User's Manual

300-Watt Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT830A, Rev. 1 4-13
(digital) calibration control that is
adjusted for a 100% digital reading, to
amplifier U2B. The amplified digital
output is connected to comparator U2C.
The other input to U2C is the level set by
aural null adjust R51, which is not used
with digital. Pots R51 and R20 should be
set full CCW. The offset null adjust R48
is adjusted for 0% digital power with the
transmitter in Standby. The adjusted
output is amplified by U3D and
connected to the other input of U2C. The
output of U2C connects to J6, pins 2 and
3, which supplies the forward digital level
output to the front panel meter for
monitoring.
The scrambling circuits are not used with
digital operation.
4.1.8.2 Reflected Level Circuit
A reflected power sample is applied to J3
of the board and is detected by diode
detector CR7 and U3B. The detected
output is fed through R39, the reflected
calibration pot, which can be adjusted to
control the gain of U3C. The output of
U3C connects to J6, pin 7, which supplies
a reflected power level output to the
front panel meter.
4.1.8.3 Voltages for Circuit Operation
The ±12 VDC is applied to the board at
J5. +12 VDC is connected to J5, pin 3,
and is isolated and filtered by L4 and C34
before it is connected to the rest of the
board. The +12 VDC also connects to U5,
a 5-VDC regulator that provides the
voltage needed to operate U4. The -12
VDC is applied to J5, pin 1, and is
isolated and filtered by L5 and C35
before it is connected to the rest of the
board.
4.1.9 (A11) UHF Upconverter Board
(1265-1310; Appendix D)
The UHF upconverter board provides
upconversion processing by mixing the IF
and LO signals in mixer Z1 to produce
the desired RF frequency output. The RF
output is connected through J3 to an
external filter and applied back to the
board at J4 where the gain is set by R10.
The RF is amplified and connected to the
RF output jack of the board at J5.
The IF signal (0 dBm) enters the board at
J1, an SMA connector, and is applied
through a filter circuit consisting of L10
and C25 to C28 to a matching pad. This
pad consists of R1, R2, and R3, which
presents a relatively good source
impedance, and feeds the signal to pins 3
and 4, the I input of mixer Z1. The local
oscillator signal (+13 dBm) from the x8
multiplier connects to the board at jack
J2, an SMA connector, through a UHF
channel filter and is connected directly to
pin 8, the L input of the mixer.
The frequency of the LO is the sum of the
IF frequency above the required visual
carrier. For instance, in system M, the IF
visual frequency is at 45.75 MHz and the
relative location of the aural would be 4.5
MHz lower, or 41.25 MHz. For digital
applications, the LO is the center
frequency of the digital channel added to
the 44-MHz IF frequency. By picking the
local oscillator to be 45.75 MHz above
the visual carrier, a conversion in
frequency occurs by selecting the
difference product. The difference
product, the local oscillator minus the IF,
will be at the required visual carrier
frequency output. There will also be other
signals present at the RF output
connector J3 at a lower level. These are
the sum conversion product: the LO and
the IF frequencies. Usually, the output
product that is selected by the tuning of
the external filter is the difference
product: the LO minus the 45.75-MHz IF.
The difference product has its sidebands
flipped so that the visual carrier is lower
in frequency than the aural carrier.
If a bad reactive load is connected to the
mixer, the LO signal that is fed through it
can be increased because the mixer no
longer serves as a double-balanced
mixer. The mixer has the inherent
property of suppressing signals that may