User Manual

UHF Analog Driver/ Transm it ter Chapter 4, Circuit Descript ions
LX Series, Rev. 0 4- 5
through C152 to the amplifier U23A. The
output of U23A is split with one part
connecting to Q26 that shuts down the
45.75 MHz oscillator. Another path
connects to Q25 the conducts and lights
the LED DS7, Alternate IF, viewed on the
front panel. The final path connects
through R268 to Q22 that is biased on
and energizes the relay, K2. The
external I F CW Input at contact 14 now
connects through the closed contact to
the splitter network consisting of L31 and
L32.
Either the internal or external CW I F from
the K2 relay is split with one path
through L31 to the am plifier U28 to the L
input of Z1 the Aural IF Mixer. The other
path is through L32 to the amplifier U29
to the L input of Z2 the Visual IF Mixer.
4.1.1.11 Visual Modulat or Circuit
The video signal is heterodyned in mixer
Z2 with the visual I F CW signal (45.75
MHz). The visual I F CW signal from L32
of the splitter connects to U29, where it
is am plified and wired to pin 1, the L
input of mixer Z2. Adjustable capacitor
C168 and resistor R275 are set up to add
a small am ount of incidental carrier
phase modulation (I CPM) correction to
the output of the m ixer stage to
com pensate for any non- linearities
generated by the mixer.
The modulated 45.75- MHz RF output of
mixer Z2, at pin 4 the R output, is
am p lified by U30 and is fed to J17
through W8, the external cable
assem bly, “WB” , to J13 on the board.
J17 is the visual I F loop- through output
jack that is norm ally jumpered to J13 on
the board. The m odulated visual IF
through J13 connects to J41C pin 17C
the Visual IF Output of the board.
4.1.1.12 Aural Modulat or Circuit
The 4.5 MHz aural m odulated signal is
heterodyned in mixer Z1 with the 45.75
MHz I F CW signal. The m ixer Z1
heterodynes the aural- modulated, 4.5-
MHz signal with the 45.75- MHz CW signal
t o produce the modulated 41.25- MHz aural
IF signal. The audio m odulated 4.5 MHz
from 4.5 MHz VCO IC U16 connects, “J”, to
the I input at pin 5 of Z1. The visual IF CW
signal from L31 of the splitter connects to
U28, where it is am plified and wired to pin
1, the L input of m ixer Z1. The R output of
the mixer at pin 4 is fed to a bandpass
filter, consisting of L18- L21, L25- L28 and
C136, C137 and C142- 144, that is tuned to
pass only the m odulated 41.25- MHz aural
IF signal. The filtered 41.25 MHz is fed to
t he am plifier U27. The am plified 41.25-
MHz signal is connected by a coaxial cable,
W9, from J21, “ WC”, to J18 on the board.
The modulated 41.25- MHz aural IF signal
from J18 is connected to J41C pin 6C the
Aural IF Output of the board.
4.1.1.13 Com bining t he 45.75 MHz Visual
I F and 41.25 MHz Aural I F Signals
The Visual IF connects back to the board at
J41C pin 3C, through a Visual I F jum per
cable connected to the rear chassis of the
exciter/ driver. I F processing equipm ent
can be connected in place of the jum per if
needed. The visual I F is connected to J12,
through jum per W7, “WA”, to J14. The
visual IF is amplified by U24 and filtered by
FL1 with T1 and T2 providing isolation.
The filtered IF is amplified by U25 and
adjusted in level by R2 14 before it is
connected to a sum m ing circuit at the
com m on connection of L16 and L17.
The Aural I F connects back to the board at
J41C pin 23C, through an Aural I F jum per
cable connected to the rear chassis of the
exciter/ driver. I F processing equipm ent
can be connected in place of the jum per if
needed. The aural IF, “F” , is connected
through C132, R234, R235 and adjusted in
level by R243 before it is connected to a
summing circuit at the com m on connection
of L17 and L16.
The Aural I F and Visual IF signals are
com bined through L16 and L17. The
frequency response of the com bined 41.25
MHz + 45.75 MHz signal is set by R238
and R239 and associated com ponents. The
corrected com bined I F signal is amplified
by U25 and connected to a splitter