Ramsey Electronics FM25A Manual do Utilizador descarregar pdf (Página 5)

Transmitter

Once transmitter receives the information from the PIC, the PLL U1 takes

the 6MHz crystal frequency and divides it by 60 to obtain a stable frequency

of 100KHz. Your desired frequency is sent to the PLL in a string of binary

bits. The PLL is used to constantly check the output frequency so that it

matches the desired frequency. The PLL also takes the output frequency

from U3 and divides it by a number N. N is your desired frequency in

megahertz times 10.

The frequency from U3 is divided by N and compared to 100 KHz. If N is

less than 100KHz, the PLL will increase the output voltage (pin 13). This

will increase the voltage across the varactor diode D21 that will in turn cause

a decrease in the capacitance. The final result is an increase in U3’s RF

oscillator, and keeping the output frequency on track to match your desired

frequency. The opposite occurs when N is more than 100 KHz.

Adjustable Components

There are five potentiometers found in the hardware; three located in the

transmitter circuit and two located in the receiver circuit.

The transmitter

potentiometers R11 and R16 allows for the adjustment of the audio levels.

The transmitter potentiometer R6 permits adjustment of stereo balance. The

receiver potentiometer RP1 is used in place of 22ohm resister and should

never be adjusted. The receiver potentiometer RP2 allow for adjustment of

the FM tuning level.

Information concerning the FM25A is taken directly from FM25A user manual.

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Ramsey Electronics FM25A Manual do Utilizador Página 5