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HMC440 - Problem at low frequencies

Category: Hardware
Product Number: HMC440


I use the evaluation board of the HMC440 with a division of 10. I have modified the DC blocking capacitors to match our working frequencies.

If i use a reference frequency higher than 25MHz (CW 6dBm typ.) and so, a VCO frequency higher than 250MHz (CW 6dBm typ.), there is no problem, i.e, i see a sawtooth waveform on ND if Fref < Fvco / 10 or on NU if Fref > Fvco / 10 (ND and NU are connected directly on an oscilloscope (high impedance mode))

But it doesn't work for lower frequencies. In this case, depending on the frequencies and the level of the signals, i see the ND always at 5V or sawtooth waveform on ND and NU at the same time.

Using a square wave for the reference, and an amplitude quite high of 1.6Vp-p, it works with a phase detector frequency of 20MHz but not under.

But we need to use it at 10MHz.

What can explain this behaviour?

How can i do about it?

Best regards,