User's Manual

3300-Watt VHF Low Band Transmitter Chapter 4, Circuit Descriptions
335B, Rev. 0 4-1
Chapter 4
Circuit Descriptions
4.1 (A4) Low Band VHF Exciter
(1070820 or 1304463 w/P.F.;
Appendix C)
NOTE: The 1304463 VHF Exciter is used
with the precise frequency system and
will contain the VCXO Assembly (1145-
1206), in place of the Channel Oscillator
Assembly (1145-1202), and the IF VCXO
Board (1248-1131), in place of the IF
Oven Oscillator Assembly (1191-1404).
4.1.1 (A4) Aural IF Synthesizer
Board, 4.5 MHz (1265-1303;
Appendix D)
The aural IF synthesizer board amplifies
and controls the levels of the three
possible audio inputs and provides a
single audio output. The balanced audio
or the composite 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
audio inputs to the board is a 600Ω-
balanced audio input (+10 dBm low gain
or 0 dBm high gain) that enters through
jack J2, pins 1 (+), 2 (GND), and 3 (-).
The input is then buffered by U1B and
U1C. The diodes CR18 and CR19 are
transient voltage suppressors that
protect the board from any surges or
transients that may occur on the
balanced audio input lines. The Diodes,
CR1 to CR4, protect the input stages of
U1B and U1C if an excessive signal level
is applied to the board. The outputs of
U1B and U1C are applied to the
differential amplifier U1A that eliminates
any common mode signals (hum) on its
input leads. A pre-emphasis of 75 µs is
provided by R11, C11, and R10 and can
be eliminated, if not needed, by
removing the jumper W5 on J5. The
signal is then applied to the amplifier
U1D whose gain is controlled by the
jumper W3 on J11. The Jumper 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 J11. If
the input level is approximately +10
dBm, the mini-jumper should be in low
gain position which is between pins 2 and
3 of J11. The balanced audio is then
connected to the 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 BNC jacks J3 and J13. The two
jacks are loop-through connected.
Therefore, the audio can be used in
another application by connecting to the
unused jack and moving the jumper W4
to J12 between pins 2 & 3. The jumper
W4 on jack J12 provides a 75Ω-input
impedance when the jumper is on J12
pins 1 and 2 and a high impedance when
it is on pins 2 and 3. The diodes CR20
and CR21 are transient voltage
suppressors that protect the board from
surges and transients on the composite
audio input lines. 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 connected to the differential amplifier
U2C, which eliminates any common