Tait T830 Series, Service Manual

Page: 42 / 284

31/01/96 Copyright TEL
2.2 Receiver Front End
Figure 2.2 T835 Front End, IF and Mute Block Diagram
The incoming signal from the N-type antenna socket is fed through a 7-pole, low pass
filter with a cut frequency of approximately 200MHz. This low loss filter (typically less
than 0.5dB over 136-174MHz) provides excellent immunity to interference from high
frequency signals.
The signal is then further filtered, using a notched doublet (L304, L305) which provides
exceptional image rejection, before being amplified by approximately 12dB (Q302). The
signal is then passed through a further doublet (L309, L310) before being presented to
the mixer.
Each sub-block within the front end has been designed with 50 ohm terminations for
ease of testing and fault finding. The overall gain from the antenna socket to the mixer
input varies from 0-2dB.
t
∆
Mixer
2-Pole
Crystal
Filter
Buffer
Amp
RSSI
Comparator
RF In
136-174MHz
Harmonic
Filter
Doublet
Filter
Doublet
Filter
RF
Amp
Local Oscillator
(From Synthesiser)
Diplexer
Post
Mixer
Amp
4-Pole
Crystal
Filter
IF
Amp
2dB Pad
21.4MHz
Mixer
(NE612)
Ceramic
Filter
IF
Amp
Limiter
(NE614)
FM
Detector
(NE614)
Low
Pass
Filter
21.4MHz 455kHz
2nd LO
20.945MHz
Amp
Active
Filter
LF
Amp
Diode
Detector
Smoothing
Filter
Gain
Adjust Mute
Adjust
Gating
Delays
Comparator
Comparator
Mute
Adjust
Buffer
Amp
Rx Gate Out
(To Audio
Processor)
Audio Section
Option 1
Option 2
Link
RSSI Output
Voltage
(To D-Range
Socket)
Audio
(To Audio
Processor)