Ramsey Electronics FM25B Assembly And Instruction Manual page 6

Crystal X1 along with C9 and C12 form the timing reference of the transmitter.
The signal from the crystal network is divided down and used to generate the
stereo components of the transmitted signal as well as by the chip's Phase
Lock Loop (PLL) circuitry to provide 'rock solid' frequency stability. More on the
PLL side of U3 in a moment.
Inductor L1, C13, and D8 form the Voltage Controlled Oscillator (VCO) stage.
The RF oscillator is a tuned circuit formed by these elements that sets the base
frequency of our RF transmitter. The DC voltage applied to varactor D8 causes
its capacitive value to change up or down as needed. This allows us to vary the
frequency output of the circuit up or down simply by changing the DC voltage
feed to D8.
Transistors Q2 and Q3 work in conjunction with C6, C8, and R17 to filter the
PLL correction pulses coming from U3 pin 7. The inverting LPF they form pass
a DC voltage component, as a function of the correction pulses, to the VCO to
vary the final operating frequency of the RF oscillator.
Varactor D7 is used to 'wiggle' the VCO voltage in accordance with the applied
audio signal. The resistor network around D7 serves a multitude of functions. In
a nutshell, D7's capacitive value changes slightly with a given applied AC signal
(the processed composite audio signal) causing the VCO to deviate from its
base frequency. This is how we get our FM (Frequency Modulation) signal. By
using a dual varactor diode modulation scheme, we can achieve a very tight
deviation tolerance across the entire FM band and improve the overall perform-
ance of the system greatly!
Resistor R25 varies the combined modulated RF signal from U3 before its final
amplification stage. R25 gives the user full control to vary the final output level
to match their custom applications and coverage.
Amplifier U4 (the Gal-5 is phenomenally rated from DC to 4GHz operation!)
boosts the output level of U3 in one super clean stage without introducing har-
monics or other spurious signals even before the signal goes through a low
pass filter!
The low pass RF filter consisting of C34, L2, C37, L4, and C38 allows the fun-
damental (operating) frequency to pass through while rejecting any unwanted
harmonics. Harmonics are multiples of the desired fundamental frequency and
in this case, they can cause unwanted emissions in critical areas of the RF
spectrum. A cleaner RF output means happier neighbors and the 'piece of
mind' that you are not causing unwanted interference.
U1 is an LM358 opamp. This little work-horse can be found in use for all types
of different applications. Our application for this circuit gives the user feedback
about the operating status of the unit. The combined stages used with their
support components act as a dual purpose pulse detector. The opamp (U1) cir-
FM25B • 6