Agilent / Keysight 85685A RF preselector is designed to operate with the Keysight 8566B

The Keysight 85685A RF preselector is a companion module for the 8566B and 8568B spectrum analyzers that enhances measurement sensitivity and dynamic range through integrated tracking filters and preamplifiers. It effectively filters out-of-band signals and mitigates image responses, enabling accurate signal analysis in complex RF environments. The unIt has 20 dB nominal preamplifier gain and extends measurement range by 30 dB when analyzing signals in the presence of high-level interference.

The 85685A is a discontinued product.

Technical Specifications

• Input 1: 20 Hz to 50 MHz


• Input 2: 20 MHz to 2 GHz


• Bypass Mode: DC to 18 GHz (nominal) Performance:


• Preamplifier Gain: 20 dB (nominal) at 0 dB RFP attenuation


• RFP Attenuation: 0 to 50 dB in 10 dB steps (nominal)


• Stopband Attenuation: >40 dB (nominal) Filter Bandwidth (3 dB):


• Fixed: 20 MHz (nominal) for 500 MHz to 2000 MHz input


• Tracking: 1 MHz (nominal) for input <2 MHz; 10% of tuned frequency for 2 MHz to 55 MHz; 5% of tuned frequency for 55 MHz to 500 MHz Input Power Handling:


• Average Power: +30 dBm (1 Watt) with ≥10 dB RFP attenuation


• Peak Power: 100 Watts peak (10 µs pulse) with ≥10 dB RFP attenuation; 2,000 Watts peak (10 µs pulse) with ≥20 dB RFP attenuation


• Impulsive Signal Tolerance: Input 1 withstands LISN transients; Input 2 withstands CISPR calibration pulses Amplitude Accuracy (with 8566B/8568B and 85650A QPA):


• ±2.0 dB at 10 dB spectrum analyzer attenuation (+10° to +30°C, 0–20 dB RFP attenuation)


• ±2.5 dB at 0 dB spectrum analyzer attenuation (+10° to +30°C, 0–20 dB RFP attenuation) Standing Wave Ratio: <1.5:1 across all attenuation modes

Key Features

• Fully programmable remote operation via HP-IB (IEEE-488)


• Dual-input architecture optimized for low-frequency and microwave bands


• Bypass mode maintains DC to 18 GHz access when preselector filtering is not required


• DC voltage tolerance: 0 Volts

Physical & Electrical

• Dimensions: 133 mm H × 425 mm W × 508 mm D


• Weight: 19 kg (net)


• Power: 100/120/220/240 V RMS (±5%, −10%), 50–400 Hz, 35 VA (nominal)

Compatibility & Integration Designed for Keysight 8566B and 8568B spectrum analyzers. 8566B units with firmware ≥5.5.85 are compatible without modification; earlier firmware requires 85685A Option 066. Compatible with 8566A across all firmware versions.