko byeongguk

Part Number: LMX2594

Tool/software:

Hello,

I am writing with an additional question about the LMX2594.

Currently, I need to output frequencies from 13.9GHz to 14.7GHz in 1kHz increments.

However, when I start at 13.9GHz and change to 13900.001, 13900.002, 13900.003MHz, at 13900.003MHz, I get outputs at 13900MHz, 13900.003MHz, and 13900.006MHz as shown in the attached image.

At 13900.004MHz, I get outputs at 13900MHz, 13900.004MHz, and 13900.008MHz.

This happens every time I move by 1kHz.

(In integer mode, there is no problem. ex : 13.9GHz)

I am using the following settings:

PFD: 100MHz
MOD: 1000000

Could you please explain why this issue is occurring?

Thank you.

4 days ago

0 Noel Fung 4 days ago

TI__Guru* 85500 points

Hi KO,

Those are fractional spurs as you are operating it at fraction frequency.

We could reduce the spurs level with a smaller charge pump current setting and use a loop filter with loop bandwidth much less than 1kHz.

0 ko byeongguk 3 days ago

Prodigy 100 points

Hi noel,

I've set the charge pump current to the lowest possible value (3mA), but the improvement is minimal.

When calculating the loop filter bandwidth to 1kHz, the resulting values are impractical to apply.(too large capacitance values and too small resistance values)

Are there any other solutions?

Regards,

+1 Noel Fung 3 days ago in reply to ko byeongguk

TI__Guru* 85500 points

Hi Ko,

To make loop filter component practical with small loop bandwidth, we should use a small charge pump current and phase detector frequency.

Anyway, even with a loop bandwidth <1kHz, I think the fractional spurs still exist, as the offset frequency is too close to the carrier frequency. If you are targeting for -80dBc spurs, I think that is not possible.

0 ko byeongguk 3 days ago in reply to Noel Fung

Prodigy 100 points

Thank you for your response. It was very helpful.

Regards,

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LMX2594: PLL Question