User Manual
AMPLIFIER CONTROLLER BOARD REV.5
PUB12-030 Rev. 1 Feb 14, 2013 12-030-24 Amplifier Controller Board Rev.5
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. Jumper E10 is a
three position jumper that can select this filter. The second position of E13 allows a filtering capacitor to be place
in the circuit to eliminate the high peaks in broadcast signals such as DVB which have high Peak to Average
ratios.
The inverted and amplified video signal is fed to a peak detector comprised of CR4, C22 and R47 through resistor
R42 (1k ohm). R42 will effectively form a RC charging circuit that averages the signal and eliminates any
momentary peaks.
Because a single supply op-amp is used at U5, the output seen on U5 pin 1 and TP3 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 three available feedback resistors
R56 (2.7K ohms) or R59 (8.2K ohms) or R146 (7.5K ohms) using jumper E11. When the lowest gain is desired, E11
is placed in the LO position selecting R56 in parallel with R146. When the middle gain is desired, E11 is placed in
the MID position selecting R59 in parallel with R146. When the highest gain is desired, E11 is not installed leaving
only R146 in the feedback path.
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.
5.7 J13 CHANNEL – REFLECTED
The RF sample is applied to input J13 and is terminated by R74 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 E12 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. Jumper E13 is a three position jumper that can select this filter. The second position of
E13 allows a filtering capacitor to be place in the circuit to eliminate the high peaks in broadcast signals such as
DVB which have high Peak to Average ratios.
The inverted and amplified video signal is fed to a peak detector comprised of CR8, C30 and R71 through either
resistor R66 (1k ohm) where R66 will effectively form a RC charging circuit that averages the signal and
eliminates any momentary peaks.
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.