Instruction Manual

6X Series Liquid-Cooled Transmitter Chapter 4, Circuit Descriptions
Instruction Manual, Rev. 0 4-1
Chapter 4
Circuit Descriptions
4.1 Exciter, Upconverter, Driver Tray Boards
4.1.1 Frequency Agile Upconverter Board (1309695)
The board takes a 44 MHz or 36 MHz IF signal and converts it to a TV channel in the
range of 54-860 MHz. The 44 MHz or 36 MHz IF input signal, (≈-8dBm level), is
connected to J6 on the board. The IF first passes through a frequency response pre-
corrector, consisting of R145, C188, R 146 and C189. The pre-corrector circuit
compensates for any response variation in the ceramic filter used to pick the
appropriate conversion sideband. The pre-corrected signal is then converted to a
second IF centered at 1044 MHz using the mixer U16, the 1.044 GHz filter U18 and
associated components. The mixer IC U16 has a 1 GHz LO1 connected to the LO
input. The 1.044 GHz signal is next applied to a second mixer, U15, where it is
mixed with the LO2 signal and converted to the final RF channel frequency. The RF
output is then sent to a low pass filter that removes unwanted conversion products
above 1 GHz, amplified by U21 then passed to another low pass filter. The filtered
output is amplified by U20 and connected to J7 the RF output jack for the board at
≈-3dBm level.
The upconverter has two local oscillators, LO1 and LO2. The LO1 oscillator consists
of U1, U2, U5, U6 and amplifiers U3 and U4. The LO1 oscillator operates at 1 GHz
for 44 MHz IF inputs and is used to convert the signal to 1044 MHz. In 36 MHz IF
systems, this oscillator circuit operates at 1.008 GHz which also converts the signal
to 1044 MHz. The Red LED DS4 will light if the PLL for the LO1 oscillator is not
locked.
The second LO, LO2, consists of two VCOs, U26 and U31, that are used to generate
the LO2 signal. One VCO operates from 1.1-1.5 GHz, for the VHF frequency band,
and the second from 1.5-1.9 GHz, for the UHF band. The Red LED DS2 will light if
the PLL for the LO2 oscillator is not locked.
Both of the LOs, LO1 and LO2, are locked to an on board 10 MHz VCXO reference
signal. The 10 MHz VCXO circuit consists of U36, U39, the VCTCXO Y1 and
associated components. When an external 10 MHz reference signal is applied to J10
on the board, the internal VCXO is locked to the external 10 MHz. Otherwise, the
internal VCXO is free-running. The Red LED DS6 will light if an External 10 MHz
reference is present. The Red LED DS3 will light if the PLL for the 10 MHz oscillator
is not locked.
4.1.2 Downconverter Board (1311103)
A sample of the transmitter’s RF output is applied to J6 on the downconverter board
at an input level of –10 to +10 dBm. The signal is fed through a matching network
and connected to the pin diode attenuator circuit consisting of DS1, DS2 and
associated circuitry which allows the operator to adjust the gain of the
downconversion path using R1. The RF is then amplified by U2 and converted to the
first conversion IF of 1044 MHz by the mixer U4 and the filter IC U5. Also connected
to the U4 mixer is the variable LO, 1.5 to 1.9 GHz for UHF, which is generated on the
external frequency agile upconverter board that mixes with the output RF frequency
to produce the 1044 MHz first conversion IF output at J9, typically -25 dBm.