Fluke PM6685R / 776 Universal Frequency Counter / Calibrator. Rubidium Timebase, 4.5 GHz C Channel, GPIB

The Fluke PM6685R / 776 is a universal frequency counter and calibrator built around a rubidium atomic timebase for precision measurement and calibration work. It delivers 10 digits per second measurement speed across Input A (10 Hz to 300 MHz) and an extended C channel reaching 4.5 GHz with optional modules. The rubidium reference provides exceptional short-term stability of < 2.5 × 10⁻¹¹ (Root Allan Variance) and long-term aging of 1 × 10⁻⁹ per 10 years, locking within 5 minutes to operational specification.

## Technical Specifications

**Timebase & Reference**
• Rubidium atomic reference with 10 MHz buffered output
• Short-term stability: < 2.5 × 10⁻¹¹ (Root Allan Variance)
• Stability within 10 minutes: 4 × 10⁻¹⁰
• Aging rate: 1 × 10⁻⁹ per 10 years or 2 × 10⁻¹⁰ per year
• Warm-up to lock: approximately 5 minutes
• Warm-up to 5 × 10⁻⁹ of final value: 10 minutes

**Frequency Measurement**
• Input A range: 10 Hz to 300 MHz
• Input C range: 4.5 GHz (with applicable option)
• Burst frequency A range: 100 Hz to 80 MHz
• Measurement resolution: 10 digits per second
• Display capability: 11th and 12th digits on overflow
• Period, ratio (A/E, C/A), pulse width, duty factor, and event totalization functions

**Input Specifications**
• Input impedance: 1 MΩ // 25 pF or 50 Ω (VSWR < 2:1)
• Manual trigger sensitivity: 10 mV rms to 10 V rms (variable)
• Trigger slope: positive or negative selection
• Low pass filter: 100 kHz nominal 3 dB point
• Auto trigger with intelligent input optimization
• Input A damage level: 350 V (DC + AC peak) at 1 MΩ

**Display & Control**
• Analog bar graph indicator for input level or sensitivity
• Digit blanking function available
• Nulling capability for reference-based measurements

## Key Features

• Intelligent AUTO trigger mode eliminates manual setup iterations
• Rubidium timebase enables calibration of any application-specific frequency standard
• Extended C-channel frequency coverage to 4.5 GHz
• 10 digits per second measurement throughput

## Typical Applications

Frequency calibration and verification, precision frequency analysis, frequency standard characterization, automated test integration in calibration laboratories and metrology environments.

## Compatibility & Integration

IEEE-488 (GPIB / SCPI) optional interface with front-panel-readable, rear-panel-selectable GPIB address assignment.