B2.4
T855 Circuit Operation
M850-00
31/09/98 Copyright TEL
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 6.3.)
Figure 2.2 T855 Front End, IF and Mute Block Diagram
The incoming signal from the N-type antenna socket is fed through a 9-pole, low pass
filter with a cut frequency of approximately 600MHz. This low loss filter (typically less
than 0.5dB over 400-530MHz) provides excellent immunity to interference from high
frequency signals.
The signal is then further filtered, using a high performance helical resonator doublet
(FL410) which provides exceptional image rejection, before being amplified by approxi-
mately 8dB (Q410). The signal is then passed through a further helical filter doublet
(FL420) before being presented to the mixer via a 2dB attenuator pad.
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 2dB.
t∆
Mixer
Comparator
RF In
400-530MHz
Harmonic
Filter
Helical
Filter
Helical
Filter
RF
Amp
Local Oscillator
(From Synthesiser)
Diplexer
Post
Mixer
Amp
4-Pole
Crystal
Filter
IF
Amp
4dB Pad
45MHz
2-Pole
Crystal
Filter
45MHz
Mixer
(MC3361)
Ceramic
Filter
IF
Amp
455kHz
2nd LO
44.545MHz
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
Link:
Low
Pass
Filter
Buffer
Amp
RSSI
Limiter
(MC3361)
FM
Detector
(MC3361)
RSSI Output
Voltage
(To D-Range
Connector)
Audio
(To Audio
Processor)
Carrier Mute
Noise Mute
To Next Page
Loading...
Need help?
General
