Page 69 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

• C2: 0.0018 IJF
• C4: 470 pF
• CS, C6, C7: 0.01 IJF
• C8, C9, C10, Cl1, C12: 1 IJF, 35 volts
• Rl: 1,200

• R2, R12: 1-k

multiturn pot
• R3, Rll: 100

• R4: 100 k

• R5: 1 k

• R6, R7, Rl0: 4,700

• R8, R9: 20 k

• R13: 1,800

• Ul: 74HC14

• U2: 74HC05
• Vreg 1: LM780S 5-volt positive regulator
• Ll: 330 IJH
With the exception of the 4S5-kHz band-pass filter consisting of R8, Ll, C4, C2, R9,

and RlO, we have two nearly identical circuits for the CW generator and AM
modulator. So we will start with the 535-kHz CW generator. A Schmitt trigger
oscillator is formed from UlA, Cl, Rl, and R2. The frequency is adjusted via R2,
which is measured with a frequency counter at pin 2 of U1A or its buffered output
at R3. As stated earlier, the frequency can be calibrated to 535 kHz via a frequency
counter by grounding one lead of the frequency counter and connecting the other

lead to R3.
The 535-kHz CW pulse signal is sent to the input of an open collector inverter U2A
pin 1, which has a pull-up or load resistor R6 at the pin 2 output. The actual biasing
voltage for R6 is provided through a filtered version of the output of the audio
oscillator circuit (Schmitt trigger oscillator) UIE, R4, and C2. RS and C8 provide a

filtered version of a pulse signal, which is a triangle waveform, to act as a
(modulating) voltage source for R6. Since the triangle waveform varies, the output
at pin 2 of U2A provides a pulse-amplitude-modulated signal at 535 kHz and 1,605
kHz with about a 1-kHz modulating frequency. Finally, a capacitor C7 is connected