Astronics / EADS / Racal 2251 VXI Counter Timer

The Astronics / EADS / Racal 2251 is a single-slot, C-size VXI counter timer module engineered for precision measurement applications requiring extensive arming and triggering control. Installed in a VXIbus-compatible mainframe and controlled remotely over VXIbus, it delivers frequency, period, time interval, pulse width, rise/fall time, frequency ratio, phase, and totalizing measurements across multiple input channels with nanosecond-level resolution. ## Technical Specifications Form Factor & Control • Single-slot, C-size VXI module • VXIbus System Specification Revision 1.4 compatible • Remote VXIbus control only; no local control • NSN: 6625-01-371-8269 Frequency Measurement • Channel A: DC to 200 MHz • Channel B: DC to 160 MHz • Channel C (Option 41): 40 MHz to 1.3 GHz • Resolution: Up to 9 digits per second at 1 GHz; 10 nHz at 10 Hz in one second Time & Period Measurement • Period: 5 nS (Channel A), 6.25 nS (Channel B) • Time Interval: −2 ns to 1 × 10⁹ sec; 1 ns (single-shot), 100 ps (average mode) • Pulse Width: 5 ns to 20 ms Input Conditioning • Sensitivity: <100 MHz: 25 mVrms, 75 mVp-p • Input Voltage Range: ±5.1 V or ±42 V • Trigger Resolution: 2.5 mV (manual mode) • Low Pass Filters: 50 kHz per channel Measurement Functions • Frequency Ratio: Channel A/B; Channel C/B (Option 41, to 1.3 GHz) • Phase Measurement: Phase A relative to B; 0.10 to 360 degrees • Totalizing: Total A by B; manual totalize modes • Rise/Fall Time: Programmable trigger level −5.1 V to +5.1 V • Check Mode operation Accuracy & Performance • Accuracy: ± Resolution ± Timebase Error × Frequency • VXIbus CLK10: 100 ppm typ. ## Typical Applications Frequency and period measurement; time interval and pulse width characterization; rise/fall time analysis; phase relationship determination; event counting and frequency ratio computation. ## Compatibility & Integration Operates within VXIbus mainframe environments. No standalone operation.