ILOD is an acronym for “Injection Locked Oscillating Detector”. The schematic below shows on the left side figure the ILOD Block Diagram, while the right side figure show the actual circuit implementing the ILOD concept to develop a +- dc error voltage. In this implementation ILOD circuits were used to demodulate 4 subcarriers to produce dc error signals to drive analog servos.
The supply voltages for this circuit were+4.8, +2.4, and 0 vdc. The principal part of the circuit is transistor T3 that is connected to coil, L, and cap, C, as an oscillator. The resistor Rx is selected so that the voltage across the LC is about 2.4 volts peak i.e. it swings between ground (0vdc) and +4.8volts. The frequency is set at the selected subcarrier frequency. Note the cap labeled Cinj. This cap couples energy into the oscillating LC and if a component due to the desired subcarrier is present, the oscillating voltage across the LC locks to that signal but shifted +90 degrees at the center frequency. Now the transistors T1 and T2 are to operate as switches driven by the signal taken from a secondary on L with a turns ratio stepped down 4.5 to 1. At +90 degrees, the resultant voltage across the Cmf cap will be +2.4Vdc. (This assumes the dc component of the Soft-Limited input is +2.4Vdc). For the servo demodulator Cmf was set at 6Mf. Now as the frequency of the selected subcarrier is shifted, the +90 degrees will swing both positive and negative. When it reaches +180 degrees or 0 degrees the oscillator can be pulled no further and the circuit no longer responds to produce a servo error signal. This is defined as the “pullable” range. By adjusting the value of Cinj the “pullable” range can be increased or decreased as desired.
ILOD can be thought of as a “homodyne” receiver when Vsig has locked the frequency and the phase detector output contains the heterodynes from the other subcarriers. In other applications the Cmf was replaced by a 2 or 3 pole LPF.
Notice the Q of the coil didn’t get considered at all. In actual practice the four subcarriers each are adjusted to be 2 volts peak-to-peak and the resultant servo voltage was about +-0.7 volts with a frequency shift of about 70% of the “pullable” range. Serendipity was that as the subcarrier amplitude values were collectively varied some 10%, the “pullable” range varied some 10% but the demodulated servo voltage did NOT change at all.
VOILA --- a perfect frequency detector.
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