Codan NGT 2010 RF Technical & Service Manual page 118

NGT—Technical description (2010)
Automatic level control
The ALC is controlled by:
•
•
•
•
•
•
Drawing
ALTERNATIVE TEXT
All the ALC inputs (except relay detection) are applied to V7 to V10. The ORed output
is connected via R50 and D14 to the positive input pin of the ALC level comparator
IC5/A pin 3. The reference voltage connected to pin 2 is firmware controlled by the
microprocessor IC2.
The microprocessor IC2 and R19 are used as a 5-bit DAC with an output range of 5 to
2.5 V. The output from R19 pin 1 is buffered by IC6/B and then divided and filtered by
R55, R56 and C151. The output is then buffered again by IC6/A and used as the
reference voltage for IC5/A and IC201/B. This allows the microprocessor IC2 to reduce
the PA output at the higher operating frequencies and ensures that the intermodulation
distortion remains within specified limits.
The output voltage of the RF bridge formed by T1 and associated components is rectified
by D1 for the forward power and by D2 for the reflected power.
The output of the forward rectifier D1 is applied via R37 to the input of V7. The sum of
the resistors between the base of V7 and ground form a resistor divider network. This
determines the nominal PEP output level when matched to a 50 Ω load.
For high power, the resistors R34 and R35 are shorted out when +5 V is applied to the
gate of the FET switch V6 and do not contribute to the chain. The nominal high-power
output, which is measured at low frequency, is set by R32 (SET HI PWR).
For low power, V6 is off, so R34 and R35 are added to the divider. R35 (LOW PWR)
sets the power to 12 W PEP.
The output of the forward detector D1 is also applied to V8 via an averaging detector
circuit formed by R40, R43 and C147. When the average of the signal exceeds the peak
detector circuit V7, V8 takes control and reduces the output power (this occurs on single
tones such as Tune mode, CW (Morse) and in some data transmissions).
When the handset PTT is active, V11 is switched off, which disables the average ALC
circuit. In these circumstances, the transceiver produces full PEP on a single tone. This is
for measurement reasons only.
The peak output drain swing is rectified by D202 and applied to the transistor V10. V10
conducts and limits the drain peak to 34 V to prevent damage to the output FETs.
84
forward power
reflected power
drain voltage swing of output stage
battery voltage
overtemperature of heatsink
failure of transmit/receive or filter relays
04-03169
NGT Transceiver System Technical Service Manual