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Controle Problems with the AD9525

Question asked by BTS on Apr 15, 2013
Latest reply on Apr 19, 2013 by BTS

Hi I'm using the AD9525 eval board to controle a 3.5GHz VCO

The point is, we want a very stable ref clock for a DDS-driver, therefor we thougt of controling a 3.5GHz VCO with the AD9525.

We use an OCXO with 100 MHz as ref clock at  the single ended entrance ref C (also tried ref A).

afterwards we want to use the chargepump to controle the VCO tuning voltage.

We also built a Loopfilter with a bandwith of 154kHz.

The chargepump should now (after passing the loopfilter) controle the VCO.

 

Our VCO is a ROS 3550+ from minicicuits and has a k faktor of 70MHz/V, which is pretty much, but we could find another cristal which could be delivered in time.

Our Problem now is, that the charge pump tries to controle the VCO with very high voltage differences, and so cant lock in at the right voltage.

(the 3,5GHz voltage should be araound 3V) It oszillates with about 1MHz, and iwth an amplitude of around 2V. I atached some pictures of the AD9525 output and the chargepump voltage.

IMAG0018.jpg

here the center of the picture is at 3.5 GHz one unit is 50MHz, so the frequencies in the signal are from 3.35MHz to 3.6MHz (this is the maximum freq of the VCO)

the chargepump voltage looks like the following:

IMAG0024.jpg

I hope you can identify the colors... the white slope (bigger amplitude) is the chargepump with the standard usage of chargepumpcurrent 8x

the yellow smaller one with CPC 1x (the littlest possible). with the littlier CPC also the spectrum gets thinner, but still has a rage of 100-150MHz.

We also tried other frequencies, and we observed, that the center of the freqrange shifts, but still is very broad.

wo the AD9525 tries to controle the VCO but cant do it properly.

We also tried to run the VCO with a stabilized constant voltage. here the spectrum is very thin, but drifts to much for  our issues, so we would need the AD9525 to contole it.

 

Is there a possibility to controle the chargepump votlage tuning sinsibility or some other thing that would help us?

 

Thanks and with best regards

Daniel

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