User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-5
1, the L input of mixer Z1. The
adjustable capacitor C78 and resistor R53
are set up to add a small amount of
incidental carrier phase modulation
(ICPM) correction to the output of the
mixer stage to compensate for any non-
linearities generated by the mixer.
The modulated 45.75-MHz RF output of
mixer Z1, at pin 4, the R output, is
amplified by U5 and is fed to J18 the
double sideband visual IF output jack.
The level at this output jack is adjusted
by R70, typically –4 to –10 dBm. J18 is
the visual IF loop-through output jack
that is normally jumpered by W10 to J19
on the board. If the (Optional) visual IF
loop-through kit is purchased, the visual
is connected out of the board to any
external IF processor trays and then back
to the board.
After any external processing, the
modulated visual IF, double-sideband
signal re-enters the board through J19.
The visual IF from J19 is amplified by
U10 and U11 and routed through the
vestigial sideband filter network,
consisting of T1, FL1, and T2, that
produces a vestigial sideband visual IF
signal output. The filtered vestigial
sideband visual IF is amplified by U7 and
connected to a T-type attenuator. R62 is
adjustable to set the visual IF gain, which
is the amount of visual IF signal that is
coupled to the amplifier IC U8. R63 and
C30 are adjusted for the best VSBF
frequency response. The amplified IF
signal output of U8 is fed to the input of
the diplexer circuit that consists of R76,
L13, and L12. A detected voltage sample
of the visual IF is available at the test
point TP5.
4.1.2.5 41.25-MHz Aural IF Circuit
The modulated 41.25-MHz aural IF is
created on this board by mixing the
modulated 4.5-MHz aural intercarrier
signal, produced by the aural IF
synthesizer board or by the optional
composite 4.5-MHz filter board, with the
45.75-MHz CW signal produced by the
45.75-MHz IF carrier oven oscillator
board or the IF VCXO board, in a precise
frequency system. The modulated
baseband 4.5-MHz aural intercarrier
signal enters the board at J14, baseband
4.5-MHz Input, and for normal operation
connects through the jumper W8 on J32
pins 1 and 2, IF Relay Disabled, through
the jumper W3 on J7 pins 2 and 3,
Internal 4.5-MHz, to Z2 pin 5, the I input
of the Mixer.
If both composite 4.5-MHz and baseband
4.5-MHz is to be used in your system,
then the relay K1 must be enabled. This
is accomplished by moving the jumper
W8 on J32 to between pins 2 and 3.
Both inputs, the modulated baseband
4.5-MHz aural intercarrier input from J14
and the Composite 4.5-MHz Input from
J28 connect to the IF relay K1. The
jumper W2 on J30 must be between pins
1 and 2, baseband select off, for external
operation of the relay. Now the relay is
controlled by a baseband select, Low,
that connects to J31 Pin 3. With the
baseband select, Low, present at J31 pin
3, the relay is energized and applies the
baseband 4.5 MHz to the mixer Z2 and
DS1 the baseband indicator will be lit.
With the baseband select removed, not
present at J31 pin 3, the relay is de-
energized and applies the composite 4.5
MHz to the mixer Z2 and DS1 the
baseband indicator will not be lit.
The Jumper W3 on J7 determines
whether the 4.5-MHz used by the board
is internally generated, either the
baseband or composite inputs, or from
an external source. With the jumper W3
connected between pins 2 and 3, the 4.5
MHz from the aural IF synthesizer board
or from the 4.5-MHz composite input is
connected to the mixer Z2. With the
jumper W3 connected between pins 1
and 2, the external 4.5 MHz input is
connected to the mixer Z2. The external
4.5-MHz signal enters the board at J12
and is fed through the gain pot R88 to
the amplifier IC U13A. The amplified 4.5-
MHz is then connected to J7 through the
jumper W3 to the mixer.