VCO Tuning at 27MHz

Before we went to 27MHz, we started at 40MHz. We didn’t have this issue at 40MHz for a long time, then one day we did. I suppose its possible that board capacitance played a part, but I dismissed it when the 243 worked without a problem. It just seems as though the inductors all want to lock at different values that could be as much as 1MHz difference. I tried to tighten the tolorances as much as possible but it didn’t help.

For the record the TCXO we are using is 16.368MHz crystal. So it could be modeled closely to the 16MHz design used in the 5043 examples.

However your probably right, as when we use the internal filter we still get the loss of sensitivity. Only issue is if we decrease the inductance by 20nH it locks to high. I don’t like the idea of adding extra capacitors as its can add more tolerance but I guess I don’t really have an option.

I don’t suppose you have any better method than just soldering capacitors and guessing and checking?
What do you suggest best method for determining the lock? Just using the external filter with the default PLLLOOP (0x80) value and adjusting the carrier frequency till it has the perfect lock?

As you said, I really don’t have a better way to solve this problem. Like you I also encountered the same situation, before this I used AX5043, AX5243, and AXM0F243 chips respectively, AX5243 and AXM0F243 used without the problem of VCO not locking. When using AX5043 also appeared not easy to lock, initially I suspected PCB Layout problems, and tried to use different brands of inductors, but the consistency is still not good.
I’m sure the locking is different from yours, maybe your method is more efficient. My method is to measure the Vctrl (VCO Control voltage), if it is out of lock, the voltage is 1.8V, you can determine the external inductor value is large; when the voltage is 0V, you can determine the inductor value is small. vctrl voltage control internal varactor diodes to adjust the oscillation frequency, in the VCO discrete device circuit LC match is not good, not easy to oscillate is also very easy to appear, so I added capacitors in parallel.
If you find a good solution, please let me know, thank you!


Awesome thanks for the re-assurance, I have spent many hours looking for possible solutions.

I’m wandering if you might be able to help me with another issue. Just running some of the examples available at lower frequency’s sometimes causes the device to get stuck in a loop. I think its getting stuck in the interrupt handler. It has happened when re-programing or more often under Brown Out conditions. I think its getting stuck in the Hard Fault handler, more specifically GPIO_Exception_handler(). Theory is that it is that it is referencing the wrong clock on start up (TCXO). I know you were asking about it in a separate post. Is this something you have seen before?