NGT—Technical description (2010)
88
NGT Transceiver System Technical Service Manual
Second mixer
Mixer IC101 is a combined amplifier/balanced mixer. The 45 MHz signal is mixed with
the local oscillator LO2 (44.545 MHz) applied to pin 5. The mixing produces a second
IF centred at 455 kHz.
DC to the mixer output is provided by the +10 V rail, which is connected to the centre
tap of the 455 kHz output transformer T202 (sheet 2). In Receive mode, the +10 VRx
rail provides DC via R132 to IC101 pin 4, and bias via R131 to IC101 pin 11.
Noise limiter
The 455 kHz output of IC101 pins 3 and 14 (sheet 1) is applied to the balanced input of
the high-gain amplifier IC201 pins 4 and 6. The balanced outputs at pins 1 and 8 are
connected to a tuned auto transformer T201 (455 kHz). The output of T201 goes to the
base of the active rectifier V201. Noise bursts produce positive-going pulses at the
collector of V201, and via V202, trigger the monostable flip-flop IC202/B/C. The pulse
width is determined by C210, R210 and R211 (nominally 250
µ
s).
The outputs of IC202/B/D pins 11 and 4 produce complementary pulses that are
connected to the FET gates of V204 and V205. These gate out the noise bursts from the
455 kHz signal. With V204 on and V205 off, the IF signal is passed to the sideband filter
Z201. When a noise burst is present, V204 is switched off and V205 is switched on,
blocking the IF signal for the period of the gate pulse.
The average DC component of the collector current of V201 is used as an automatic gain
control to IC201. It is applied via R206 to pin 5. This ensures that only the noise burst
signal triggers the flip-flop IC202/B/C. The AGC decay constant is set by the
components C206 and R205.
In Transmit mode, IC202 is disabled (see
).
The 455 kHz output of IC101 pins 3 and 14 (sheet 1) is applied to the balanced input of
the high-gain differential amplifier IC201 pins 2 and 13. The balanced outputs at pins 7
and 8 are connected to a tuned auto transformer T201 (455 kHz). The output of T201
goes to the base of the active rectifier V201. Noise bursts produce positive-going pulses
at the collector of V201, and via V202, trigger the monostable flip-flop IC202/B/C. The
pulse width is determined by C210, R210 and R211 (nominally 250
µ
s).
ALTERNATIVE TEXT
, or
NOTE
If the PCB issue status is -06 or earlier, see drawings
and
ALTERNATIVE TEXT
ALTERNATIVE TEXT
NOTE
If the PCB issue status is -06 or earlier, see drawings
and
Summary of Contents for NGT 2010 RF
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