User Manual

MXi AMPLIFIER CONTROLLER BOARD REV.2
PUB06-82 Rev 1 July 17, 2007 06-82-44 MXi Amplifier Controller Board Rev.2
Because a single supply op-amp is used at U14D, the output seen on TP4 will contain a small DC offset which must
be minimized because low level signals are near ground/earth potential. With no RF input, this offset voltage is
adjusted by potentiometer R80 as near as possible to ground/earth. The setting can be seen when using a
DC-coupled scope. Adjust R80 to move the DC level toward ground/earth potential; stop turning the potentiometer
immediately the DC ceases moving. A residual voltage offset of 20 millivolts can be expected for the LM358 family of
op-amps.
The gain of the amplifier stage at U14D can be adjusted by selecting one of the two available feedback resistors
R79 (6.2K ohms) or R82 (20K ohms), using jumper E15. When a lower gain is desired, E15 is placed in the LO
position selecting R79 and when a higher gain is desired, E15 is placed in the HI position selecting R82.
Outputs from unity gain op-amps U14A, U14B and U14D drive the reflected power metering circuits. The output of
U14A provides telemetry to the CPU A/D converter input. The output of U14B provides telemetry to the CUTBACK
and VSWR circuitry. The output of U14C provides telemetry to the remote controls. Bench test calibration consists of
adjusting R72 with rated, properly modulated input while observing the voltage at TP5, which should read 4.0 volts
DC for full scale calibration.
6.10 J12 CHANNEL – AURAL FORWARD
Note: This description only applies when the aural metering option has been included in the transmitter system.
The aural metering circuit takes a sample of detected video signal from the forward port and provides DC level
proportional to the amplitude of the aural carrier. The sample of this DC level is used to compensate visual
forward reading affected by presence of the aural carrier at the forward port.
The inverted and amplified video signal is fed to sync separator U9-2. When sync is detected, the sync output at
pin 1 delivers a positive-going composite sync pulse, which turns on Q11 whose collector then goes LOW. If no
sync is detected by U9, its pin 1 remains LOW and Q11 remains off.
After U9-1 pulse has finished, Q11 turns off and its collector output goes HIGH. This LOW to HIGH transition
activates blanking multivibrator U7 and an active low pulse is fed to Q10, turning it on.
Q2, C4, and U2A form a sample-and-hold circuit that samples the signal originating from the emitter of Q1 and which
is buffered by Q18. Sampling occurs during the back porch, and holds during the subsequent horizontal line. This
DC sample is amplified in U2A .
The sample of the signal detected by CR1 from J1 port is buffered by Q3. The wideband visual/aural signal drives
an aural bandpass ceramic filter FL3. The filter is chosen for the broadcast standard in use. C12 couples the
filtered aural CW signal to amplifier Q7. Following is a buffer Q6 and RF amplifier U4. The aural signal from U4
output is then fed to peak detector consisting of CR2, C15, R45. Q8 buffers signal before applying to sample and
hold circuit consisting of Q9, C16 and R52. Sampling aural signal during the back porch provides level
independent of video content.
Because a single supply op-amp is used at U3B, the output seen on TP3 will contain a small DC offset which
must be minimized because low level signals are near ground/earth potential. With no RF input, this offset voltage
is adjusted by potentiometer R34 as near as possible to ground/earth. The setting can be seen when using a DC
coupled scope. Adjust R34 to move the DC level toward ground/earth potential; stop turning the potentiometer
immediately the DC ceases moving. A residual voltage offset of 20 millivolts can be expected for the LM358
family of op-amps.
U3B provides DC amplification of detected signal. Following is another amplifier U5A which gain is set by
potentiometer R59, so DC level at TP4 is in the range of 7VDC. This provides enough “safety” range for the signal
not to saturate or not being high enough for 4VDC calibration at TP5. The sample of aural output is fed to
differential amplifier U3A to compensate for aural carrier influence on visual metering in internally diplexed
systems. R42 is used to minimize this influence. Jumper JPH3 set in position 1-2 enables this compensation,
otherwise JPH3 must be set in position 2-3.