FM Transmitter
Sistemas Electrónicos S.A EM 250 COMPACT DIG
Technical Manual - v1.1 - February 2006 24
2.3 Stereo encoder (optional).
The encoding circuit uses an 8-step switching technique,which ensures excellent performance with a
relatively simple circuit. In addition, by this technique, the first harmonics that are associated with the
switching devices are the 7
th
and 9
th
(266 and 342kHz); this simplifies the design of the low-pass filter on
the multiplex signal.
The signal is filtered beyond 15kHz by the two precision active low-pass filters built around IC1 ~ IC4. It
is then buffered by IC3d and IC4c and applied to the encoding circuit comprised in IC8. Another
higher frequency low-pass filter follows to remove higher order harmonic products. This filter is also
made with highly precise active circuitry built around IC5 and IC6a,b. The latter section (c) of IC6
performs phase equalization.The four analog switches comprised in IC7 permits to select the mono or
the stereo-encoded signal and to slightly vary the encoder gain to adjust for the 90%audio
modulation in stereo vs.100% in mono. Two jumpers on BD1 permit to select either Left or Right or
Left+Right channel for mono operation,with no output level change. As factory configured, both
jumpers are installed, to mix Left and Right channel for "MONO L+R" operation.
Circuits IC10, IC11 make the encoder’s time base; IC9 synthesizes the 19kHz pilot frequency, which is
filtered and buffered by IC5a. A separate 1Vpp output is provided on J2 to drive carrier synchronization
on a possible external RDS generator.
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