Power Control Circuit & Pa - Tait T880 II Series Adjustment Manual

C2.12 T881 Circuit Operation
2.8 Power Control Circuit & PA
(Refer to Figure 2.1 and the power control and PA circuit diagrams (sheets 5 & 4) in Sec-
tion 6.2.)
The output power of the PA is maintained at a constant level via a power control loop
applied to the bias pin of the RF power module (#IC400 pin 2). The forward and reverse
RF power levels are sensed via a dual directional coupler and detector diodes (D400,
D402 in the PA cavity). The detected DC signals are buffered (IC500 pins 3 & 5),
summed with a very small bias current and then fed to the control integrator op-amp
(IC510 pin 9). The purpose of the small bias current (provided by R559) is to raise the
voltage potential slightly at the summed node. This is necessary to ensure the output
voltage at IC510 pin 8 is zero when the transmitter is not keyed on.
Note: Forward and reflected power signals are summed so that, under high
VSWR, the power control will turn the output RF level down.
To reduce the spurious output level when the synthesiser is out-of-lock, the Tx-Reg. and
Lock-Detect signals are gated to inhibit the PA control circuit and to switch off the RF
signal at the input to the RF power module. This is achieved by a PIN diode switch
(D308). There is a 5.5dB (R370, R372, R383) and a 7.5dB (R390, R391, R392) pi-attenuator
at the input and output of the PIN switch to attenuate the level of the VCO output and
also provide good isolation between the VCO and RF power module. A level shifter is
also implemented to enhance the transient performance by improving the dynamic
range of the module bias voltage.
Cyclic keying control is provided by additional circuitry consisting of a ramp, several
gate and time delay stages:
• Q505, Q508, IC510
• Q500, Q501
• Q502, Q506, Q510, Q512, Q513
• Q507, Q511, Q515
This is to allow the RF power circuits to ramp up and down in a controlled manner so
that minimal adjacent channel interference is generated during the transition.
A temperature sensor (R450) is mounted on the input flange of the RF power module to
monitor the flange operating temperature. When a pre-determined temperature thresh-
old is exceeded, a protection circuit (IC510 pin 7, Q516) switches on to reduce the RF
output power to a preset level. The purpose of the protection circuit is to prevent over-
heating, as the RF power module is rated for a maximum flange temperature of 100°C.
The RF power module is a 5W device which requires an input drive of approximately
+6dBm. L402 and C430 are provided to match the impedance of the output low pass fil-
ter to the impedance of the module. A DC control signal is applied to the RF signal path
via %L405 if cyclic keying is required with a Tait power amplifier.
10/07/00
trapezoidal power ramping generator
Tx-Reg. and Lock-Detect gate
PIN switch drive plus delay
level shifter plus delay.
M880-00
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