7904: Difference between revisions

The '''Tektronix 7904''' is a 500 MHz, non-storage 7000-series oscilloscope mainframe with four bays. It accepts two [[7000-series_plug-ins#Vertical plug-ins|7000-series vertical plug-ins]] and two [[7000-series_plug-ins#Horizontal plug-ins|7000-series horizontal plug-ins]]. It was introduced in late 1971/early 1972 (Note 1). The successor model 7904A was introduced in 1984. There is also a rack-mount version, the [[R7903]], albeit with only one horizontal bay.

Performance is largely determined by the plug-ins. The [[7A19]] vertical amplifier and [[7B92]] time base were introduced along with the 7904 to match the 500 MHz system bandwidth.   

The 7904 had an elaborate 5-decade built-in calibrator with variable rate that was dropped from the 7904A. In the 7904A, The Control Illumination switch moved to the rear panel, and the [[7000 series readout system|readout system]] gained a pulsed mode in which it is synchronized with the sweep instead of interrupting the display at random times. This pulsed mode is not compatible with all time base plug-ins (see manual p. 1-12).

The 7904 has a many modes of operation. One mode is called "Independent-Pairs Display" in the manual, where the input to the left vertical plug-in is displayed using the B horizontal time-base plug-in, and the input to the right vertical plug-in is displayed using the A horizontal time-base plug-in. This resembles the trace-slaving "ALT" mode that is produced by an [[1A4]] in a [[547]]. Many other modes are available. It is possible to put vertical plug-ins in the horizontal slots for X-Y operation. It is even possible to put a horizontal plug-in in one of the vertical slots to produce a raster scan.   

A special plug-in, the [[7A21N]], provides direct access to the deflection plates up to 1 GHz at an uncalibrated sensitivity of 4 V/Div. This plugin requires modifications to the mainframe to bypass the Y output amplifier (thus disabling readout and the other vertical plugin bay).

The 7904 uses a switch-mode power supply and has no fan. It incorporates a 65 ns Y delay line. This delay is too short for some older time bases such as [[7B53]] to display the trigger event.

[[Thor Hallen]] designed the main vertical amplifier and the [[7A19]] plugin (Note 1). It used bridged T coils and the "[[Transit frequency doubler|f_t doubler]]" circuit patented by [[Carl Battjes]] to achieve its bandwidth.

The 7904 makes extensive use of custom integrated circuits and hybrid circuits, inter alia, the [[155-0064-00]] 470-ps output amplifier, [[155-0022-00]] analog multiplexer for channel switching, [[155-0059-00]] HF amplifier, [[155-0095-01]] and [[155-0095-03]] differential HF amplifiers, and [[155-0067-02]] SMPS controller.   

The signal path is completely differential and the internal signal wiring is 50 Ω coax. The CRT ([[154-0644-05]]) has [[distributed vertical deflection plates]].

File:Tek-7904-front.jpg | 7904  (non-A) front

File:7904-left.jpg | Left side. Left top: Vertical amplifier; left bottom: switch-mode power supply; right center: delay line; right bottom: plug-in bays

File:7904-right.jpg | Right side.  Top left: calibrator; bottom left: plug-in bays; top center: [[7000 series readout system|readout board]];  top right: Z amplifier  

File:7904-right-front.jpg | Right side detail. Left: calibrator; right: readout.

File:7904 interface 1.jpg|Plug-in backplane board front (left trigger selector piggyback board removed)

File:Tek7904-trig-selector.jpg | B Trigger selector piggyback board (A6)

File:Tek7904-vertical-interface.jpg | Vertical interface piggyback board (A7).  The two [[155-0095-03]] amplifiers (large metal cans) are used to select the signal from one vertical plug-in bay, and drive the delay line.

File:7904 interface 2.jpg|Plug-in backplane board rear

File:Tek7904-backplane-bottom.jpg | Backplane board sandwich seen from below

File:7904-x-amp.jpg | Horizontal (X) amplifier board

File:7904-y-amp.jpg | Vertical (Y) amplifier board

File:7904-z-amp.jpg | Z amplifier board

File:7904-controls.jpg | front-panel controls

File:7904_block.gif|7904 block diagram

File:Tek7904-7a19-7b92a-1010mhz-hfsync.jpg | A 7B92A (B098xxx) displaying a 1.01 GHz sine in HF Sync mode ([[7904]] mainframe, [[7A19]] amplifier).

File:7904-psu-internal.jpg    | 7904 switchmode power supply, covers removed

File:7904-psu-right.jpg        | View from right (as seen from front of instrument)

File:7904-psu-primary.jpg      | Primary side circuit board

File:7904-psu-primary-caps.jpg | Primary side capacitors

File:7904-psu-transformer.jpg  | Detail view of transformer from the side

File:7904-psu-rectifier.jpg    | Secondary side rectifier board

File:7904-psu-regulator.jpg    | Secondary side post-regulator