The Fluke PM6681R Rubidium Universal Counter is a precision frequency measurement and time interval analysis instrument built around an ultra-stable rubidium atomic clock reference. It delivers 11-digit resolution (1 µHz) across a standard 300 MHz range, expandable to 4.5 GHz with optional prescalers, making it suitable for metrology, telecommunications, and laboratory calibration work. The instrument combines exceptional time-base accuracy (±5 × 10⁻⁹) with dual-channel inputs, single-shot time interval resolution to 50 ps, and high-speed measurement capability of up to 40k readings/second in period mode.
## Technical Specifications
**Frequency Measurement**
• Standard range: 300 MHz
• Optional range: Up to 4.5 GHz (PM9625B), 2.7 GHz (PM9624)
• Minimum frequency: 10 × 10⁻¹⁰ Hz
• Resolution: 11 digits/second or 1 µHz
• Internal memory rate: Up to 8k readings/s
• GPIB rate: Up to 250 readings/s
• Period measurements: Up to 40k readings/s (back-to-back)
**Time Interval**
• Single-shot resolution: 50 ps
• Averaged resolution: 1 ps
• Time delay range: 3.3 ns to 1010 s
• Trigger hold-off range: 60 ns to 1.34 s (10 ns resolution)
**Input Characteristics**
• Impedance: 1 MΩ // 30 pF or 50 Ω (VSWR ≤2:1)
• Sensitivity: 20 mV rms 200 mV rms
• Reference output: 5× 10 MHz & 1× 5 MHz >0.5 V rms into 50 Ω
**Physical**
• Dimensions: 315 mm (W) × 86 mm (H) × 395 mm (D)
• Weight: 4.8 kg net; 7.8 kg shipping
• Power: 100–240 V AC, 50/60 Hz
## Key Features
• Two independent measurement channels with arming input support
• GPIB interface and SCPI command compatibility for remote automation
• Vertical resolution: 1.25 mV
• Optional reference output buffer (PM9671B) with six buffered 1 V rms outputs
• Prescaler options extend capability to 4.2 GHz (PM9625B)
## Typical Applications
Precision frequency calibration, time interval measurement, phase noise analysis, atomic clock characterization, and telecommunications signal validation.
## Compatibility & Integration
GPIB interface included. Supports external 10 MHz time-base input. Optional rackmount kit (PM9622) and high-stability oven time-base (PM9691) available.
