Page 63 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

a relaxation oscillator for generating CW waveforms. Basically, the tripping-point

voltage at the input changes depending on what the output state is.
When the output is high at pin 2, the input at pin 1 rises slowly owing to the
low-pass filtering effect of R2, Rl, and Cl. Note: R2 preferably is a multiturn trim
pot Eventually, the voltage at pin 1 rises sufficiently to cause the inverter to output
a low signal. Once the output is low, though, the input trigger point at the input is
also lower. Thus capacitor Cl has to discharge to that lower trigger pOint,

Vlow_trigger. When it does, the inverter goes high but also causes the input to
trigger at a higher voltage, Vhigh_trigger. As a result of the dynamic nature of how
the input trigger voltage changes, an oscillating signal occurs between the voltages
of Vhigh_trigger and Vlow_trigger.
Both Vhigh_trigger and Vlow_trigger will vary as a function of the supply voltage.

Thus a supply voltage that is not regulated will cause a shift in oscillating
frequency. Therefore, it is advised to use a regulated 5-V supply when stability of
the preset frequencies is desired.
The 74HC14 inverter gate can provide oscillating waveforms to at least 10 MHz and
at nearly but not perfectly symmetric square waves. The frequency of oscillation is
about l/[(Rl + R2)Cl] in Figure 4-5. R2 is adjusted to 535 kHz via measuring with

a frequency counter (e.g., a digital voltmeter with frequency counter).
If a frequency counter is not available, use a radio with digital readout tuned to
1,070 kHz (second harmonic of 535 kHz), connect a wire to the R3 terminal, and
place the wire near the radio. Listen for the radio's hiss level to go down when
tuned to 1,070 kHz while adjusting R2.

For higher oscillation frequencies, one can use a 74AHC14 gate instead, but the
74HC14 type is more common. However, when using a 74AHC14 gate, values of
Cl, Rl, and R2 may change.
Other inverters can be used, such as the 74C14, which is slower in speed than the
74HC14 but should work. Also, the 74HCT14 will work, but the

frequency-of-oscillation formula is not the same as for the 74HC14, so some
experimentation by the reader is required.
For really high-frequency oscillations, a 74AC14 or 74ACT14 will work up to
frequencies well beyond 30 MHz (probably up to about 70 MHz or 100 MHz), but
the reader will have to experiment to determine the resistor and capaCitor

combinations.
As a starting point, if one wants to experiment with inverter gates other than the
74HC14, try using Cl = 0.001 ~F, and replace Rl with a 470-

resistor and R2 with a 5-k