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
  • 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 (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 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 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 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
  • 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 Vp-p)
Rear panel

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

Components

Some Parts Used in the 491

Part Part Number(s) Class Description Used in
155-0157-00 155-0157-00 Monolithic integrated circuit digital storage vertical control 7L5 7L14 7L18 491 492 492A 492BP 492PGM 494 494A 495 496 497P
155-0158-00 155-0158-00 Monolithic integrated circuit digital storage horizontal control 7L5 7L14 7L18 491 492 492A 492BP 492PGM 494 494A 495 496 497P
1N3714 152-0081-00 Discrete component 2.2 mA, 25 pF germanium tunnel diode 546 547 556 21A 22A 3B1 3B2 3B3 422 491 283
1N3717 152-0381-00 152-0125-00 Discrete component 4.7 mA, 25 pF tunnel diode 1L40 1S1 1S2 11B1 11B2 11B2A 147A 1470 148 21A 22A 3B4 3B5 408 432 434 453 453A 454 466 491 5T3 544 RM544 546 RM546 547 RM547 556 RM556 7B70 7B71 7D11
STD704 152-0125-00 Discrete component 4.7 mA tunnel diode 1L40 1S1 11B1 11B2 11B2A 147 R147 147A R147A 1470 148 R148 148-M 149 R149 149A R149A 21A 22A 3B4 3B5 408 432 434 453 453A 454 464 465 466 491 5T3 544 RM544 546 RM546 547 RM547 556 RM556 7B70 7B71 7D11
T4910 CRT CRT 491
TD714 152-0402-00 Discrete component 2.2 mA, 25 pF tunnel diode 1401 1401A 283 R283 3B2 3B3 422 491 546 RM546 547 RM547 556 RM556 SPG11 SPG12
Y1641 154-0506-00 154-0510-00 Vacuum Tube (Triode) SHF cermet triode 1L20 491