From the pilot-navigator's viewpoint, there have been insignificant improvements in the automatic direction finder (ADF) since World War II. While new navigation aids have been developed with digital frequency selection, tuning of the ADF has remained a “coffee grinder” operation.
Recent developments in voltage-variable capacitors (var-actors) for circuit tuning and in integrated digital logic circuits for precise multichannel frequency generation makes the digitally tuned (ADF) practical.
Discussed are various methods which have been or may be used to obtain crystal-electronic tuning: frequency control and circuit tuning from a variable tuning voltage; crystal matrix frequency synthesis and circuit tuning from a variable tuning voltage; and crystal frequency synthesis using a stabilized master oscillator (SMO) and circuit tuning using the voltage derived in the SMO. The last two methods can provide the electronic tuning and frequency stability necessary for digital frequency selection in an ADF.
Digital selection allows the pilot to select the desired frequency prior to entering the area covered by the ground transmitter. As an added bonus, the pilot receives the capability of instantaneous transfer of control between two heads in which frequencies have been preset.