The Astronics/EADS/Racal 2251A is a single-slot, C-size VXI counter timer delivering high-resolution timing and frequency measurements for VXIbus-compatible systems. Microprocessor-controlled and remotely operated via VXIbus, this instrument provides twelve automatic measurement functions with resolution down to 1 ns for time intervals and 10 nHz at 10 Hz. The 2251A synchronizes to the VXIbus CLK10 system clock, minimizing timebase error in multi-instrument test configurations.
## Technical Specifications
Frequency measurement spans DC to 200 MHz on Channel A and DC to 160 MHz on Channel B, with optional Channel C (Option 41) extending to 40 MHz–1.3 GHz. Resolution reaches 1 Hz at 1 GHz within one-second gate times and 10 nHz at 10 Hz. Period measurement begins at 5 ns (Channel A) and 6.25 ns (Channel B). Time interval measurement supports separate and common modes with ranges from 200 µs to 800 ms or 200 µs to 99.999 s; single-shot resolution is 1 ns, averaging mode achieves 100 ps.
Additional measurement functions include pulse width (5 ns to 20 ms), rise/fall time on Channel A, frequency ratio (A by B, or C/B with Option 41), phase measurement (0.10 to 360 degrees), totalize modes (manual, pulse, cycle, timed), peak signal amplitude, and DVM (MAX, MIN, DC voltages). Math mode supports [(Reading − X) × Y] / Z calculations.
## Key Features
• Input impedance programmable to ×1 or ×10 attenuation
• Voltage range ±5.1 V (standard); ±42 V for high-voltage measurement
• Sensitivity <100 MHz: 25 mVrms (5 ns pulse, ×1 attenuation)
• Channel C 50 Ω impedance for microwave applications
• VXIbus Specification Revision 1.4 compliance
• Accesses CLK10, Analog SUMBUS, and TTLTRG signal lines
## Typical Applications
Phase measurement, frequency ratio analysis, pulse characterization, time-interval precision measurement, and integrated VXI rack-based automated test systems.
## Compatibility & Integration
Single-slot C-size module; remote control only via VXIbus command set. No local front-panel control.
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