IF System
70 MHz Conversion
Overview
The C41 unit is used in the First IF (I-IF) Conversion. The C41 unit is divided into C41-A and C41-B. The basic function of the C41 PIU is to convert the R.F. signals received from the Front End into 70MHz frequency. Both the C41-A and C41-B units perform the same operation.
The C41 PIU provides attenuation and Bandwidth selection facility for the IF system.
For the circuit diagram of the PIU C-41 -- click here
Circult Diagram
First Mixer
The first Mixer unit converts RF signals into the fixed Fi 70MHz. For this conversion, Amplitude Modulation is used. A device SRA2010MH is used for modulation. The SRA2010MH is Double Balanced Mixer Module. Amplification of the signal is also done with the help of MAR3 Amplifier. The Mixer module is referred as C61 and named as IMIX. The default gain of mixer IC is 20dB.
For the datasheet of the mixer IC ..Click Here
Programmable Attenuater
The Programmable Attenuator serves the purpose of having 0-30dB attenuation in the steps of 2dB. An IC AT001D4-25 is used for this purpose.The IC is an 8-bit controlled one and the controls can be adjusted to have a gain of 2,4,8,16 dB.
Analog 8-bits are designed in the same unit using Op-Amp cirsuitry.The 8-bits are controlled by using 4 control bits.
Some of the important characteristics are enlisted below.
100 MHz Low Pass Filter
The Low Pass Filter 100MHz is a Chebyshev type filter. The filter is of order 3 and the filter is called LC Filter. The unit is recognised as C63. The cut-off frequency is 100MHz. Some of the important characteristics are enlisted below.
SAW Filter Bank
SAW stands for Surface Acoustic Wave. The First IF (I-IF) Conversion is done from the R.F. signals to 70MHz. For 70MHz frequency, different bandwidth SAW filter is used. These filters are commonly used for commercial purpose. In the unit of SAW filter Bank (C64); 6, 16 MHz filter module is used and for 32MHz selection, the signal is directly sent through particular attenuator to produce compensation of filter module loss. Pin Diode is used for selection of bandwidth. +9 to +12 volts can make required section ON. The unit is called as C64.
Some specifications about the 70MHz SAW filter:
- -Insertion loss=22.44 dB
- -Delay=1.660 nSec
- -Bandwidth=20MHz
Some of the important characteristics are enlisted below.
Some important components of the SAW Filter Bank are enlisted below:
Power Supply
The power supply for the components is generally provided in the chassis itself for the IF system.