penrico
Full Member level 5

Help modulating in FM
Hey!
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:
+-------+......VCC
|...CD...|....... \
|..4046..|---->/--------| |------ Audio in
|...........|...... \
+--------+......Ground[/code]
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)
Hey!
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:
+-------+......VCC
|...CD...|....... \
|..4046..|---->/--------| |------ Audio in
|...........|...... \
+--------+......Ground[/code]
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)