LMX2487E-EVM: Problem of not being able to control the frequency

Ryota Watanabe

Part Number: LMX2487E-EVM

We are currently running an evaluation board (LMX2487E-EVM) to evaluate the LMX2487E, but it is not working properly.

We are applying a voltage of 5 V to the evaluation board, inputting a 100 MHz sine wave at Vp-p1.0 V, and observing the output waveform with an oscilloscope. (We use an oscilloscope with a frequency bandwidth of 4 GHz.)We are connecting our evaluation board to a PC via USB2ANY and controlling registers from TICS Pro v1.7.5.15. (See pictures below)

I thought this setup would work, but the following problems occurred.

1. The signal is not output as controlled by the register. (e.g., changing the value of N in the frequency divider does not change the frequency of the output signal)

2. The signal is always oscillating at 4 GHz even without input reference signal.

How can we solve this problem?

over 1 year ago

0 Noel Fung over 1 year ago

TI__Guru* 99340 points

Hi Watanabe-san,

As far as I know, we may have different EVM versions.

Could you confirm that you receive below version EVM and you have the little adaptor board?

0 Ryota Watanabe over 1 year ago in reply to Noel Fung

Prodigy 30 points

Dear Noel

Thank you for your quick reply.

I use this EVM board and have this little adapter board. However, the adapter board is not connected when operating EVM.

0 Noel Fung over 1 year ago in reply to Ryota Watanabe

TI__Guru* 99340 points

Hi Watanabe-san,

This version of EVM does not need the adaptor board.

The EVM uses active loop filter, you need to change the polarity of the charge pump to negative.

0 Ryota Watanabe over 1 year ago in reply to Noel Fung

Prodigy 30 points

Dear Noel

Thanks for your advice !

We changed the polarity of the charge pump to negative and became able to control the frequency !

One question, why does the polarity need to be negative for active loop filter?
I would like to know more about active loop filter.

0 Noel Fung over 1 year ago in reply to Ryota Watanabe

TI__Guru* 99340 points

Hi Watanabe-san,

The polarity of VCO Vtune is usually positive. That is, high Vtune voltage, higher output frequency.

The active loop filter is basically an inverting amplifier, so the "gain" is negative. That is, higher input voltage, lower output voltage.

So the polarity of the charge pump needs to be set to negative. The amplifier will invert the characteristic and as a result, the VCO gets positive Vtuen.

0 Ryota Watanabe over 1 year ago in reply to Noel Fung

Prodigy 30 points

Dear Noel

I apologize for the late reply.

Thank you so much. Your explanation really helped. Now I understand active loop filter.

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LMX2487E-EVM: Problem of not being able to control the frequency