Fluke PM6681R Rubidium Universal Counter

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.