[SOLVED] problem: BH1417 transmitter frequency unstable

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circema

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Hi,

I've build two identical circuits based on BH1417 as stated in ROHM data sheets. One works perfectly well, with a solid rock transmitter frequency. My other one is very unstable. It looks like the PLL does not work. The frequency shifts when I approach my hand from the circuit or when I touch the inductor. My levels on pins 15 to 18 are right, but changing them does not have effect on a frequency change.

Any idea on what could cause that kind of problem?

Thanks
 

What test equipment do you have?

Make sure the reference clock at pin 13 is correct and stable at 7.6 MHz.
Monitor the charge pump output with a scope on pin 7 where you should only see very narrow pulses when the PLL is locked. If you see wildly varying pulses here that indicates an unlocked state.

You can also have a questionable chip.
 

Thank you for the quick answer!

I have a complete test bench, including a numeric 100 Mhz scope and a communication analyser (General Dynamics R-8000). The oscillator on pin 13 is fine. I will take a look this morning at pin 7, and post the result.
 

Ok, I have spent some time this morning, and you were right, the signal on pin 7 was wrong, so I knew for sure something was wrong with the PLL... so I looked carefully and noticed two wiring errors in the circuit near the PLL section (it was not a PBC, it was a veroboard soldered with the components. Now everything works perfectly, with the frequency exactly as expected, stable and with the frequency correctly changing with the change of the dip switches on pin 15 to 18.

Thank you for the hint that brought me on the right track
 

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