LMX2595: Locked but RFout dirty

jinjing zuo

Prodigy 230 points

Part Number: LMX2595
Other Parts Discussed in Thread: USB2ANY

My lmx2595 board locked but has very dirty signal.

My refrence is sinewave, about 7dBm.

12.2G.tcs

over 5 years ago

0 jinjing zuo over 5 years ago

Prodigy 230 points

HI,

BTW, is It necessary to put resistances(12K on EVM) between LMX2595 and MCU(or FPGA)?

If connect FPGA and LMX2595 directly, is it possible to cause signal dirty ?

0 Noel Fung over 5 years ago in reply to jinjing zuo

TI__Guru** 108550 points

Hi jinjing,

OSCINP is floating, you should AC-couple it to ground via a 50Ω resistor.

Did it lock?

Is the reference clock clean? You can try with a 1GHz reference clock, disable OSC_2X, set PLL_R_PRE = 5 to keep the same fpd.

The 12kΩ resistors are used for 5V logic, they are not required for 3.3V or 1.8V logic.

0 jinjing zuo over 5 years ago in reply to Noel Fung

Prodigy 230 points

Hi Noel Fung,

The reference I used is Single-ended, and very clean. As below.

Soldered the R24(0Ω),R25(49.9Ω) and test. Pin 20 is high(locked), The RF output as below.

0 jinjing zuo over 5 years ago in reply to jinjing zuo

Prodigy 230 points

I tried with a 1GHz reference clock, but it can not be locked, and the frequency output is out of control.

The reference shown below.

0 Noel Fung over 5 years ago in reply to jinjing zuo

TI__Guru** 108550 points

Hi Jinjing,

So, it is locked, phase noise is good but there are spurs at 3kHz - 30kHz. Are you using DC/DC power supply? How is your loop filter layout? Are there strong-signal traces close to the loop filter?

0 jinjing zuo over 5 years ago in reply to Noel Fung

Prodigy 230 points

-I am using a LDO power supply.

-the layout as below(top->GND->power->bottom).

-It seems no strong-signal traces close to the loop filter.

0 jinjing zuo over 5 years ago in reply to jinjing zuo

Prodigy 230 points

The LDO I used is ADP7158ACPZ-3.3-R7.

0 Noel Fung over 5 years ago in reply to jinjing zuo

TI__Guru** 108550 points

Hi Jinjing,

Where is the SPI signal come from? USB2ANY? Can you remove the SCK, SDI wire after programming the device? If you use USB2ANY, unplug the USB cable to power down the USB2ANY.

0 jinjing zuo over 5 years ago in reply to Noel Fung

Prodigy 230 points

Hi Noel Fung:

I used the USB2ANY, The signal output will be better when unplug the USB cable.

I want to know the reason.

Next step, I will use a custom FPGA board to control the LMX2595. I am worried that the problem will appear again . What should I do to avoid the problem?

0 Noel Fung over 5 years ago in reply to jinjing zuo

TI__Guru** 108550 points

Hi Jinjing,

in your layout, SCK trace is running in parallel with the loop filter trace, there may be some coupling. Another possibility is, did you connect the GND pin of USB2ANY to your board?

0 jinjing zuo over 5 years ago in reply to Noel Fung

Prodigy 230 points

Hi Noel Fung,

I will change my layout to have a try.

The GND is connected.

0 jinjing zuo over 5 years ago in reply to jinjing zuo

Prodigy 230 points

Hi Noel Fung,

I changed my layout and test again. The signal is quite clean.

Thanks a lot.

Clock & timing

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LMX2595: Locked but RFout dirty