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of signal information, it measures RF power, and it updates the display. The
microcontroller also does important tasks like converting the frequency value
to text on the display that you can read, and signals that the stereo modulator
IC needs to go to that frequency. It also sends the proper signals to the level
control IC U7, which is just a digital to analog converter, U4, the digital pot,
and DS1, the LCD display. In all there is a lot going on inside of the
microcontroller, and be glad you don’t have to do things with a bunch of dip
switches any more.
Now here comes the fun part, the stereo modulator IC, U3. This IC is a great
little part, and contains a lot of circuitry that helps us get on the air with
minimal fuss, and great sound. This IC not only contains a stereo multiplexer,
but also a phase locked loop (PLL), audio filters, and equalizations.
The audio filter portion of the part adds a little more low-pass filtering to the
audio to increase quality more than what our external low pass filter does on
its own. The equalization portion enhances the high frequencies for the radio
standard of 75uS in the US. 75uS is just a term used for a high-pass filter to
enhance the high frequency audio before transmission to help reduce noise
upon reception.
The PLL is the portion of the IC which locks your chosen transmission
frequency to the crystal reference X1. The PLL portion also contains an
oscillator circuit which works in conjunction with the external parts of D5 and
the STUB (that weird trace on the back of the board). D5 is called a varactor
diode, and is a special variety of diode that is connected backwards. As a
reverse DC voltage is applied across the diode, its capacitance varies. The
higher the voltage, the less the capacitance. This is due to depletion layers of
the diode junction, but we wont get into details here. This variable capacitor in
conjunction with the stub, which is actually an inductor (coil) is the basis of our
voltage controlled oscillator! As the voltage increases across D5, the
frequency of oscillation increases.
The stub is just acting like a coil in this circuit, and because it is part of the
circuit board layout, it has a very predictable value. This prevents us from
having to tune anything in this section like our old products. Just turn the unit
on, and you are good to go! The best part of the stub is it’s low sensitivity to
microphonics. Microphonics is a term used for mechanical vibrations that are
picked up in a VCO circuit by vibrating components changing in value
because of mechanical shock. The sub value is very hard to alter from
mechanical shock, and therefore quite immune to vibration.
So what actually tunes the VCO you may ask? Good question. A PLL uses a
phase comparator to compare the crystal frequency with the oscillator
frequency combined with some internal dividers which are programmable. If
the frequency is too low, U3’s PLL tells pin 7 to turn up the voltage. This signal
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