491

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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, the 491 was replaced by the completely redesigned 492 in 1980.

Key Specifications

Frequency Range	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 Range	(plus 0 Hz) 200 kHz/div to 10 MHz/div in 1−2−5 sequence
Resolution Bandwidth	1 kHz to 100 kHz (6 dB)
Input impedance	50 Ω
Input Power	max. −30 dBm for linear operation, +15 dBm (25 mW) diode mixer power limit
IF Attenuator Range	0 dB to 51 dB, in combinations of 1, 2, 4, 8, 16, and 20 dB
Sweep Speed Range	0.5 s/div to 10 µs/div in 1, 2, 5 sequence; Uncalibrated to approx 1.25 s/div
Triggering Modes	Internals, 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 Frequency	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
Front panel inputs/outputs	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) N connector (50 Ω, 12.4 GHz 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 Vp-p)
Rear panel inputs/outputs	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

Prices

1979: $10,700 ($41,800 in 2022 Dollars)

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

Pictures

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