Circuit Operation - Ramsey Electronics FM100B Instruction Manual

CIRCUIT OPERATION

Here is where we get into a little circuit analysis. If you just want to plug it in and
start broadcasting you can skip this section and read the FCC regulations at the
end. Otherwise read on to learn how the FM100B works. We will reference the
schematic often as we analyze the circuit.
Let's begin with the power supply. It is arguably the most important part of the
FM100B. Instead of having you build a discrete power supply which can be
difficult and bulky, we decided to include a very versatile switching power
supply which is pre-built for you. These power supplies have several
advantages over conventional supplies; they are lightweight for their power
output, they are efficient (usually better then 80% efficiency), and they will
operate anywhere from 85 VAC to 264 VAC without setting any jumpers! This
means the European folks will be just as happy as the U.S. folks!
The only drawback with this power supply is that our particular unit only
generates +/-12 VDC. We need +5 VDC for some of the digital parts so it was
necessary to add a voltage regulation stage to step down the positive supply
voltage. This is performed with VR1, C1, and C2. These parts bring down the
+12V to a very smooth +5V for our digital components.
Now let's move on to the display circuit. It was designed in a very specific way
in order to reduce the introduction of noise in our final transmission. We needed
a display that produced as little noise as possible but was also easy to read.
LED displays provide the best choice for visibility, but we couldn't use normal
display drivers that are multiplexed. Multiplexing means that all of the digits are
never on at the same time. Each one is turned on in turn at a high enough rate
so that to your eye it looks as if all are lit at the same time. The switching
between the displays produces more noise than we want to deal with, so we
chose a method where all the displays are constantly lit. While this may not be
the most efficient method of lighting a display, it is the least noisy. U11, the
MM5451 is a large serial shift register made specifically for driving LED
segments.
By itself, the display wouldn't do much without a smart device to send it some
meaningful data. In this case we are using a Motorola microcontroller (U1) to do
the job for us. This microcontroller performs most of the operations on the unit
such as checking for button presses, setting the transmitting frequency,
checking the PLL voltage for lock, muting audio lines at the appropriate times,
as well as updating the display.
When the frequency is changed, we access U2, a serially programmed PLL
(Phase Locked Loop) and stereo modulator IC. When the frequency needs to
be set, the information is sent serially to U2. This information is a string of
binary data (1's and 0's) that is sent one bit at a time to U2. The frequency
information takes 16 bits of data and is fairly straight-forward. You may think
FM100B • 5