differences between direct and indirect method of FM generation

what are the main differences between direct and indirect method of FM generation?

Dear firas90
Hi and welcome to EDA board!
I think , that means this: at direct generation , we will use a miller fm modulator! ( with a simple bjt oscillator and BE capacitance (according to the miller effect of that , the capacitance of BE junction will change with variation on message signal (Audio).)) and indirect fm modulators: i think that means , in some step , we can provide Fm signal , such as reactance modulator. that , in that , the audio signal , will change the reactance that is in paralle with the tank circuit of oscillator.
( i think )
Best Wishes
Goldsmith

thanku goldsmith
but can u explain what the difference ?

Again hi
In fact ( i think ) no thing special . both of them are fm signals . but indirect has good quality but direct is cheaper , but it's quality is not about indirect.
Best Wishes
Goldsmith

thank you goldsmith

I have worked in Telecomms for 40+ years and am struggling with the term "indirect". The problem with FM, is not the modulation but to keep the mean carrier frequency correct. The smart thing is to generate your FM at IF (~39 MHz), then to mix it up to the output frequency. This way you can supply a transmitter to a different output channel with just changing the mixer crystal and leaving the modulator alone. So the thing is to generate your FM at IF and use a PLL to keep the modulated oscillator at the right centre frequency.
There is the "Armstrong" method of generating FM, this has a stable centre frequency but is technically very elaborate to scoot around problems that were about in the 1930's. Basically it is generate AM at a low mod index with no carrier, re-insert the carrier at a phase of +90 degrees, multiply its frequency by X 32 limiting at every stage. LO and behold!! you have broadcast quality FM!
Frank

Usually, "indirect method" for generating FM refers to Armstrong method described by Chukey.
The AM with no carrier or DSB-SC (double side-band with suppressed carrier) is generated with a balanced mixer. When the 90° carrier is added, we have a PM (phase modulation) signal with low phase deviation. If the modulating signal is integrated before the DSB modulation, then we have a low-index FM modulation.
After that, frequency multiplication increases (multiplies by the value of the multiplier) the modulation index, and frequency translation allow to place ths final signal at the desired carrier.
It is called "indirect" because it does not generate "directly" a frequency modulation, but instead synthesizes it.
Sometimes, in direct method using varactors, reactance modulators, etc, linearity (instantaneous frequency vs. modulating signal) is an issue.
One advantage of Armstrong method is linearity and accuracy.
Regards

Z