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Help me with modulating in FM

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Full Member level 5
Aug 28, 2001
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Help modulating in FM


I'm designing a system in FM, in 1.000 Mhz to 2.000 Mhz (yes Mhz 1000Khz), to modulate audio.

In first time, i used the VCO of a CD4046, so I connect a potentiomenter to it in a way to set the frecuency like this:

|...CD...|....... \
|..4046..|---->/--------| |------ Audio in
|...........|...... \

The audio is added in to the VCO input througth a capacitor.

When I made this the audio in the receptor is very good quality and band with.

So, then I change the potentiometer to set the frecuency to the complete PLL system (using the same CD4046), I put a reference frecuency in the phase comparator input, and a digital divider in the VCO out, and a pasive low pass filter in the feedback networtk. The poles in the network are lower than 1Hz or less. I tried many poles frecuencies, and number of poles (1 to 3).

Using this circuit, the audio in the receptor is very noisy (like no signal noisy in wireless analog telephonic system, or AM noisy), I don't know what's happen. Why with the first circuit audio is OK and second no?

Any suggestion?

Thanks. 8)

To my experience, if the deviation is low ( i.e. 5-10 KHz ) should be very low the level of the modulating signal ( in the range of 10 mV ); in this case the noise is probably due to the internal noise of the op-amp you are using for the loop filter; for example if you use a 324 the noise is terrific.
Try to use a very low noise op-amp and also use a metal film resitors in the loop filter.
Another suggestion is to add a low pass filter in between the loop filter and the input of the 4046, with a cut-off frequency about 10-20 times higher the frequency of the loop filter ( to avoid to corrupt the phase ).
If you can access a modulation analyzer try to see the demodulated signal ( the spectrum ) to understand if the noise is white probably the problem is this.

This is from the locking of PLL. When u put ur VCO free runing.
It can follow what the signal inpput to change its frequency.
Once u turn-on the PLL. This locking loop will try to lock ur frequency.
That is to make distortion on the linearity of modulation voltage to frequency transfer. It's also often present in modern broadcasting
station's FM exciter. The old solution is to multiply the specific fundamental frequency to make the frequency deviation muplier of orginal. The recent solution is to use the direct digial frequency synthsizer.

U can improve ur quality by lower ur VCO frquency and mulitply (or use multi-rating CMOS/TTL) them. But it's hard to modulate it in such wide frequency deviation. Try to make it free-runing and sample the error-signal back in the long time or in quiet time(no modulation input) .

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