Nautel XR12, Руководство по устранению неисправностей, Страница: 89 / 283

XR12 Troubleshooting Manual Detailed Circuit Descriptions
Issue 3.0 2009-07-28 Page 2-21
RF drive b+ current limiter
The RF drive B+ current limiter is effectively a voltage controlled variable resistor in the RF drive
amplifier's B+ voltage path. The magnitude of the control voltage applied to the base of the reference
power transistor is determined by circuitry which monitors the RF drive signal's voltage and current.
No Limiting:
• Under normal conditions, the ‘reference’ power transistor will be turned on and operating in
its saturation region due to the voltage across R31. The reference transistor’s emitter-collec-
tor voltage will be minimal (0.3 V) and unrestricted current from the RF drive volts supply
will flow to the RF drive amplifier (see
“RF drive amplifier” on page 2-20 ). If the RF load on
the RF drive amplifier is not tuned properly, current will be limited by one of two out of tol-
erance parameters - excessive RF drive current or RF drive voltage.
High RF Drive Current Limiting:
• Resistors R20, R29 and transistor Q3, in conjunction with the ‘reference’ power transistor,
form a circuit which limits the current drawn by the RF drive amplifier to 1.0 A. When the
RF drive current flowing through the parallel combination of R20/R29 exceeds 1.0 A, Q3's
base-emitter voltage will exceed 0.64 V. Q3 will turn on and conduct current, resulting in an
increased voltage across resistor R31. This voltage, proportional to the level of current flow-
ing through R20/R29, is applied to the ‘reference’ transistor's base. The ‘reference’ transistor
drops out of saturation and enters its linear region. The voltage across its emitter-collector
junction will increase, limiting the current drawn by the RF drive amplifier.
High RF Drive Voltage Limiting:
• Diode CR15, resistors R30, R31 and capacitor C19, in conjunction with the ‘reference’
power transistor, form a circuit which acts to limit the tuned RF drive peak voltage to
approximately 100 V. When the tuned RF drive voltage exceeds approximately 61 V, CR15
begins to conduct during the tuned RF drive signal's positive half-cycles. The current con-
ducted through CR15 is low-pass filtered by R30 and C19, resulting in an increased voltage
drop across resistor R31. When the peak tuned RF drive voltage reaches 100 V, the voltage
across R31 is sufficient to cause the reference transistor to enter its linear region. The voltage
across its emitter-collector junction will increase and cause the current drawn by the RF drive
amplifier to decrease, limiting the tuned RF drive voltage to approximately 100 V.