Indirect method of generation of FM signal is also known as Armstrong method .Here a crystal oscillator generates carrier signal , which provides very high stability compared to Direct method. this method generates a WBFM signal, i.e a phase modulator generates a NBFM signal in the first step , then in the second step NBFM will be converted to WBFM signal using a frequency multiplier.
In NBFM modulation index is small and the distortion is very low in NBFM ,here we prefer phase modulator to generate NBFM as it’s generation is easy, the frequency multiplier multiplies incoming frequency along with frequency deviation . Hence NBFM will be converted into WBFM with large frequency deviation as well.
The frequency multiplier consists of a non-linear device followed by a Band Pass Filter, the non-linear device is a memory less device.
If the input to a non-linear device is an FM wave with frequency and deviation then output consists of DC component and ‘n’ frequency modulated waves with carrier frequencies and frequency deviations . The BPF designing is in such a way that it passes the FM wave centered at the frequency with frequency deviation and to suppress all other FM components. Thus a frequency multiplier generates a WBFM wave from a NBFM wave.
Generation of WBFM by Armstrong’s method:-
This Armstrong’s method is indirect method used to generate WBFM signal.It is used to generate FM signal having both the desired frequency deviation and carrier frequency.
The block diagram consists of two stage multiplier and an intermediate stage of frequency translator .