Assembly Instructions Chapter 4

4-6 kW UHF Translator Chapter 4, Circuit Descriptions
837B, Rev. 0 4-7
by jumper W3 on J11. Jumper W3 on
jack J11 is positioned according to the
input level of the audio signal (0 or +10
dBm). If the input level is
approximately 0 dBm, the mini-jumper
should be in the high gain position
between pins 1 and 2 of jack J11. If the
input level is approximately +10 dBm,
the mini-jumper should be in low gain
position between pins 2 and 3 of jack
J11. The balanced audio is then
connected to buffer amplifier U2A
whose input level is determined by the
setting of balanced audio gain pot R13.
The output of the amplifier stage is
wired to the summing point at U2D, pin
13.
4.2.1.2 Composite Audio Input
The second possible audio input to the
board is the composite audio (stereo)
input at BNC jacks J3 and J13. The two
jacks are loop-through connected; as a
result, the audio can be used in another
application by connecting the unused
jack and removing W4 from J12.
Jumper W4 on jack J12 provides a
75Ω-input impedance when the jumper
is between pins 1 and 2 of jack J12 and
a high impedance when it is between
pins 2 and 3. Diodes CR9 to CR12
protect the input stages of U6A and
U6B if an excessive signal level is
applied to the board. The outputs of
U6A and U6B are applied to differential
amplifier U2C, which eliminates
common mode signals (hum) on its
input leads. The composite input signal
is then applied to amplifier U2B; the
gain of this amplifier is controlled by
composite audio gain pot R17. The
composite audio signal is connected to
the summing point at U2D, pin 13.
4.2.1.3 Subcarrier Audio Input
The third possible input to the board is
the SCA input at BNC jack J4. The SCA
input has an input impedance of 75Ω
that can be eliminated by removing
jumper W2 from pins 1 and 2 of J14.
The SCA input is bandpass filtered by
C66, C14, R22, C15, C67, and R23 and
is fed to buffer amplifier U3A. The
amplified signal is then applied though
SCA gain pot R24 to the summing point
at pin 13 of U2D.
4.2.1.4 Audio Modulation of the VCO
The balanced audio, or the composite
audio and/or the SCA-buffered audio
signals, are fed to the common junction
of resistors R14, R20, and R27 that
connect to pin 13 of amplifier U2D. The
output audio signal at pin 14 of U2D is
typically .8 Vpk-pk at a ±25-kHz
deviation for balanced audio or .8 Vpk-
pk at ±75-kHz deviation for composite
audio as measured at TP1. This signal
is applied to VCO U10. A sample of the
deviation level is amplified, detected by
U7A and U7B, and connected to J10 on
the board. This audio-deviation level is
connected to the front panel meter
through the transmitter control board.
The audio is connected to CR13 to
CR16; these are varactor diodes that
frequency modulate the audio signal
onto the generated 4.5-MHz signal in
U10. U10 is the 4.5-MHz VCO that
generates the 4.5-MHz continuous
wave (CW) signal. The output
frequency of this signal is maintained
and controlled by the correction voltage
output of U5 PLL integrated circuit (IC).
The audio-modulated, 4.5-MHz signal is
fed to amplifiers U11A and U11B. The
output of U11B is connected to the 4.5-
MHz output jacks at J7 and J8.
4.2.1.5 Phase Lock Loop (PLL) Circuit
A sample of the signal from the 4.5-
MHz aural VCO at the output of U11A is
applied to PLL IC U5 at the F
in
connection. In U5, the signal is divided
down to 50 kHz and is compared to a
50-kHz reference signal. The reference
signal is a divided-down sample of the
visual IF, 45.75-MHz signal that is
applied to the oscillator-in connection
on the PLL chip through jack J6 on the
board. These two 50-kHz signals are