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In cases where alias filtering is not necessary the E1437A can be programmed to
bypass the anti-alias filter. This allows the system to take advantage of the full 40
MHz sampler bandwidth. To avoid incorrect results, the alias filter bypass mode
should be used with caution; it is not recommended for normal operation.
Sampling ADC
The heart of the E1437A is a precision Analog-to-Digital Converter (ADC). The ADC
generates 23 bit outputs at a sample rate up to 20.48 MHz. It has very low noise
density and very low distortion levels.
Zoom and Decimation Filtering
This section uses digital circuitry to allow programmable changes in the center
frequency and signal bandwidth of the E1437A (zoom). This is done at high speed
for real-time operation.
Bandwidth is controlled by a chain of digital low-pass filters (see the diagram
below). Each of the filters reduces the bandwidth by a factor of two (decimation).
With the ADC sample rate (Fs) set to the standard internal 20.48 MHz rate, the
bandwidth choices are 10 MHz, 5 MHz, 2.5 MHz,...0.289 Hz around the programmed
local-oscillator (LO) frequency.
Real and imaginary components of the signal are each computed to 32-bit precision,
so the complex output of the decimation filtering block contains 64 bits. Whether or
not all of these bits are stored in memory is programmable.
Zoom and Decimation Filtering
Local
Oscillator
Input
23
from
/
ADC
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26
/
Fs/4
Decimate
21
/
DATA OUTPUT SELECTION AND MULTIPLEXING
90
Phase Shift
26
/
Fs/4
Decimate
2x
2x
Fs/8
Decimate
2x
2x
Fs/8
Decimate
E1437A User's Guide
Module Description
26
FS/2
32
/
Real
32
Imag
/
26
FS/2
6-7
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