Ramsey Electronics STC1 Instruction Manual download pdf (Page 8)

FM25B • 8

ence crystal (X1) as we discussed before. All the internal operations of U3 are

truly amazing! The reference signal of X1 is divided by 4 and then again by 19

to obtain a stable reference frequency of 100KHz that will be used to keep our

transmitter on frequency. U3 then internally samples the RF output and di-

vides it by a number (N). N is the frequency data that was sent by U2 and is

always equal to the desired frequency in Megahertz times 10. Using the previ-

ous example, a frequency of 95.3 MHz gives an N of 953. This means the

sampled RF output signal will be divided by 953 by U3 and then compared

with the reference frequency of 100 KHz. If the desired RF output frequency

happens to be too low (lower than the calculated reference frequency), U1

sends a series of controlled pulses close to the chip’s 0 Volt rail from a 1/2

VCC midpoint on pin 7. The inverting LPF (Q2, Q3 in conjunction with C6, C8,

and R17) in turn raises the DC control voltage on D8. As the voltage across

the varactor increases, it causes a decrease in capacitance (Increasing re-

verse 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 U3’s RF oscillator (f

= 1/[2π(LC)

]), bring-

ing the FM25B’s output frequency back on frequency to match that of the ref-

erence. If the desired frequency is higher than the reference, U1 does just the

reverse and sends a series of controlled pulses close to the chip’s 5 Volt rail

from a 1/2 VCC midpoint on pin 7. If the frequency is just right then pin 7 floats

at a constant DC level, keeping the VCO voltage constant on D8. In this way

the output frequency of U3 is "locked" to that desired by U2. When the fre-

quency 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 (assuming of course that

no audio is being applied). If the frequency starts to drift for any reason (such

as a temperature change) then U3 instantly corrects the tuning voltage to

bring it back to the proper frequency.

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Ramsey Electronics STC1 Instruction Manual Page 8