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Page 36
4.3 Tuner Board (IRIG band).
Overview:
The Tuner board includes a double heterodyne RF to IF down conversion with 110 dB gain
adjustment range to provide a 70 MHz pre-filtered and regulated signal. Its design is mainly based on
the S band architecture (cf. 4.2 )
Description:
The RF input bandwidth for the different band (S/L/Cif/P) is delimited by a bank of RF
filters with a Low Noise Amplifier to perform the input stage noise figure. The synthesizer and a first
mixer perform the RF input bandwidth frequency conversion down to IF-1 fixed frequency. A ceramic
filter delimits the bandwidth to 40 MHz and a Variable Voltage Attenuator achieves a part of the AGC
gain (30dB).The local oscillator and a second mixer performs the IF-1 frequency down conversion to
70MHz (IF-2). An efficient IF pre-filtering is performed by a bank of octave SAW filters from 500 KHz to
40 MHz, software selectable. Two Variable Voltage Attenuators achieve the last part of the AGC gain
(80dB). Some High Linearity gain blocks are properly located along the entire receiver chain, and
warrant the compromises between total gain, noise figure and linearity.
Three IF outputs are available. Two could be used as test analog outputs or as outputs towards an IF
recorder and the last one is used to be digitized by the ADP3 board. The Variable Voltage Attenuators
are digitally controlled by the tuner board across FPGA and DAC. The attenuator linearity response is
performed by a compensation table stored in flash memories. Calibration is done at factory and cannot
be modified by the user.
The synthesizer is designed around a typical low phase noise Fractional-N/Integer-N PLL architecture
with a frequency step of 1MHz. The local oscillator follows the same design but with a frequency step of
100 kHz. Hence, the global frequency step of the RF board is 100 kHz. A smallest resolution can be
achieved digitally by the ADP3 board.
The FPGA is used as an interface to the PCI express bus and makes easier the control of the board in
the RTR system. The main functions are OL programming, reading and writing data in flash memory
