User's Manual

2650 Watt VHF High Band Transmitter Chapter 4, Circuit Descriptions
435B, Rev. 0 4-1
Chapter 4
Circuit Descriptions
4.1 (A4) VHF High Band Exciter
(1070901; Appendix C)
4.1.1 (A4) Aural IF Synthesizer
Board, 4.5 MHz (1265-1303;
Appendix D)
The aural IF synthesizer board amplifies
each of the three possible audio inputs
and connects them to a summing point
and amplifier circuit that provides the
single audio output. Either the balanced
audio or the composite stereo audio input
is connected to the board while the
subcarrier audio (SCA) input can be
connected at the same time as either of
the other two inputs. The board has the
4.5-MHz voltage-controlled oscillator
(VCO) and the aural modulation circuitry
that produces the modulated 4.5 MHz
output. The board also contains a phase
lock loop (PLL) circuit that maintains the
precise 4.5 MHz separation between the
aural (41.25 MHz) and the visual (45.75
MHz) IF frequencies.
4.1.1.1 Balanced Audio Input
The first of the three possible baseband
inputs to the board is a 600Ω-balanced
audio input. The balanced audio input
(+10 dBm) enters through jack J2, pins 1
(+), 2 (GND), and 3 (-) and is buffered
by U1B and U1C. The Diodes CR1 to CR4
protect the input stages of U1B and U1C
if an excessive signal level is present on
the input leads of jack J2. The outputs of
U1B and U1C are applied to differential
amplifier U1A, which eliminates any
common mode signals (hum) that may
be on its input leads. A pre-emphasis of
75 mS is provided by R11, C11, and R10
and can be eliminated by removing
jumper W5 on J5. The signal is then
applied to amplifier U1D whose gain is
controlled 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.
4.1.1.2 Composite Audio Input
The second possible audio input to the
board is the composite audio (stereo)
input at the BNC jacks J3 or 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 the jumper 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 the differential amplifier U2C,
which eliminates common mode signals
(hum) on its input leads. The composite
input signal is then applied to amplifier
U2B whose gain is controlled by the
composite audio gain pot R17. The
composite audio signal is then connected
to the summing point at U2D, pin 13.
4.1.1.3 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 the
buffer amplifier U3A. The amplified signal