User's Manual

Digital UHF Transmitter Chapter 4, Circuit Descriptions
DT835A, Rev. 1 4-4
The x8 circuitry consists of three
identical x2 broadband frequency
doublers. The input signal at the
fundamental frequency is fed through a
6-dB pad consisting of R21, R24, and
R25 to amplifier U3. The output of the
amplifier stage is directed through a
bandpass filter consisting of L8 and
C32, which is tuned to the fundamental
frequency (67 MHz to 132 MHz). The
voltage measured at TP1 is typically +.6
VDC. The first doubler stage consists of
Z1 with bandpass filter L9 and C34
tuned to the second harmonic (134 MHz
to 264 MHz). The harmonic is amplified
by U4 and again bandpass filtered at
the second harmonic by C38 and L11
(134 MHz to 264 MHz). The voltage
measured at TP2 is typically +1.2 VDC.
The next doubler stage consists of Z2
with bandpass filter C40 and L12 tuned
to the fourth harmonic of the
fundamental frequency (268 MHz to
528 MHz). The fourth harmonic is then
amplified by U5 and fed through
another bandpass filter tuned to the
fourth harmonic consisting of L14 and
C44 (268 MHz to 528 MHz). The voltage
measured at TP3 is typically +2.0 VDC.
The final doubler stage consists of Z3
with bandpass filter C46 and L15 tuned
to the eighth harmonic of the
fundamental frequency (536 MHz to
1056 MHz). The signal is amplified by
U6 and U7 to a typical value of from +2
to +4 VDC as measured at TP4. The
amplified eighth harmonic is then fed to
the SMA output jack of the board at J3.
Typical output level of the signal is +16
dBm nominal.
The +12 VDC for the board enters
through jack J4-3 and is filtered by L22
and C54-C58 before being distributed to
the circuits on the board.
4.3.3 (Optional) (A14-A1) 10-MHz
Reference Generator Board (1519-
1126; Appendix D)
The 10-MHz reference generator board
is located in (A10) the (Optional) 10-
MHz reference kit (1286-1108). The
board contains a high-stability crystal
oscillator that provides a 10-MHz
output that is used as reference
frequency for the transmitter. The
board is mounted within an enclosed
assembly that helps to maintain the
operating temperature of the oscillator
board.
The oscillator operates at 10 MHz.
Transistor Q1 is the oscillating
transistor with the frequency of
oscillation set by the crystal Y1. L2, C2,
and C3 have second-order effects on
the frequency, with C2 and C3 used to
pull the oscillator exactly on frequency.
Capacitors C4 and C5 provide the
positive feedback necessary for
oscillation.
The output of the oscillator is buffered
by Q2 and Q3. L4 and C12 form a 40-
MHz bandpass filter. The 40-MHz signal
passes through a digital divider IC U4,
which divides the signal down to 10
MHz. Transistor Q5 provides a buffered,
10-MHz output at jack J1.
The crystal is heated in an enclosed
crystal oven (HR-1) that is internally
set at 60° C. The oscillator board is
heated by a separate oven that is set at
50° C. U1 is a temperature
sensor/controller IC that monitors the
temperature of the oscillator assembly
and controls the operation of Q4 and
U2. The operating temperature of the
assembly is set by adjusting R15. If the
temperature of the assembly falls
below 50° C, U1 will bias Q4 on, which
in turn increases the amount of current
flow through U2.
The flange of U2 is thermally connected
to the heatsink of the assembly. The
temperature of the heatsink will
increase as the current through U2
increases. As a result, U2 will dissipate
more power in the form of heat, and
the temperature of the assembly will
increase. If the assembly temperature
rises above 50° C, the opposite action