Fm10C Circuit Description; Circuit Description - Ramsey Electronics FM10C Manual

CIRCUIT DESCRIPTION

Power Supply
The FM10C can be powered with a standard 9 Volt battery or an external DC
supply. When a standard external female 2.1MM power plug is inserted into
J3 its SW contact is removed from its RING contact. This disconnects the
battery from the FM10C's circuit. S1 applies power to the circuit. L3 and
C3 are a low pass filter designed to prevent RF (Radio Frequencies) from
entering the unit through the power cord. C4 is a filter capacitor that reduces
60 or 120Hz ripple from an external supply. VR1 is a voltage regulator de-
signed to provide 2.6 Volts DC output with a 5 to 20 Volt DC input. Regula-
tors will also provide low frequency filtering. C9 is a low frequency output
filter. C15, C16, C17, C21, and C25 are high frequency filter caps that are
placed physically as close to the supplied devices as possible.
U1 FM Stereo Transmitter IC
FM stereo transmitter IC (U1) is at the heart of the FM10C. It contains a ste-
reo modulator that develops a main L+R signal, a sub (L-R) signal, a 19kHz
pilot signal, and RF Carrier (88-108 MHz). The control of U1 is determined
by its surrounding circuitry as described in the following paragraphs.
Audio
We will look only at the left channel circuitry since the right channel is
identical to the left. J1 is an RCA connector that brings the left channel au-
dio into the FM10C. L2 and C2 form a low-pass filter that reduce any RF
brought in by the audio cables. R2 is a dropping resistor used to decrease
the audio level into R4. R4 allows the user to set the level of audio that
reaches U1. C6 and R6 set the pre-emphasis characteristics for the region
you intend to operate (75 µs for USA, 50 µs for Europe). C8 is a coupling
capacitor that allows the audio to pass but prevents DC bias from the IC from
being affected by the setting of R4.
Multiplexer Balance
R7 sets the multiplexer balance of U1. This control should be set to mid-
range.
Stereo Generator
C11, C12, X1, and U1 form a 38 kHz oscillator that is used by U1 to create
the 19 kHz pilot frequency for stereo generation.
Multiplexed Audio/Pilot
U1 pin 13 is the pilot output. U1 pin14 is the multiplexed audio output. R8
sets the pilot signal level and R9 sets the multiplexed audio level going back
into U1 at pin12.
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