User Manual

MXi AMPLIFIER CONTROLLER BOARD REV.2
PUB06-82 Rev 1 July 17, 2007 06-82-43 MXi Amplifier Controller Board Rev.2
Finally, Q4 serves as a low impedance video source. Jumper E9 is used to introduce color subcarrier trap FL1. An
alternate trap consisting of C43, C25 and L1 (marked with asterisks) can also provide this function. The alternate
trap can also be easily changed in values to provide different frequency traps for other systems (like PAL).
Note: only one of either FL1 or the Inductor/Capacitor traps would be installed but never both.
Transistor Q2 serves as a buffer and Q1 serves as another buffer. Following is filter FL2, which removes the aural
carrier. An alternate Inductor/Capacitor trap consisting of L2, C44 and C26 is also provided.
The inverted and amplified video signal is fed to a peak detector comprised of CR4, C22 and R47 through either
resistor R42 (1k ohm) or resistor R46 (33 ohm). Jumper E8 will determine which resistor is in the path and is silk
screened as Pk (peak) or Av (average) on the circuit board. If the RF signal is Digital, then E8 is installed in the
Av position where R42 will effectively form a RC charging circuit that averages the signal and eliminates any
momentary peaks. If the RF signal is analog, then E8 is installed in the Pk position, where R46 will effectively
pass the entire detected envelope to the peak detector.
Because a single supply op-amp is used at U5, the output seen on TP1 and TP2 contains 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 R58 as near as possible to ground/earth. The setting can be seen when using a DC
coupled scope. Adjust R58 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 U5A can be adjusted by selecting one of the two available feedback resistors R56
(5.1K ohms) or R59 (10K ohms) using jumper E11. When a lower gain is desired, E11 is placed in the LO position
selecting R56 and when a higher gain is desired, E11 is placed in the HI position selecting R59.
The two test points TP1 and TP2 are provided to support the external aural detector. When this option is not
installed, a jumper is placed between J11 pin 5 and J11 pin 6 which placed these two test points at the same
potential.
Outputs from unity gain op-amps U13A, U13B, U13C and U13D drive the forward power metering circuits. The
output of U13A provides telemetry to the CPU A/D converter input. The output of U13B provides telemetry to the
AGC (VSWR) circuitry. The output of U13C provides telemetry to the remote controls and U13D provides telemetry
for the optional transmitter switch connections. Bench test calibration consists of adjusting R48 with rated, properly
modulated input while observing the voltage at TP3, which should read 4.0 volts DC for full scale calibration.
6.9 J13 CHANNEL – REFLECTED
The RF sample is applied to input J13 and is terminated by R74 in parallel with R75 for an impedance of around
50 ohms. CR9 and Q6 form an envelope detector. CR9 is forward biased slightly by R67 and R76 to overcome
CR9 conduction threshold voltage, thereby improving detection linearity. Q6 is forward biased by R67 as well, and
when RF is applied, Q6 is driven in the direction of turn off during each positive-going half cycle, thus causing its
emitter voltage to become more positive and in effect forming a linear envelope detector.
C32 utilizes the lead inductances of CR9 and Q6 to form a Tee network, which provides a matching section that
improves the UHF signal transfer between the devices. Q4 and CR5 have similar temperature coefficients and the
opposing connection of the two in this back-to-back configuration provides temperature compensation.
Finally, Q6 serves as a low impedance video source. Jumper E13 is used to introduce color subcarrier trap FL3.
An alternate trap consisting of C33, C45 and L3 (marked with asterisks) can also provide this function. The
alternate trap can also be easily changed in values to provide different frequency traps for other systems (like
PAL). Note that only one of either FL1 or the Inductor/Capacitor traps would be installed but never both.
Following is filter FL4 which removes the aural carrier. An alternate Inductor/Capacitor trap consisting of L4, C46
and C32 is also provided.
The inverted and amplified video signal is fed to a peak detector comprised of CR8, C30 and R71 through either
resistor R66 (1k ohm) or resistor R70 (33 ohm). Jumper E12 will determine which resistor is in the path and is silk
screened as Pk or Av on the circuit board. If the RF signal is Digital, then E12 is installed in the Av position where
R66 will effectively form a RC charging circuit that averages the signal and eliminates any momentary peaks. If
the RF signal is analog, then E12 is installed in the Pk position where R70 will effectively pass the entire detected
envelope to the peak detector.