491

Manuals – Specifications – Links – Pictures

The Tektronix 491 is a spectrum analyzer with a frequency range of 10 MHz to 12.4 GHz (up to 40 GHz with external waveguide mixers). The 491 was introduced in August 1966. Due to severe performance limitations − lack of input attenuator, display dynamic range of only 40 dB, minimum resolution bandwidth 1 kHz, ambiguous display due to low IF − the 491 was replaced by the completely redesigned 492 in 1980.

Key Specifications

Frequency	Coaxial input bands: 10 MHz to 12.4 GHz (Band A-1 10 MHz to 275 MHz; Bands B-2 & 3: 275 MHz to 2 GHz; Bands C-4, 5, & 6: 1.5 GHz to 12.4 GHz) External waveguide mixers: 12.4 MHz to 40 GHz (Band 7 (Ku) 12.4 GHz to 18 GHz; Band 8 (K) 18 GHz to 26.5 GHz, (Ka) 18 GHz to 40 GHz)
Dispersion	200 kHz/div to 10 MHz/div (1−2−5) and 0 Hz
Resolution Bandwidth	1 kHz to 100 kHz (6 dB)
Input	50 Ω, max. −30 dBm for linear operation, +15 dBm (25 mW) diode mixer power limit
IF Attenuator	0 dB to 51 dB, in combinations of 1, 2, 4, 8, 16, and 20 dB
Sweep Speed	0.5 s/div to 10 µs/div (1−2−5); Uncalibrated to approx 1.25 s/div
Triggering Modes	Internal, External, and Line; 100 V maximum input (DC + peak AC)
Dynamic Range	80 dB (Max sensitivity of −110 dBm minus Max input power of −30 dBm); display range ≥40 dB
Calibrator	200 MHz
Weight	13.6 kg / 30 lbs (Rackmount version 18 kg / 40 lbs)
Power	90−136 and 180−272 VAC in six ranges, 48−440 Hz. At 115 VAC, 60 Hz, 55 W

Connectors

Front

• RF INPUT (A) BNC connector (50 Ω, 10 MHz to 275 MHz)
• RF INPUT (B) N connector (50 Ω, 275 MHz to 2 GHz)
• WAVEGUIDE (C) − receptacle that accommodates either a coaxial mixer, 119-0096-00, with an N connector input for 1.5 to 12.4 GHz, or a 119-0104-00 waveguide mixer adapter connecting to external mixers for up to 40 GHz.
• MARKERS OUT/EXT REF FREQ IN BNC connector: 1 MHz marker output when INT REF FREQ set to ON, otherwise PLL reference frequency input (1−5 MHz, 1−5 V_p-p)

Rear

• TRIG IN BNC connector (20 Hz to 100 kHz, ≥0.2 V)
• SAW OUT BNC connector (75 mV/div)
• RECORDER BNC connector (≥4 mV/div of displayed signal in LIN mode, DC-coupled, into 600 Ω)

Options

• Opt. 01: Limit Frequency Range to 10 MHz to 2 GHz
• Opt. 02: Limit Frequency Range to 1.5 GHz to 40 GHz

Links

Documents referencing 491

Document	Class	Title	Authors	Year	Links
Service Scope 48 Feb 1968.pdf	Article	Spectrum Analyzer Techniques for Frequency Stability Measurements		1968	491 • 1L20 • 1L30
TekWeek (partial) October 10, 1975.pdf	Article	Spectrum analyzers require high technology	Thor Hallen • Dave Friedley	1975	Spectrum Analyzers • Pentrix • 7L5 • 5L4 • 7L12 • 7L13 • 1401 • 491 • 1405 • 1502 • 1503

Patents that may apply to 491

Prices

Year	Catalog price	In 2026 dollars
1967	$4,200	~$42,000
1979	$10,700	~$49,200

According to an internal memo, in 1979 annual sales were estimated at 172 units.

Pictures

• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 
• 

Components

Some Parts Used in the 491