RFL 9780 RFL Electronics Inc.
April 8, 2003 12-4 (973) 334-3100
Voltage-controlled oscillator U4, varactor CR1, and their associated components form a tunable VCO
that is controlled by the signal at U3-4. The VCO output frequency is controlled by varactor CR1,
which serves as a tuner. Capacitor C34 and resistors R13 and R14 form a filter, which sets the
dynamics of this loop.
The output of the VCO is fed back to the synthesizer through U3-1. When divided by the frequency set by
SW1 and SW2, the resultant frequency equals 500 Hz, and the loop is stable. Capacitor C23 attenuates very
high-frequency noise, as well as harmonics of the reference frequency. The output of the VCO is also fed to
the input mixer.
c. Input Low-Pass Filter. The incoming carrier enters the RFL 97 PLC IF/BF module through edge
connector terminals C15 and C16. It then passes through the input low-pass filter formed from
capacitors C2 and C3 and inductor L1. This filter has a cutoff frequency of 600 kHz.
d. Input Mixer (First Mixer). Mixer U1 takes the input signal and converts it to the 5.12-mHz if, by
combining it with the output of the tunable VCO controlled by the synthesizer.
e. If Filter. FL101 is a ten-pole crystal filter that allows the lower 3.7-kHz sideband of the 5.12-mHz if
to pass. It is used to clean up the output of the if mixer before it is fed to the if amplifier.
f. If Amplifier. Operational amplifier U2 is used to boost the output of the if filter and supplies most of
the gain in the demodulator section. Its gain is controlled by potentiometer R18.
g. Product Detector (Second Mixer). The output of the if amplifier (U2-6) is fed to U6-6. U6 is the
second mixer, which serves as a product detector. U6 combines the if signal with the 5.12-mHz signal
generated by the crystal oscillator. The result is an audio signal, consisting of a 2-kHz carrier with
upper and lower sidebands. The output of the product detector is fed through low pass filter/amplifier
U12.
h. Output Modulator (Third Mixer). The carrier from the product detector is fed through
potentiometer R28 to the output modulator or third mixer, formed from modulator/demodulator U7 and
its associated components. The output modulator converts the carrier signal to a 4-kHz baseband signal,
using the 6.00-kHz or 6.25-kHz signal provided by the second local oscillator. The modulator's output
(U7-16) is buffered by operational amplifier U8.
i. Second Local Oscillator. Xilinx programmable logic chip U9, crystal Y2, and their associated
components form the second local oscillator. This oscillator produces the 6.00-kHz or 6.25-kHz signal
needed by the output modulator. The oscillator frequency is controlled by a selectable divide-by-N
binary counter in U9 and is selected by DIP switch SW1-1.
j. Onboard Regulators. Linear voltage regulators U10, U11 and U13 serve as onboard voltage
regulators. U13 converts the incoming +15-volt supply voltage to a regulated +8 volts, U11 converts it
to a regulated +5 volts, and U10 converts it to a regulated +10 volts.
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