7L12

The Tektronix 7L12 is a 1.8 GHz Spectrum Analyzer plug-in for 7000-series scopes. It is a double-width plugin to be installed in the right vertical and left (or only) horizontal bay.

Early models had a mechanical reference level readout. Later, front panel outputs for 1st and 2nd LO were added to allow connecting a TR501 or TR502 tracking generator. Late models had LED reference level readout.

The 7L12 does not have the UNCAL indicator typical for spectrum analyzers that alerts the operator when the sweep speed is too fast for the given resolution. The manual advises to use the variable control to reduce sweep speed until the signal amplitudes don't change.

The Tek catalogs suggested the 7603 Opt.06 or 7613 Opt.06 as suitable mainframes.

Key Specifications

Input frequency	100 kHz – 1.8 GHz (Opt.39: 2.5 GHz)
Span	500 Hz/Div – 100 MHz/Div
Sweep	1 μs/Div to 5 ms/Div (1-2-5)
Resolution bandwidth	300 Hz – 3 MHz in decade steps
Sensitivity	-115 dBm at 300 Hz
Vertical scale	10 dB/Div, 2 dB/Div or linear
Reference level	-100 dBm – +30 dBm
Video filters	30 kHz, 300 Hz or 3 Hz
Features	Automatic phase lock Built-in calibrator Base line clipper

Internals

The 7L12 is a traditional triple-mixing spectrum analyzer design. The input signal runs through a step attenuator (0 to 60 dB in 10 dB steps), a 1.8 GHz low-pass filter and a 3 dB pad (to isolate the mixer from LPF reflections) to a 1^st mixer. The 1^st LO is a YIG oscillator running at 2.095 GHz to 3.9 GHz, resulting in a 2.095 GHz 1^st IF.

The 1^st IF passes through a "traveling wave damper" (isolator), a 2.2 GHz LPF and a 10 MHz wide 2.095 GHz bandpass filter before reaching the 2^nd mixer. The 2^nd LO oscillates at 2.2 GHz (variable by ±1 MHz) for a 2^nd IF of 105 MHz, which is amplified, then filtered in a 3-stage helical filter with 3 MHz bandwidth before passing through a 3^rd mixer to the final 10 MHz IF, variable resolution filters, and post-resolution amplifier to the "Function IF amplifier" that implements the linear or logarithmic detector.

A phase lock circuit controls the 1^st LO at spans of 0.1 MHz/Div and less, tying its frequency to a multiple of 2.21 MHz using a diode sampler employed as a mixer. The sampling gate is driven by an avalanche pulser. In phase lock, the 2^nd LO is swept instead of the 1^st. The fine frequency control is only operational in phase lock mode, one of the key limitations of this design.

The 7L12 includes a standard sweep circuit built around the 155-0056-00 Sweep Control (U1140) and 155-0042-02 (U1160) Miller Integrator custom ICs. The 7L12 also uses the TEK-custom quad opamp 155-0035-00 in several places.

Prices

Year	1971	1980	1984	1988
Catalog price	$4,850	$6,500	$10,250	$12,500
2020 Dollars	$31,000	$20,400	$25,500	$27,350

Mods

• Tektronix 7L12 Mod 139U Increases Bandwidth to 2.5 GHz (PDF)

Links

• Tekscope Vol. 4 No. 1, Jan 1972
• Tek 7L12 @ amplifier.cd
• Tek 7L12 @ barrytech.com
• 7L12 videos: #1 / #2 / #3 / #4 #5

Pictures

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  7L12, later version with LED reference level display, front

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  Left side

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  Top view

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  Right side

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  Right side toward front. The 1^st mixer (bottom left) and 1^st LO (large cylindrical can) are visible.

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  Bottom view

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  Bottom front

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  Bottom center. The 3-stage 2^nd IF 105 MHz helical filter is visible in the middle (black cylinders).

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  Top front

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  Tektronix 7L12, center board extended

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  Tektronix 7L12, center board extended, back side. 2nd IF (105 MHz) helical filter seen near bottom.

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  7L12 105 MHz Helix filter. From Tekscope, Jan 1972

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  later and early model side by side

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  early model

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