I’m past the age where most engineers would refer to me as old school. I’m nearly Jurassic. I still have HP spectrum analyzers on the bench. But over the years I’ve learned a few things about debugging and bringing up a new hardware design.
Recently a young engineer working for one of my clients was having trouble getting her microwave PLL to lock. She asked me what could be wrong and showed me her loop filter calculation with all the pole-zero calculations carefully plotted. Her concern was that her stability analysis was somehow wrong and that was what was causing the no-lock problem.
Microwave oscillators and PLLs can be a bit daunting and the textbook theory can be complex. My experience with debugging is that it is usually something basic and simple that is wrong. In most cases you can usually find the bug with nothing more than VOM. Furthermore, almost all phase lock loops will lock even if the loop filter is way wrong.
I asked her if she’d checked all the IC pin voltages in the associated circuit and she said she had not but assumed that wasn’t the problem since the power regulators were operational. She came back with a still grim look and said all the voltages were fine and wondered if she should replace the chip. I asked her to check the PLL reference clock to see if it was present. She came back this time with a big smile and told me the clock was present but it was 100MHz and her PLL reference divider was programmed for a 10MHz reference clock. When she changed her code to set the reference divider correctly, the microwave oscillator snapped to the correct frequency and LED lock light came on.
The point is: Stay calm, check the simple stuff first, this too will pass.
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