Agilent Technologies E1439 User Manual page 225

Sampling ADC
The heart of the E1439 is a precision analog-to-digital converter (ADC). The ADC generates 12
bit outputs at a sample rate up to 95 MHz. This raw unfiltered data can be output via the E1439D's
fiber optic interface.
Zoom and Decimation Filtering
This section uses digital circuitry to allow programmable changes in the center frequency and
signal bandwidth of the E1439 (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 95.0 MHz rate, the bandwidth choices are 40 MHz, 20 MHz, 10 MHz,...76
Hz around the programmed local-oscillator (LO) frequency.
Real and imaginary components of the signal are each computed to 24-bit precision, so the
complex output of the decimation filtering block contains 48 bits. Whether or not all of these bits
are stored in memory is programmable.
16
Local
/
Oscillator
Input
12
from
/
ADC
90 deg
Phase
Shift
Memory Controller and SDRAM Memory
The E1439 can be programmed to save the real component of the signal or to save the complete
complex signal. The data precision can be set to 12 bits or 24 bits. Thus, each sample occupies
from 1.5 to 6 bytes of memory in the SDRAM. The memory controller block packs the selected
data into 72-bit words, which are stored in the SDRAM memory. Since the standard SDRAM
depth is 2M × 72 bits, it is possible to hold up to 12-Msamples in memory at one time.
16
Fs/4
/
DATA OUTPUT SELECTION AND MULTIPLEXING
16
Fs/4
/
2x
Fs/8
Decimate
2x
Fs/8
Decimate
Module Description
Block diagram and description
2x
19
Fs/2
Decimate
24
Real
/
24
Imag
/
2x
Fs/2 19
Decimate
215