IF System

130 MHz Conversion
Overview

The output of the C-41 PIU is given to the input of C42 & C43 PIU simultaneously.The C-42 PIU acts as a 130 MHz converter converting the 70 MHz frequency signal at its input into 130 MHz signal. Initially it mixes the signal from Local Oscilator 2 by means of a mixer.

Then the signals are fed to a 32 MHz Band pass SAW filter.The Band Pass SAW filter allows a band of frequency 32 MHz to pass. The output of this stage is now fed to a Low Pass Filter.

The Low Pass Filter passes al the signals who have the frequency less than 150 MHz. The processed signals are fed to the Voltage Controlled Amplifier with Pre-Attenuator. The gain of this amplifier is 9 dB Default but which can be controlled by using feedback from the next circuit i.e. Automatic Leval Control.

The Automatic Leval Control (commonly refferd as ALC) controls the level of the signal at its input. The gain to be achieved by using ALC can be varied and is set by proper adjustment of 2dB,4dB,8dB, 16dB controls on the control panel. The ALC gives feedback to the preceding stage i.e. Voltage Controlled Amplifier and the gain of amplifier is adjusted to have proper output

Block Diagram
Second Mixer

The second Mixer unit converts RF signals into the fixed Fi 130MHz. For this conversion, Amplitude Modulation is used. A device SBL1MH is used for modulation. The SBL1MH is Double Balanced Mixer Module. Amplification of the signal is also done with the help of MAR3 Amplifier. The Mixer module is referred as C65 and named as IIMIX. The default gain of mixer IC is 20dB.

For the datasheet of mixer IC Click Here

Saw Filter Bank

The SAW stands for Surface Acoustic Wave. The SAW Filter Bank used in C42 PIU is made for 130MHz.The unit is called as C66.

Some specifications about the 130MHz SAW filter:

  • -Insertion loss=28.14 dB
  • -Delay=0.582948 microSec
Some of the important characteristics are enlisted below.

Some important components of the SAW Filter Bank are enlisted below:

150 MHz Low Pass Filter

The Low Pass Filter 150MHz is a Chebyshev type filter. The filter is of order 3 and the filter is called LC Filter. The unit is recognised as C68. The cut-off frequency is 150MHz. Some of the important characteristics are enlisted below.

Automatic Level Controller

The new design incorporates a new unit AD8367. This device can also used in Variable Gain Amplifier Mode (VGA).

The new ALC circuit has been installed in the IF system.

The ALC IC is in the middle portion of the Gain-Loss Circuit followed by MAR4 amplifier.The Amplifier subsequent to ALC is MSA-0520 which provides the proper operating point, helps in preserving the dynamic range due to its high 1dB compression specification. the input and output transformers are for matching the ALCIC to 50 ohms, since the device is originally designed to perform at 200 ohms. Different paths are needed for ALC ON and OFF as the different requirements of gain/loss distributions. So in case of ALC ON path, the matching is done by attenuator and not by transformer.

The ALCIC is used as Variable Gain Amplifier in ALC OFF mode. The gain control interface of ALCIC runs from 50mV to 950mV to get the gain variation of +42.5dB to -2.5dB. Seven bits of DAC are used with a range from 0 to 1.28V so that we get 10mV step for each LSB and in turn 0.5dB gain variation.

Power Supply

The power supply is for the components generally provided in the chassis itself for the IF system..