Total posts : 45366
John was referring to a problem some transmitters have called carrier shift. When modulation is applied the carrier frequency may shift due to poor design allowing the oscillator to change frequency for one reason or another.
One of my Talking House units had a noticable “warble” when the received signal was weak. A check on the station monitor found the frequency was about 800 Hertz low and when modulated the carrier would shift about or “FM”. Adding a “netting” capacitor brought the frequency right on and the “warble” or “FM’ing” disappeared.