FM25A • 6
may think that all this would take a long time but in fact the whole process from
the time you press the program switch until the data is completely sent is less
than 1/100th of a second!
U1 is a phase locked loop (PLL) synthesizer IC. It takes the 6 MHz crystal fre-
quency 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 U2 each time the program button is pressed or the loop is out
of lock. U1 also takes the output frequency of U3, and divides it by a number,
N. N is the frequency data that was sent by U2. N is always equal to the de-
sired frequency in megahertz times 10.
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 fre-
quency U1 increases the output voltage on pin 13. This increases the voltage
across diode D21, a varactor diode. As the voltage across the varactor in-
creases, it causes a decrease in capacitance (Increasing reverse bias essen-
tially 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 U3’s RF oscillator (f
o
= 1/[2π (LC)
½
]), bringing the FM25A’s out-
put frequency back on frequency. If the desired frequency is higher than the ref-
erence, 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 D21. The voltage changes on pin 13 are filtered by
R8, C16, R10, and C19 to provide a steady, noise free tuning voltage for D21.
In this way the output frequency of U3 is "locked" to that desired by U2. When
the frequency is locked, U1 will cause led D1 to be brightly lit. If D1 is dim or off
there is a problem and the frequency is not locked. If the frequency starts to
drift for any reason (such as a temperature change) then U1 instantly corrects
the tuning voltage to bring it back to the proper frequency.
Notes on Operation
Any time the unit is turned "on" or the "program" switch is pressed the micro-
controller reads the frequency switches and sends the data to U1.
To change the frequency simply set the switches to the desired frequency and
press "program". The frequency is limited to within 88 to 108 MHz. Any setting
outside this range will be programmed as 88.1 MHz. If you set your frequency
and press program but don’t hear the signal on the frequency you expected try
listening on 88.1 MHz. If the signal is here there is an error in the switch set-
tings or maybe a solder bridge somewhere making the frequency setting inva-
lid.
A switch must be closed to be active. Add up the positions 1, 2, 4, and 8 to
make any number between 0 and 9. 10 -15 are invalid and will be read as 0.
(29 pages)
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