FM25B
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CIRCUIT DESCRIPTION
We will begin by talking about the power supply of your new FM25B. While a
DC power source is provided with the kit, its DC output isn’t ‘clean’ enough to
provide us with the low-noise, stable supply that we would like for good audio
quality. We wouldn’t want our FM transmitter ‘Humming’ with the music now
would we! I mean… it should know the words!
Special care has been taken to filter the input DC signal to maximize our work-
ing voltage while still offering a clean, stable supply. Take a look at the sche-
matic as we cover the kit’s circuit description. Right off the bat our input DC
voltage is channeled through an RF filter network composed of bypass caps
C35 & C36 and RF chokes L3 & L5. This filter isolates the plus and minus sup-
ply feeds coming from the wall transformer and helps to remove any unwanted
RF that might be coupled into your transmitter. The large electrolytic capacitor
(C32) that follows the RF filter stores energy so instantaneous peaks in de-
mand for power do not cause dips in the supply voltage.
The final parts of the FM25B’s power supply section are composed of a 12 Volt
ripple filter and a 5 Volt regulator to obtain a clean well-filtered power source.
The capacitive multiplier formed by Q1, R4, and C4 together make a handy low
loss ripple filter by in effect multiplying C4’s capacitive value by the DC current
gain of Q1. This increases C4’s ripple filtering capability to roughly that of a
10,000uF capacitor without the typical 2 to 3 volt overhead loss you would get
with a 12 Volt regulator! Now that we have a clean power source, let’s dive into
the rest of the kit!
The custom FM stereo IC (U3) is the heart of the FM25B. U3 is a microproces-
sor controlled FM stereo generator with lots of built-in performance features.
The surrounding support circuitry configures U3 for proper operation under a
variety of conditions.
Potentiometers R27 and R28 allow for adjustment of the input audio levels to
match a wide sampling of audio sources for the best possible sound.
Capacitors C24 and C27 set the pre-emphasis characteristics for that of the re-
gion you intend to operate in (75 µs for USA, 50 µs for Europe).
Capacitors C22 and C25 are part of the 15kHz audio low-pass filter (LPF) that
is internal to the chip. The Bessel filter has flat delay characteristics and re-
moves the higher frequency elements that would interfere with the stereo sig-
nal.
Capacitor C29 acts as a ripple filter for U3’s internal audio reference voltage.
This reduces any chance of audio distortion due to internal demands on its
power buss.