AM25
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6
amount of power. It is not recommended that a common 9V battery be used
to power this kit. Instead, a battery “pack” consisting of eight 1.5 volt cells, a
12V sealed battery, or other external 12V DC supply may be used.
The Phase-Locked-Loop portion of the circuit is responsible for maintaining
the frequency of the transmitter. U2 is the brains of the whole circuit. This
chip looks at the settings of each switch in the dip switch S1 and from this
information, calculates the desired frequency. These switches are
programmed in Binary, you simply add up the closed switch positions: 1, 2,
4, 8 all the way up to 128 to make any number between 0 and 255. The final
output frequency will be the programmed number times 10 KHz. For the AM
broadcast band, 540 - 1710 KHz, we’ll only need to program between 54 and
171. For example, if a broadcast frequency of 550 KHz if desired, we’ll need
a programmed number of 55 (55 times 10 KHz equals 550 KHz). Closing
switches 32, 16, 4, 2 and 1 all adds up to 55, our desired number. As
another example, what would we need for 1520 KHz? Our desired program
number is 152 and we’ll have to close switches 128, 16, and 8 which adds
up to 152! It really is quite easy and fun to find the right switches to close!
The phase locked loop (PLL) synthesizer IC takes the 10.24 MHz crystal
frequency and divides it internally by 1024 to obtain a stable reference
frequency of 10 KHz for the PLL’s phase detector. U1 also takes the output
frequency from the VCO (Voltage Controlled Oscillator) and divides it by a
number that we’ll call ‘N’. N is the frequency data that was sent by U2, and N
is always equal to the desired frequency in megahertz times 10.
Using the example from above, a frequency of 550 KHz gives an N of 55, so
the frequency from the oscillator is divided by 55 and then compared with the
reference frequency of 10 KHz. If the desired frequency is less than the
reference frequency U2 increases the output voltage on pin 7. This increases
the voltage across diode D2, 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 the VCO’s RF frequency (f = 1/[2
π
(LC)
½
]), bringing the AM25’s output frequency back on frequency. If the desired
frequency is higher than the reference, pin 7 is driven low. If the frequency is
just right then pin 7 becomes a high impedance, basically disconnecting it
from the circuit so it will cause no change in the voltage on D2. The voltage
changes on pin 7 are filtered by R25, C26, R28, and C28 to provide a
steady, noise free tuning voltage for D2. In this way the output frequency of
the oscillator is "locked" to that desired by U2. When the frequency is locked,
U2 will cause led D3 to be brightly lit. If D3 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 U2 instantly corrects the tuning voltage
to bring it back to the proper frequency.
Содержание AM25
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Страница 8: ...AM25 8 PARTS LAYOUT DIAGRAM...
