Dear all,
I am using a LMX2492 EVM board to test the performance of the PLL on my application. I am using an external reference clock instead of the provided clock to lock the PLL and an external VCO. I have followed the EVM reference manual instructions for connecting an external reference and care has been taken to disconnect every power supply for the onboard VCO and the Vtune input and RFout. The RFout SMA has been directly connected to the PLL Fin pin. I have mounted an active 2nd order type B filter on the board and the output of the filter is connected to the Vtune pin of an external VCO through a wire. The parameters for the filter and VCO are as follow:
Loop Filter: Type B 2nd Order (1.9 MHz BW, 40 deg Phase Margin) [Component Topological names according to PLLatinum SIM]
OpAmp: AD8065 -> Vs+ = 18V, Vs- = GND
C1 = 27 pF
C2 = 0.18 nF
C3 = 1 nF
R2 = 1.2 kOhm
R3 = 15 Ohm
VCO:
K = 280 MHz/V
Cin = 15pF
I have programmed the PLL N value to 60. I am changing the external reference frequency (generated by a high end bench RF synthesizer with a very clean, low spurious output) between 160 and 200 MHz to make the PLL lock to different frequencies (9.6 GHz to 12 GHz) and it is working like a charm except for a few specific frequencies on the Ref pin. These frequencies are 196, 197, 198 and 200 MHz. What happens in these frequencies is that the PLL makes a wideband not uniform FM modulation and after a few seconds (the exact time is not constant between tries) it finally locks on to the frequency it should be locked on to. In the case of 197 MHz specifically (11.82 GHz) I cannot get it to lock, it just outputs an FM modulation. According to basic PLL theory, I don't understand the mechanism behind this behaviour, as it is not an oscillating, unstable response, but a temporary unstability followed by correct locking, and it also just depends on the PFD frequency and not its amplitude or any other factor that may affect its response.
Also, this FM modulation and eventual locking may or may not happen (and if it happens it takes an arbitrary amount of time for the system to eventually lock) on any of the working frequencies for Ref IN if I switch off and on the Ref IN frequency.
I hope you can help me find the cause for this strange behaviour.
Kind Regards,
Alejandro