Receiver Front End - Tait T880 II Series Adjustment Manual

B2.4 T885 Circuit Operation
2.2

Receiver Front End

(Refer to the front end, IF section and audio processor circuit diagrams (sheets 4, 3 and 2
respectively) in Section 6.2.)
RF In
800-960MHz
Harmonic Helical
Filter Filter
2-Pole
Crystal
Filter
45MHz
Active LF Active
Filter Amp Filter
Gain
Adjust
The incoming signal from the N-type antenna socket is fed through a 7-pole, low pass
filter with a cut frequency of approximately 1.2GHz. This low loss filter (typically less
than 0.5dB over 800-960MHz) provides excellent immunity to interference from high
frequency signals.
The signal is filtered again, using a high performance helical resonator doublet (#H900)
which provides exceptional image rejection, before being amplified by approximately
7dB (Q401). The signal is then passed through a further helical filter doublet (#H400),
after which it is amplified again by 8dB (Q403). It is finally filtered by #H401 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 is approximately 8dB.
10/07/00
RF Helical RF
Amp Filter Amp
Mixer IF Ceramic
(MC3372) Amp Filter
455kHz
2nd LO
44.545MHz
Link:
Diode
Detector Comparator
Smoothing
Filter
Figure 2.2 T885 Front End, IF and Mute Block Diagram
Local Oscillator
(From Synthesiser)
3dB Pad
Helical
Filter Diplexer
Mixer
Limiter/
Detector
(MC3372)
RSSI
Comparator
Carrier Mute
Noise Mute
t ∆
Gating
Delays
Mute
Adjust
M880-00
Post 4-Pole
Mixer Crystal IF
Amp Filter Amp
45MHz
Low
Pass
Filter
Audio
(To Audio
Processor)
RSSI Output
Voltage
(To D-Range
Connector)
Buffer
Amp
Mute
Adjust
Rx Gate Out
(To Audio
Processor)
Comparator
Audio Section
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