Receiver Back-End - Ericsson EF738 Service Manual

Technical Description

Receiver Back-End

The IF/AF circuit is an integrated bipolar circuit containing a mixer, IF amplifier,
limiter, RSSI circuits, and FM detection circuit. The rest of the required functions
are located in BERTINDY.
The 1st IF frequency of 45 MHz is fed to the 2nd mixer where it is down-converted
to 450 KHz (2nd IF) in the IC. The 2nd LO signal at 44.55 MHz for the mixer
comes from BERTINDY. Third harmonic of the reference oscillator frequency
(14.85 MHz) is used as the 2nd LO signal.
The 450 KHz signal is filtered through a ceramic 2nd IF filter and fed to the input of
the IF amplifier stage. The output signal from this stage is filtered through a second
ceramic 2nd IF filter and is then applied to the limiter. The resulting 450 KHz
square-wave signal is fed to the digital discriminator in BERTINDY where it is
demodulated and de-emphasized before being passed to the ANTON IC for further
audio processing.
Band Pass 1st IF Filter (2nd Image Filter)
This crystal filter is needed for spurious response rejection (suppression of the 2nd
image frequency) and inter-modulation rejection. The crystal filter is a band pass
filter with a center frequency of 45 MHz. Some channel filtering is also performed
in this filter.
Channel and Noise Reduction Filters
Two bandpass 2nd IF filters are needed for broadband noise reduction and desired
channel selection. Each filter is a ceramic 4-pole bandpass filter with a center fre-
quency of 450 KHz.
450 KHz Output
From the limiter output, the 450 KHz square-wave signal is fed to the digital dis-
criminator in BERTINDY for demodulation. This signal is also used by the AFC
algorithm in determining the frequency error of the 14.85 MHz reference relative to
the received signal.
RSSI Output
A voltage which is logarithmically proportional to the RF input power of the
received signal is produced in the IF/AF IC.
RSSI Range
The lower end of the useable range is controlled primarily by the RF/IF gain of the
receiver while the upper end of the RSSI voltage is controlled by the supply voltage
of the IF/AF chip.
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