Vectronics VEC-1292K - Manual (Page 18)

VEC-1292K Owner’s Manual Stereo Transmitter Kit
18
THEORY OF OPERATION
The BA1404 transmitter IC contains a number of specialized circuits for
generating the on-air FM stereo signal. Independent left and right audio inputs
are pre-emphasized for 75 uS (US standard) and buffered through identical
amplifiers. The L and R signals are then fed into the multiplexer section, along
with 38 kHz LO from crystal-controlled oscillator Y1. Here, the two audio
channels are combined to produce a monaural-compatible L+R signal. Samples
are also subtracted to generate a second L-R difference signal. The difference
signal is superimposed over the LO in a balanced modulator circuit to generate a
38 kHz suppressed-carrier DSB signal. Unmodulated 38 kHz LO is also
sampled through a frequency-divider stage to generate a 19 kHz stereo-pilot
signal.
L+R Signal
19-kHz Pilot
L-R Signal
Carrier
Frequency
38-kHz Suppressed
Subcarrier
Multiplex Stereo FM Signal
A L/C-tuned VHF-range oscillator generates the transmitter's primary carrier
signal. This stage is user-adjustable to cover the 88-108 MHz band. To
generate a multiplex stereo signal, the IC's FM modulator stage superimposes the
L+R audio signal, the 19 kHz stereo pilot, and the 38 kHz DSB L-R signal onto
the fundamental carrier. This signal is then fed through a buffer/low-level
amplifier connected to the transmitter's RF output network.