HiThe "AD9545_BOM_05-040513-01-c.xls" recommend OCXO "OH300-61003CF-010.0M" from Connor Winfiled. This is a 10MHz oscillator.According to the AD9545 data sheet the minimum system clock is 16MHz, but with a note: "Support of oven controlled crystal oscillators (OCXOs) < 20 MHz is possible using the auxiliary DPLL for system clock frequency compensation".Why is the recommended OCXO 10MHz? Would it not be better(easier) to use an e.g. 20MHz OCXO like "OH300-61003CF-020.0M" from Connor Winfiled?BRAD
The OCXO that you refer from the AD9545 evaluation board is Y400, which is not populated. If you look at the schematic of the board, you can see this oscillator is associated with some jumpers that can direct the clock to M0 or REFB pins of the AD9545. It is true that it can also be directed to the system clock inputs (XOA, XOB), but it does not make sense to do this because it is much cheaper to use the regular crystal resonator to create the system clock.
So, if one uses the OCXO as a reference clock at pins M0 or REFB (that is, not as a source for the system clock), then it does not matter if it is 10MHz or 20MHz. The Channel PLL will lock into it and will create the output clock, whatever is desired. The influence of the system clock (that in this case created by the crystal resonator together with the system clock PLL) is compensated using the system clock compensation approaches outlined in the rev B data sheet, page 137.
Thank you for your reply. I am afraid I am missing something regarding the AD9545 functions. As mention in the data sheet "the stability of the system clock is crucial to the accuracy of the NCOs and TDCs within the AD9545." I do not understand why it is better to use a low end oscillator for the system clock, and then use an OCXO to compensate the system clock. If an application requires the stability of an OCXO (I am working with 1PPS synchronization), why not use the OCXO directly as the system clock?
You are right: we recommend using a low cost crystal resonator to create the system clock, then use an OCXO as a reference clock, move it through the channel PLL and use this to compensate the system clock.
Imagine you have another reference clock that is passed through the second channel PLL to create an output clock.
The OCXO, although more stable than a regular crystal resonator, has more phase noise. So the system clock is noisier being sourced from an OCXO and this has repercussions on all the AD9545 blocks.
So it better to use the scheme above. The output clock will reflect exactly the reference input, without an influence from the AD9545
Hi Petre, thanks for clearing that up. I was not aware of the phase noise penalty when using an OCXO.We are working with 1PPS synchronization and lock-in time is our main focus. For the evaluation part we are using a 20MHz OCXO since this frequency can be used directly as a system clock (with the phase noise penalty) for an easy board configuration, and later on as a reference clock as you have recommended above.BRAD