'Distortion' in PLL (CD4046) Active Filter

Thread Starter

hidahhusni

Joined Oct 6, 2023
2
Hello there, i am new to the forum and also to op amps.
Currently i am building an FM transmitter that uses 4046 as PLL, LB3500 as n-divider and TC9122P. I am using 4046 PC II to drive the VCO. I'm currently moving on from using passive RC filter to filter the PLL output and now using op amps as active filter as the 4046 datasheet suggests. I've build one and it kinda worked. The frequency is there i've checked that but i get some kind of 'distortion' from the filter which overmodulate the VCO and render it unusable thus the frequncy is not locked because of that effect. I am new to op amps and trying to understand what causes that. I've attached schematic and component layout. UVCO is the output from 4046.

I've tried to fix the problem by changing C22 to 47uF and it worked to remove the 'distortion' effect but the frequency is not locked yet. But when i attach 470R in series with 47uf in any points before the non inv input of the second CA3240 it removed the unwanted effect and the PLL is able to lock indicated by an led, but the locking process becomes slow. My RC filter lock time is comparably faster. Currently i'm monitoring the RF, lock condition etc using cheap SDR and SDR++ as i am waiting for my oscilloscope, spec analyzer etc to arrive. I also used MPXTool.

1696836108568.png 1696836276552.png
 

Thread Starter

hidahhusni

Joined Oct 6, 2023
2
The first parts of your lowpass filter pass very low frequencies (below 20Hz) but cut low, medium and high frequencies.
Changing C22 to 47uF causes the filter to cut all frequencies.
well the problem is i'm still figuring out where that 'noise' is coming from and trying to prevent it getting amplified or whatever it is. The components above have been calculated and it does work accordingly. I've attached a picture that this filter does work.

or it's just me using cheap 4046 'chinese' ics. Well to clear confusion i've also attached where i put that 47uf cap. I put that in series with 470R at R18. Anyway thanks for the response...
 

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