NGT—Technical description (2010)
NGT Transceiver System Technical Service Manual 79
RF unit—RF/IF PCB transmit path
Transmit/Receive switching
The PTT command from the handset via the CIB causes IC405 to drive pins 12 and 13
(Rx Switch and Tx Switch). This switches off V419 and V420 and switches on V421 and
V422, thus causing +10 VTx to be active. This switches all circuits to the Transmit
mode.
The PTT command from the handset via the CIB causes IC408 to drive pins 12 and 13
(Rx Switch and Tx Switch). This switches off V415 and V416 and switches on V417 and
V418, thus causing +10 VTx to be active. This switches all circuits to the Transmit
mode.
Transmit modulator
The audio goes to the Tx Level potentiometer R256 to set the exciter output level at J2 to
650 mV p–p. It is passed to the input of the balanced modulator IC204 pin 1. The
modulator is enabled when +10 VTx is applied via R254 to the bias input pin 5.
The audio is mixed with the local oscillator of 456.5 kHz (453.5 kHz for LSB). The local
oscillator is applied to IC204 pins 8 and 10. This produces a DSB output at pin 6. From
there it is passed via D205 to the 455 kHz sideband ceramic filter Z201 pin 1.
455 kHz filter and first mixer
(sheet 2) The 455 kHz sideband ceramic filter Z201, with a bandwidth of 2.5 kHz, filters
out the unwanted sideband and leaves only the wanted sideband at pin 3. From there it is
passed via the noise limiter gate V204 to a tuned auto transformer T202.
(sheet 1) The signal is passed from T202 via a resistor-matching attenuator network
R144, R145 and R146 to the input of the first balanced amplifier/mixer IC103 pins 12
and 13. IC103 is enabled by applying +10 VTx. This applies DC via R142 to pin 4, and
bias current via R143 to pin 11. The filtered +10 V rail is applied to the mixer output,
pins 3 and 14, via the centre tap of the output transformer T108.
ALTERNATIVE TEXT
Drawing 04-03106 (sheet 3)
ALTERNATIVE TEXT
Drawing 04-03378 (sheet 3)
NOTE If the PCB issue status is -06 or earlier, see drawing 04-03378 (sheet 3).
ALTERNATIVE TEXT
Drawing 04-03106 (sheet 2), or
Drawing 04-03378 (sheet 2)
NOTE If the PCB issue status is -06 or earlier, see drawing 04-03378 (sheet 2).
ALTERNATIVE TEXT
Drawings 04-03106 (sheet 1) and 04-03106 (sheet 2), or
Drawings 04-03378 (sheet 1) and 04-03378 (sheet 2)
NOTE
If the PCB issue status is -06 or earlier, see drawings 04-03378 (sheet 1)
and 04-03378 (sheet 2).
To Next Page
Loading...
Need help?
General
