I am working on a system that requires a 32.768MHz master clock delivered to 5 separate locations (single-ended CMOS). The system also needs to take a 1PPS signal from GPS receiver as an input to synchronize.
I do not see any readily available TCXO or OCXO oscillators with that precise frequency.
Therefore, my question is this: does AD9544 support five 32.768MHz outputs from a 20MHz OCXO oscillator (most commonly available freq. OCXO >=20MHz on digikey right now) system input and a 1PPS reference input?
The 1PPS signal may or may not always be present, hence the OCXO for quality system input in case 1PPS signal not present.
Yes, the AD9544 will accomplish this task. You can use the 20MHz OCXO as the System Clock input and the 1PPS as a reference input. When the 1PPS goes away, the device will enter holdover and the outputs…
I apologize for the delay. Responses to your questions are given below:
1) The stable oscillator (OCXO or TCXO) stabilizes the System Clock through System Clock Compensation (see System Clock Compensation…
Feedback on schematic:
1) I have some concerns about the output terminations. I can see that the 10K resistors should setup the common mode for the signal at 3.3V/2. This seems high. I'm assuming that…
Yes, the AD9544 will accomplish this task. You can use the 20MHz OCXO as the System Clock input and the 1PPS as a reference input. When the 1PPS goes away, the device will enter holdover and the outputs will continue with the stability of the System Clock OCXO. This may be adequate for your application. However there is a significant degradation (>6dB) in phase noise by using such a low frequency system clock. If phase noise is a concern (see datasheet graphs), then a better approach is to use a 52MHz Xtal on the System Clock with the internal System Clock frequency doubler enabled. The 20MHz OCXO is then a reference input like the 1PPS. The AD9444 can use the OCXO to stabilize the 52MHz Xtal to allow the low loop bandwidth (<50mHz) required for 1PPS operation. This will give excellent stability and phase noise performance. (We no longer recommend using the System Clock frequency doubler with an OCXO.)
The AD9444 datasheet will be updated soon. For more detailed operation of the AD9444, use the updated AD9545 datasheet with the understanding that he AD9444 has identical performance except that it does not support Auxiliary NCOs, Auxiliary TDCs, and Embedded Clock Modulation/Demodulation found in the AD9545.