Because RFL™ and Hubbell® have a policy of continuous product improvement, we reserve the right to change designs and specifications without notice.
Input Normalizer. The input signal at pin C15 is first attenuated by 0 to 50 dB in 10 dB steps, as
selected by J1. Diodes CR3 and CR4 provide high voltage transient protection. The signal is then
amplified by U14 and U15. The gain of the first amplifier (U14) is either 4 or 40, depending on the
selection of J2. The gain of amplifier U15 is variable between 0.1 and 5.1, as determined by the setting
of GAIN potentiometer R69.
Input Low-Pass Filter. The incoming carrier enters the RFL 9785 Receiver Downshifter module
through edge connector terminals C15 and C16. It then passes through the input low-pass filter formed
from capacitors C2, C3, C95 and C96 and inductors L1, L10 and L11. This filter has a cutoff
frequency of approximately 600 kHz.
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.
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.
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 R10.
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.
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 24 kHz baseband signal, using the
26.00 kHz or 26.25 kHz signal provided by the second local oscillator. The modulator's output (U7-
16) is buffered by operational amplifier U8.
Second Local Oscillator. Xilinx programmable logic chip U9, crystal Y2, and their associated
components form the second local oscillator. This oscillator produces the 26.00 kHz or 26.25 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.
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.
RFL 9785 RFL Electronics Inc.
June 18, 2009 11-4 (973) 334-3100
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