# Armstrong's Modulation Method vs Wideband FM

1. Apr 22, 2015

### clubgitmo

Last edited: Apr 22, 2015
2. Apr 22, 2015

### tech99

The Armstrong method generates FM using a fixed frequency stable oscillator operating at a low frequency. This is raised to the required broadcast frequency using a chain of frequency multipliers. The multiplication process also increases the deviation, so that the initially narrow FM signal becomes wideband FM, as required for broadcasting.
It is interesting that, in essence, the initial modulation is achieved by creating AM, but inserting the carrier with 90 degrees phase shift. The sideband structures for NBFM and AM are almost identical.

3. Apr 22, 2015

### JakeBrodskyPE

There are many ways to generate an FM signal. The nice thing about Armstrong's method is that modulation was very linear and broadband, and the signal center frequency was quite stable. I actually saw a transmitter at a college radio station once that used this method.

The problem with the Armstrong method is that it is complex (lots of stages), and it is not frequency agile. Deviation could be narrowband or wideband, depending on how many multiplier stages it had. There are quite a few other methods for generating a broadcast FM signal these days.