RE: Airtronics RD6000
Transmitters operate on a basic RF carrier frequency. That is the number you see on your crystal. When modulated to impart information on the carrier it is "shifted" off that frequency in either a positive or negative direction at the TX. The RX must know which way to compensate for this or it will not "see" the data. Some RX's can be set to go either way and some will do it automatically. Some TX can be programmed which way to go also but not many.