Agilent Technologies E1439 manual Sampling ADC, Zoom and Decimation Filtering

Module Description

Block diagram and description

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.

Input 12 from /

ADC

	16	Fs/4	2x	Fs/8	2x
	/					Fs/2	19
			Decimate		Decimate
Local	16
Oscillator	/
							24	Real
			DATA OUTPUT SELECTION AND MULTIPLEXING				/
							24	Imag
							/
	90 deg
	Phase
	Shift
	16	Fs/4	2x	Fs/8	2x	Fs/2	19
	/
			Decimate		Decimate

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.

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