7L12: Difference between revisions

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.

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 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. This 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.

File:Tek-7l12-led-front.jpg    | 7L12, later version with LED reference level display, front

File:Tek-7l12-left.jpg          | Left side

File:Tek-7l12-top.jpg          | Top view

File:Tek-7l12-right.jpg        | Right side

File:Tek-7l12-bottom.jpg        | Bottom view

File:Tek-7l12-bottom-front.jpg  | Bottom front

File:Tek-7l12-bottom-center.jpg | Bottom center. The 3-stage 2^nd IF 105 MHz helical filter is visible in the middle (black cylinders).

File:Tek-7l12-top-front.jpg    | Top front

File:Tek-7l12-center-1.jpg      | Tektronix 7L12, center board extended

File:Tek-7l12-center-2.jpg      | Tektronix 7L12, center board extended, back side. 2nd IF (105 MHz) helical filter seen near bottom.

File:7l12-105m-helix.jpg        | 7L12 105 MHz Helix filter.  From Tekscope, Jan 1972

File:Two 7L12s.jpg              | later and early model side by side

File:tek-7l12.jpg              | early model