These Voltage-Controlled Oscillators (VCOs) are improved versions of the original VCO series SN54LS124, SN54LS324 through SN54LS327, SN74LS124, and SN74LS324 through SN74LS327. These new devices feature improved voltage-to-frequency linearity, range, and compensation characteristics. Except for the 'LS624 and 'LS628, all these devices integrate two independent VCOs in a single monolithic chip. The 'LS624, 'LS625, 'LS626, and 'LS628 have complementary Z outputs. The output frequency of each VCO is determined by an external component (capacitor or crystal) in combination with a voltage-sensitive input for frequency control and frequency range. Each device has a voltage-sensitive input for frequency control; however, the 'LS624, 'LS628, and 'LS629 also have an input for frequency range. The 'LS628 provides more precise temperature compensation compared to its 'LS624 model. The 'LS624 has a built-in 600-ohm timing resistor. The 'LS628 requires a timing resistor to be connected between the external ReXt pins. Temperature compensation will be improved due to the temperature coefficient of the external resistor. Figures 3 and 6 contain the information needed to select the appropriate capacitor value for the desired operating frequency. A single 5-volt power supply can be used; however, a set of power supply voltage and ground pins (Vcc and GND) is provided for the enable, sync gating, and output sections, and another set (Osc Vcc and Osc GND) is provided for the oscillator and its associated frequency control circuitry to achieve effective isolation in the system. For operating frequencies greater than 10 MHz, two independent power supplies are recommended. Either VCO in the 'LS625 and 'LS627 can be disabled by removing the appropriate Osc Vcc. Enable inputs are provided on the 'LS624, 'LS626, 'LS628, and 'LS629. When the enable input is low, the output is enabled; when the enable input is high, the internal oscillator is disabled, Y is high, and Z is low. Caution! Crosstalk may occur when both VCOs in the dual devices ('LS625, 'LS626, 'LS627, and 'LS629) are operating simultaneously. To minimize crosstalk, it is recommended to take either of the following measures: (A) If the frequencies differ significantly, use a 10-μH inductor between the Vcc pins. (B) If the frequencies are close, use two independent Vcc power supplies or place two series diodes between the Vcc pins. The pulse sync gating section ensures that the first output pulse is neither truncated nor extended. The duty cycle of the square-wave output is fixed at approximately 50%.
TI SN74LS624N