Chapter 2 73
Block Diagrams
A12 Analog IF Assembly
A12 Analog IF Assembly
The A12 analog IF assembly receives a 21.4 MHZ signal from the RF
Board.The 21.4 MHz signal is amplified, pre-filtered, mixed with the
28.9 MHz third LO to provide a 7.5 MHz final IF, amplified again, and
post filtered.
The analog IF assembly also provides anti-alias filter protection, and
routes the 21.4 MHz signal, that is used in ranging, to the A10 digital
IF board.
The A12 analog IF assembly also performs the following functions:
• prefilters the 21.4 MHz second IF frequency
• uses a variable gain amplifier to provide the optimum level to the
mixer
• provides a 21.4 MHz anti-alias filter
• provides the final down conversion
• performs post down-conversion filtering
• provides calibrator oscillator and sample rate oscillator
• provides a clock generator
• performs the third LO conversion
•triggering
• provides the power supply switching frequency generator
Pre-filters
LC and XTAL prefilters are used to attenuate the out of band signals
that are not in the measurement bandwidth, in order to improve TOI
distortion and spurious responses. The pre-filters are adjusted to 2.5
times the resolution bandwidth; if the instrument resolution bandwidth
is at 1 kHz, the XTAL pre-filter will be at 2.5 kHz.
Variable Gain Amplifier
The gain of the amplifier is adjusted electronically to provide +2 dB of
gain from the front panel RF input to the A12 output.
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