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Tektronix 3L5
50 Hz - 1 MHz Spectrum Analyzer Plug-In
Tektronix 3L5 Spectrum Analyzer

Compatible with 560-series scopes

Produced from 1967 to (?) manuals=



(All manuals in PDF format unless noted otherwise)
Manuals – Specifications – Links – Pictures

The Tektronix 3L5 is a 50 Hz − 1 MHz spectrum analyzer plug-in for 560-series scopes. It is very similar to the 1L5.

In conjunction with the 015-0107-00 Swept Frequency Converter (tracking generator), the 3L5 can also be used to make scalar frequency response measurements.

Key Specifications

Center Frequency 50 Hz to 990 kHz
Dispersion 10 Hz to 100 kHz
Input Impedance 1 MΩ // 30 pF
Input Voltage Max. 300 V (DC + peak AC)
Vertical Resolution Video: 100 mV/div to 100 V/div, LIN: 1 mV to 2 VRMS
Internal Cal Signal 5 kHz ±1%
First LO Out ≥1 MHz (Sweep), ≥1 Vp-p into ≥1 kΩ
Recorder Out 5 mV to 15 mV into 600 Ω
Weight 2.7 kg / 6 lbs

A video filter switch was added to the later SNs (1971 Catalog).


The 3L5 is a full semiconductor design except for the final output amplifier, which uses a 8416 vacuum tube. The circuit is based on the swept-frontend design, i.e. the first local oscillator is swept for the spectrum scan.

75 Ω Version

There is also a 75 Ω version of the 3L5. Its input is on the rear, via a coaxial connector. A lever on the front panel switches between two frequency ranges:

  • 10 kHz − 1 MHz
  • 18.010 MHz − 19 MHz

Timebase Support

The 3L5 requires a timebase plug-in in the horizontal compartment to operate.

Timebase plug-in units with serial numbers listed below require modification 040-0413-00 to provide the sweep signal to the 3L5 (To pin 18 of the plug-in connector).

Time Base SN#
2B67 <15180
3B1 <4040
3B3 <4270
3B4 <740


1 MΩ version

75 Ω version


Some Parts Used in the 3L5

Part Part Number(s) Class Description Used in
8416 154-0413-00 157-0116-00 Vacuum Tube (Dual Triode) dual triode 1S1 2A61 3A8 3L5 W