These voltage-controlled oscillators (VCOs) are improved versions of the
original VCO family: SN54LS124, SN54LS324 thru SN54LS327, SN74LS124, and SN74LS324
thru SN74LS327. These new devices feature improved voltage-to-frequency linearity,
range, and compensation. With the exception of the 'LS624 and 'LS628, all
of these devices feature two independent VCOs in a single monolithic chip.
The 'LS624, 'LS625, 'LS626, and 'LS628 have complementary Z outputs. The output
frequency for each VCO is established by a single external component (either
a capacitor or crystal) in combination with voltage-sensitive inputs used
for frequency control and frequency range. Each device has a voltage-sensitive
input for frequency control; however, the 'LS624, 'LS628, and 'LS629 devices
also have one for frequency range. (See Figures 1 thru 6).
The 'LS628 offers more precise temperature compensation than its 'LS624
counterpart. The 'LS624 features a 600 ohm internal timing resistor. The 'LS628
requires a timing resistor to be connected externally across Rext
pins. Temperature compensation will be improved due to the temperature coefficient
of the external resistor.
Figure 3 and Figure 6 contain the necessary information to choose the proper
capacitor value to obtain the desired operating frequency.
A single 5-volt supply can be used: however, one set of supply voltage
and ground pins (VCC and GND) is provided for the enable, synchronization-gating,
and output sections, and a separate set (OSC VCC and OSC GND) is
provided for the oscillator and associated frequency-control circuits so that
effective isolation can be accomplished in the system. For operation of frequencies
greater than 10 MHz, it is recommended that two independent supplies be used.
Disabling either VCO of the 'LS625 and 'LS625 and 'LS627 can be achieved by
removing the appropriate OSC VCC. An enable input is 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 in the
dual devices ('LS625, 'LS626, 'LS627 and 'LS629) when both VCOs are operated
simultaneously. To minimize crosstalk, either of the following are recommended:
(A) If frequencies are widely separated, use a 10-μh inductor between VCC pins. (B) If frequencies are closely spaced, use two separate VCC supplies or place two series diodes between the VCC pins.
The pulse-synchronization-gating section ensures that the first output
pulse is neither clipped nor extended. The duty cycle of the square-wave output
is fixed at approximately 50 percent.
The SN54LS624 thru SN54LS629 are characterized for operation over the full
military temperature range of -55°C to 125°C. The SN74LS624 thru SN74LS629
are characterized for operation from 0°C to 70°C.
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