FM modulator using PLL working ???

[akhilpaulv]

Hi all,

In the attached FM modulator, What is the purpose of set deviation and deviation balance blocks? . Why they give the message signal to both TCXO and VCO? . Where will i get more details about this circuit?

-Akhil

 

[betwixt]

Where did you get that diagram?

To my eyes, it doesn't make sense and there is no PLL. It appears the modulating signal is fed to the reference oscillator and the VCO. It also seems the VCO has no frequency control and I have no idea what IC6 does as it has no output whatsoever.

I suspect the circuit around the two controls is wrong and the modulation should only go to the VCO with one potentiometer setting the modulation level and the other setting the static DC on the VCO to set it's center frequency, either from the supply or the output of IC6.

Brian.

 

[akhilpaulv]

Hi Brain,

IC5 may be a buffer. I got this ckt from internet. there is PLL (IC6). i saw some other circuits like FM modulator using PLL. But in the above circuit they give modulating signal to both TCXO and VCO. I saw some TCXO, which has changeing frequency option w.r.t control voltage.

AK

 

[FANT]

The fact is that under the loop frequency you must modulate the crystal of the reference frequency, otherwise the modulation will be acncelled by the loop correction, while above the loop frequency you can modulate directly the VCO.
The balance pot is used to avoid chenge in deviation passing from low to high modulating frequency. Take care that if the circuit is not very well designed, you will have very sharp changes in phase of the modulating frequency at the crossover frequency.
At the end, do not use it, unless you have the proper instrumentation to tune it.
Better use a DDS and a fractional PLL.

Mandi

 

[betwixt]

Modulation cancellation can to a large degree be stopped by suitable loop filtering. Modulating the reference oscillator, which is normally quartz based only allows small deviation and makes the PLL multiplier affect the modulation index. In any case, I can't see a PLL in that diagram at all.

Brian.

 

[biff44]

there is an input to U1 that makes no sense to me. A TCXO is rock solid in frequency...you are not adjusting it at all as far as FM deviation goes. At most you might be able to pull it +/- 1000 ppm in frequency

 

[akhilpaulv]

The fact is that under the loop frequency you must modulate the crystal of the reference frequency, otherwise the modulation will be acncelled by the loop correction, while above the loop frequency you can modulate directly the VCO.
The balance pot is used to avoid chenge in deviation passing from low to high modulating frequency. Take care that if the circuit is not very well designed, you will have very sharp changes in phase of the modulating frequency at the crossover frequency.
At the end, do not use it, unless you have the proper instrumentation to tune it.
Better use a DDS and a fractional PLL.

Mandi

Hi Fant,

Why we need Two point modulation. If we only apply the message signal to Ref then the output frequency at the VCO will create FM modulated signal ,nah?

-Akhil