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FM-100 Page 6
lit. While this may not be the most efficient method of lighting a display, it is the
least noisy. U3, the MM5451 is just a large serial shift register made for driving
LED segments.
Well, 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 (U4) 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, muting
audio lines at appropriate times, as well as updating the display.
When the frequency is changed, we access U2, a serially programmed PLL
(Phase Locked Loop). 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). In
this way data is sent one bit at a time to U2. The frequency information takes
16 bits, and there are an additional 32 bits sent for the internal control of U2.
You may think that all this would take a long time, but in fact the whole process
from the time you press a frequency control switch until the data is completely
sent is less than 1/100th of a second!
The PLL also takes the 6 MHz crystal frequency and divides it by 60 to obtain a
stable reference frequency of 100KHz. How does it know to divide the crystal
frequency by 60? That is part of the data that is sent from U4 each time a
frequency button is pressed. To set the operating frequency of U6, we simply
send U2 divide by N data to produce the desired frequency from the BA1404’s
VHF oscillator. The oscillator’s frequency is divided by this value N, producing
an output in the 100KHz region. The internal phase detector compares the
highly accurate 100KHz reference with the divide-by-N output and controls the
BA1404 oscillator so that the frequencies match precisely.
Using the example from above, a frequency of 95.3 MHz gives an N of 953, so
the frequency from U3 is divided by 953 and then compared with the reference
frequency of 100 KHz. If the desired frequency is less than the reference
frequency U2 increases the output voltage on pin 13. This increases the
voltage across diode D10, a varactor diode. As the voltage across the varactor
increases, it causes a decrease in capacitance (Increasing reverse bias
essentially increases the distance between the capacitor’s plates by increasing
the depletion region in the diode (C = kA/d). The decrease in capacitance
causes an increase in U6’s RF oscillator (f
o
= 1/[2
π
(LC)
½
]), bringing the FM-
100’s output frequency back on frequency. If the desired frequency is higher
than the reference, pin 13 is driven low. If the frequency is just right then pin 13
becomes a high impedance, basically disconnecting it from the circuit so it will
cause no change in the voltage on D10. The voltage changes on pin 13 are
filtered by R8, C9, R7, and C11 to provide a steady, noise free tuning voltage
for D10. In this way the output frequency of U6 is "locked" to that desired by the
microcontroller. Meanwhile U4 is polling pin 11, the LD (Lock Detect) output of
U2 to determine if the PLL can achieve a “lock” or not.
Содержание FM-100
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