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NGT—Technical description (2011/2012)
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 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). V17 switches
between 100 W and 125 W.
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 collector swing is rectified by D202 and applied to the transistor V10.
V10 conducts and limits the collector peak to 42 V to prevent damage to the output
transistors.
The battery voltage is monitored by V5. When the supply voltage (A rail) drops to
approximately 12 V, the voltage at the base of V5 falls and causes V5 to conduct. This
changes the threshold of the forward power ALC detector. The ALC detector then starts
to reduce the output power and since the control is linear, a further reduction in supply
voltage continues to lower the output power.
If the heatsink temperature exceeds 80°C (176°F), the PTC resistor R262 rapidly
increases its resistance. This causes V13 to conduct and thus reduces the reference
voltage R19. This lowers the output power and prevents the heatsink temperature from
exceeding 100°C (212°F).
The relay protection circuit is formed by IC5/B and associated components. It
simultaneously monitors the output transistor collector swing and forward detector
output of D1. If a relay failure results in no signal at the antenna output, the output of
IC5/B goes high. This lowers the gain of the input amplifiers V200 and V201 and thus
reduces the drive to the output transistors to a safe level.
The ALCs control the output from the summing amplifier IC5/A. This output passes to
IC201/B, which inverts the direction of the control signal to drive the variable resistor
FETs V202 and V203 in the gain stage (see
page 113, Gain control
stage).
NGT Transceiver System Technical Service Manual
115
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