Specifications

4–7
Principles of Operation
U6 is programmed with the four or five rotary switches. The binary output of the
0.1 MHz switch programs the “A” counter directly. BCD data from the 100’s, tens,
and units rotary switches is converted to binary data by U3 to set the “N” counter.
An optional fifth digit rotary switch for 10kHz spacing is available.
U5C is a differential amplifier and filter for the error signal. Audio that is out of
phase with that appearing on the error voltage is introduced by U5A, allowing for
greater loop bandwidth with less degradation of the low-frequency audio response.
U5D is an integrator. U5B is a VCO input voltage clamp.
DV71 and DV72 are hyper-abrupt varactor tuning diodes with a square-law
capacitance vs DC voltage curve, giving a straight-line frequency vs voltage curve
in a LC oscillator where the varactors are the dominant source of capacitance.
Lock and unlock status signals are available at the outputs of U4E and U4F,
respectively. Modulation is introduced to the VCO through R17 and R71 to R75.
About 4.1 millivolts across R75 produces 75 kHz deviation.
An FSK signal (used for automatic identification of FM repeaters) shifts the
frequencies of the 10.24 MHz crystal reference and the VCO. With keying, diodes
D9 and D10, are reverse biased, increasing the crystal reference frequency. At the
same time, current through R72 increases the VCO frequency. See section 5.3.4.