Receiver Part; Receiver Front End; If Part - Ericsson EF738 Service Manual

Technical Description

Receiver Part

The front-end stage is the first link in the receiver chain. It's purpose is to amplify
the RF and down-convert it to the 1st IF frequency and to set the RX system noise
figure. The desired input frequencies are within the range 917.0125 to 949.9875
MHz. The selected frequency is down-converted to 45 MHz and amplified. This
section includes a low-voltage RX front-end chip, "Low-voltage ANNIKA", com-
prised of a low noise amplifier, a mixer, and the 1st image filter.

Receiver Front End

ANNIKA: RF-amplifier
The input of the integrated low-noise amplifier is matched for optimal performance
regarding minimum noise level and second order intermodulation performance.
It also provides a 50Ω load to the duplexer.
BP-filter
This bandpass filter is a SAW filter needed for spurious response rejection (suppres-
sion of the first image frequency). The filter also suppresses the local oscillator
backwards to the antenna.
ANNIKA: Mixer
The integrated mixer in the same package as the LNA includes an internal buffer for
the Local Oscillator (LO) which allows low LO input power and immunity from
variations in LO input power. At the RF port, an inductor rejects the noise coming to
the mixer directly at the IF band. The output of the mixer is converted to a higher
impedance to match the crystal filter in the following IF circuitry.

IF Part

The IF/AF part is the second link in the receiver chain. It performs the major part of
the RF amplification and all the channel filtering. The 1st IF at 45 MHz is down-
converted to 450 KHz and FM demodulated in Bertindy. The IF IC generates a DC
voltage (RSSI), which is logarithmically proportional to the strength of the received
signal.
F r o m
A N N I K A
Mixer
14
IF/AF IC
2nd Mixer
2nd LO
4 5 M H z from
Filter B E R T I N D Y
Figure 7: IF/AF Link
IF Amp
1st 2nd
4 5 0 K H z 4 5 0 K H z
Filter Filter
To
Digit Discr
Limiter &
De-Emphasis
( B E R T I N D Y )