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.
