AD9528

Question

I have a question. I have configured AD9528 as follows. I found that when Ref A does not have a clock input, 9528 will still output the set clock normally. Is the internal VCXO clock locked at this time? Will 9528 automatically use Ref A's clock input as a reference when I input the preset clock signal into Ref A after the clock outputs normally for a period of time?

pminc48 · Accepted Answer

Hi, 
 could you please send me the stp file you created? May I suppose you ran this configuration onto an AD9528 eval board? I do not see the LD LED in the middle of PLL2 turned on, so I suppose the PLL2 is not yet locked. This should happen right away. 
 The AD9528 may output clocks even if the PLL2 is not locked because you may have tried to calibrate the PLL2 VCO and this triggers automatically a sync operation. 
 Regarding to your REFA question. See the section PLL1 reference switchover in the rev F data sheet page 26. 
 Petre)

pminc48 · Answer

HI, 
 your AD9528 generates an output because I believe the PLL2 has locked. You should verify PLL2 has locked by reading bit 1 of register 0x508. 
 You say that after AD9528 outputs a clock, then you can input a reference clock at REFA. Please look at the initialization process presented at page 41 in rev F data sheet. It states the reference clock must be applied after power up, not after PLL2 locked: 
 
 If you do not apply any reference after power up and apply it after PLL2 has locked, the AD9528 should automatically select REFA. See page 26 in rev F data sheet.

REFB is not enabled in your configuration anyway. 
 Because REFB is not enabled, I recommend manually select REFA as the active reference by setting bits 2:0 in register 0x010A to 010. 
 
 " How to determine whether the reference clock of Refa is in effect" 
 Monitor bit 2 in register 0x0509. Note that if you select REFA manually, this does not function. 
 
 "or the VCXO clock that has been locked earlier ?" 
 The VCXO status may be found in bit 5 in register 0x0508. 
 Generally, see registers 0x508 and 0x509 for various status bits or use STATUS0 and STATUS1 pins to output various status bits. 
 I looked over the stp file. I recommend using Ra=2, N1=2 instead of Ra=4, N1=4 because this will reduce a little the noise of the PLL1. The configuration is OK in any case. 
 Petre

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