4-14
888-2595-001
12/20/11
WARNING: Disconnect primary power prior to servicing.
Section 4 Theory of Operation
ZX Series
Each of these necessary conditions control the logic state at the gate of transistor Q4 via
diode-or network CR4A through CR4E. When the gate of Q4 is logic high for any
reason, no restart pulses will be generated by U14. The logic high condition at the gate
of Q4 also causes transistor Q5 to conduct, grounding the base of transistor Q1, thereby
preventing it from charging capacitor C46. Therefore, capacitor C46 will only charge
when the transmitter is in a state calling for restart pulses to be generated. Capacitor
C46 charges to a level high enough (+2.5V) to trigger latching comparator U27D after
approximately thirty seconds of restart attempts, thereby generating a restart fault and
preventing more restart pulses from being generated. If a restart condition is cleared
before U27D has flipped state, C46 will discharge within an approximately 30 minute
time frame for an almost full charge. The charge in C46 and the state of U27D can be
reset by a +5V pulse from the ON pushbutton or remote control on command via
optoisolator U30A, capacitors C67-C75, diode CR4F, and transistor Q6.
4.12.2 APC Circuit and Power Level Adjust
(page 3) A forward power sample from either the internal detectors on the I/O filter
PCB or an external system detector is selected via the position of jumper JP2 and
passed to electronically-controlled variable gain amplifier U27C. The gain of U27C is
varied by the amount of resistance provided by variable attenuation U11 in the feedback
network R8-U11-R34. The resistance setting of U11 is incremented/decremented
according to the grounding of pin 2 (UC = up command) and pin 7 (DC = down
command) which is in turn command by the local RAISE/LOWER pushbuttons or the
remote control raise/lower commands if remote control is enabled. As the LOWER
pushbutton S9 is pressed, U11 decreases its resistance, increasing the gain of U27.
Because U27 is inside the negative feedback APC loop, increasing its gain increases the
relative strength of the output sample and forces the APC to lower the transmitter power
to compensate.
U11 has 64 different attenuator steps, corresponding to a power control step of
approximately 1%. The power control steps are logarithmically weighted, with finer
steps near the top end of the power adjustment range.
The output from U27C passes through another variable gain stage: U24B with gain
being determined by potentiometers R10, R11, R12 and analog switch U6. The gain of
U24B changes with each transmitting mode (FM, FM+HD, HD/SL) to set the desired
maximum power level via R10, R11, and R12. The relationship between U24B and
U27C is described by the following statement:
Potentiometers R10, R11, R12 set the maximum power per mode and the raise and
lower buttons allow the transmitter to move between zero and full within that range.
The output from U24B passes through diode-or network CR6-CR9-CR10-CR12 to
APC comparator U25A. The gain adjusted forward power sample from U24B is
Содержание ZX10
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Страница 80: ...3 12 888 2595 001 12 20 11 WARNING Disconnect primary power prior to servicing Section 3 Operation ZX Series ...
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