AD700
VME 10 Bit 70 MSPS Digitizer Subsystem

The AD700 is a single 6U VME board that implements the front end signal conditioning and variable gain amplifiers in addition to a high speed analog to digital converter operating at a sample rate of 70 MHz.

The AD700 features an analog RF preselector to reject out of band interference that would otherwise consume dynamic range.

Two variable gain amplifier stages provide up to 80 db of gain control, allowing the board to acquire low level signals or signals that have been processed by other signal conditioning systems.

An onboard antialiasing filter restricts the input bandwidth to approximately 32 MHz.

The analog to digital converter has resolution of ten bits that are available as differential ECL signals on the front panel. The user has the option of selecting single ended or differential data and clock. The VME bus backplane is used only for control.

The AD700 is intended to be used with a wideband digital filter, digital tuner, or other high speed digital signal processor as part of a multiple board DSP system. Custom versions of the AD700 are available with different sample rates and filter cutoff frequencies. For system applications where external bandlimiting is provided, the onboard antialiasing filter can be bypassed. The user may also choose between AC and DC input signal coupling.

APPLICATIONS

• HF Communications Receivers
• Instrumentation Receivers
• Spread Spectrum Signal Processors
• Wideband Data Acquisition
• Signal Processor Front End

TECHNICAL DESCRIPTION

Analog HF signals are input through a front panel mounted BNC connector to a preamplifier that terminates the input in a 50 Ohm load and provides overload protection for subsequent stages.

The output of the preamplifier is processed by a two stage preselector with interspersed variable gain amplifiers. The preselector is configured as a set of second order lowpass filters followed by a similar set of highpass filters that allow the user to construct a set of fairly broad bandpass filters to control out-of-band interference that would otherwise diminish dynamic range.

The variable gain amplifiers provide up to 80 dB of gain variation to support high level signals from other systems or for direct connection to an antenna. The output of the second variable gain amplifier is processed by a 32 MHz sharp cutoff antialiasing filter.

A ten bit analog to digital converter is used to digitize the conditioned HF signal at a sample rate of 70 Megasamples per second.

All control parameters are software programmable using a separate VME control interface.

INPUT CHARACTERISTICS Impedance: 50 Ohms, Single Ended Coupling: DC or AC (100 Hz Cutoff) Sensitivity: 2 mV to 3 V p-p PRESELECTOR LOWPASS SELECTIONS 2 MHz 5 MHz 12 MHz Bypass PRESELECTOR HIGHPASS SELECTIONS                      3 MHz 8 MHz 18 MHz Bypass ANTIALIAS FILTER 32 MHz Equiripple Lowpass Bypass

A/D CONVERTER PERFORMANCE Resolution: 10 Bits Conversion Speed: 70 MSPS Analog Bandwidth: 175 MHz Harmonic Distortion: < 59 dBc Intermodulation Distortion: < 70 dBc Signal to Noise Ratio: > 55 dB OUTPUT CHARACTERISTICS Data: 10 bit Differential ECL, Two's Complement Clock: Differential ECL SIGNAL PROCESSING BANDWIDTH 35 MHz PHYSICAL CHARACTERISTICS Single Height 6U VME Form Factor

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