The decision to use either AM, narrowband FM (less than 15kHz), or wideband FM (greater than 30 kHz) depends on the application. For the transmission of music, FM is better because it has greater noise immunity. For speech or other noncritical applications, AM may be satisfactory. Our transmitter permits either mode by switch selection.
Audio is fed from S1a to either the FM or AM circuitry. Starting with the FM section, amplifier Q1 accepts an audio signal in the 10 Hz to 20 kHz range of about 0.5V peak-to-peak. The audio gain is adjusted via R5 to provide up to 60 kHz deviation of voltage-controlled oscillator IC1 which is set to nominally 280 kHz. IC1 and Q1 are supplied with a regulated 12V from IC2. A square-wave signal from IC1 pin 3 drives Q2, and Q2 drives the output amplifier Q3. A coupling network is used to match the nominal 45 amps output impedance of Q3 to the 10-omps ac line impedance.
In the AM mode, audio is coupled to Q8 via R24 and then amplified again by transistors Q4 to Q7. The normally stable dc voltage at test point A is thereby varied at an audio rate. Because Q2 and Q3 obtain their dc Vcc from test point A, the VCO carrier input to Q2 is amplitude modulated by the varying Vcc amplitude. That produces an amplitude-modulated output from the transmitter. Careful setting of carrier level R23 and audio level R24 provides up to 100% modulation.