NASR-M board bring-up, TCXO adventures
People who have never done a board bring-up would not understand the adrenaline rush of finding yet another bug. It’s a wild ride, just like an extreme sport. Despite your butt being glued to the chair, you get a kick just before pressing the power button on a freshly assembled circuit or when the device is finally doing what it’s supposed to do, responding perfectly to I2C control. Fasten your antistatic wrist bands, ladies and gentlemen. Our next adventure is TCXO.
What have you done wrong, old man? You would ask, and I’ll tell you that my TCXO of choice, the ECS-TXO-32CSMV-270-AN-TR, outputs a clipped sine wave of 0.8V amplitude, which is not enough to drive the CMOS input of the LMK03328RHSR PLL. I have overlooked that the amplitude shall be above 1.4V to drive the PLL IC. The future release of the board will include a TCXO with a 3V output. What should I do now? I have two choices.
The obvious choice would be to switch the reference input to the secondary and feed the clock from the external signal generator via the J13 connector. This may work, but I don’t have a nice, stable signal generator with an adjustable output level. Even if I had one, my work desk is already overcrowded with tools and cables, so I want a more elegant solution.
A less obvious choice is to find a replacement TCXO in a similar package (drumroll here) and perform a rework surgery. Our TCXO candidates are FT3MHUPM25.0-T1 and ABRACON ASTX-H11-25.000MHZ-T. But why rework? I’ll tell you why. The original package has no enable input, so both pins 1 and 2 are connected to the ground. However, most TCXOs with CMOS output reserve the first pin for the output control logic. It is still possible to isolate the first pin and connect it to the positive rail with a thin wire, but I found a better solution. Some TCXOs enable output with the first pin floating, so all I have to do is isolate the first pin by cutting the traces. Also, I shall cut the trace connecting pin 2 of JP18 to the 2.5V supply, and connect it to +3V3CLKVCC. The TCXO is ordered, and while it is in transit, I need to whip up a driver to configure the PLL chip.