Block Diagram - Tektronix 284 Instruction Manual

Type 284
SECTION 3
CIRCUIT DESCRIPTION
Change information, if any, affecting this section will be found at the
rear of the manual.
General
The Type 284 Pulse Generator circuits provide three types
of output signals. A fast pulse, with risetime and transient
response specified; a square wave with amplitude and pulse
period specified; and a sine wave with the period specified.
Each signal source uses a common trigger signal output ter-
minal for externally triggering the oscilloscope time base
generator.
BLOCK DIAGRAM
The Block Diagram is located with the schematics at the
back of this manual. It includes all circuits except the power
supply, with circuit names the same as identified on the sche-
matic diagrams. Each active component, connector, test
point and control is identified. The instrument is divided into
two major sections, with the MODE switch used to select
which circuits operate. The block diagram is drawn, the
MODE switch applies power to the Square Wave or Sine
Wave circuits, and the PERIOD switch selects the 10 us out-
put signal.
Square Wave or Sine Wave Blocks
The Low Frequency Oscillators QI, Q10 and Q20 are
individually turned on by the PERIOD switch. All three out-
put circuits remain connected to the Emitter Follower (buffer)
Q30. Q30 drives the Squarer block, that delivers both the
amplitude controlled square waves to the Attenuator and
the trigger signal to the Trigger Isolation block. The square
wave amplitude is adjusted by the Amplitude control through
the Emitter Follower Q50.
The High Frequency Oscillators, Q60 and Q70 are also
individually turned on by the PERIOD switch. Their output
circuits include a resistive matrix which allows each oscillator
output to remain connected to the Trigger Isolation and Sine
Wave Amplifier circuits at all times. The Trigger Isolation
block, and the pulser Trigger Generator block both drive
the front panel TRIGGER OUTPUT connector.
The Sine Wave Amplifier provides isolation between the
two High Frequency Oscillators and any changes in load at
the front panel SQUARE WAVE OR SINE WAVE OUTPUT
connector.
Pulser Blocks
The Tunnel Diode Pulser circuits are energized when the
MODE switch is placed toward the PULSE OUTPUT con-
nector. The 50kHz repetition rate is set by the Oscillator
& Multi block, which shapes tse oscillator waveform to
properly drive the TD Bias Driver circuits. Two halves of
®
the TD Bias Driver provide both the proper arming bias to
the TD Pulser, and a drive pulse to the Trigger Generator.
Once, during each half cycle of the oscillator signal, the
TD Bias Driver arms the TD Pulser, then the TD Tripper trig-
gers the pulser to deliver a signal to the PULSE OUTPUT
connector. About 1 us later, the TD Bias Driver removes
the arming bias from the TD Pulser and returns the pulse
output to zero.
The Trigger Generator drives both the TD Tripper block
and the front panel TRIGGER OUTPUT connector. The LEAD
TIME switch allows the operator to additionally delay the
TD Pulser firing time by inserting a signal delay between the
Trigger Generator and the TD Tripper block.
The TD Tripper applies a fast trigger pulse to the TD
Pulser to drive the TD, which then delivers a fast step pulse
to the PULSE OUTPUT connector.
PERIOD GENERATORS AND SWITCHING
See the schematic diagrams for waveforms that relate to
the following descriptions.
Low Frequency Oscillators
Two of the three Low Frequency Oscillators, Q1 and Q10
are Colpitts oscillators, operating at a 10 ys period (100
kHz) and a 1 ps period (1 MHz) respectively. The third oscil-
lator, Q20, is a crystal controlled Pierce oscillator, operating
at a 100ns period (10 MHz). Each oscillator tank circuit
operates at DC ground potential, allowing all three oscilla-
tors to drive a common resistance matrix and emitter follow-
er buffer stage.
Circuits of the 10 us and 1 ys oscillators are identical
except for component values. The basic oscillator circuit
is redrawn in Fig. 3-1, showing how the power supply by-
pass capacitor C38 connects the transistor collector to the
ground end of the parallel resonant tank circuit. Fig. 3-1
shows that the transistor collector and base signal voltages
are caused by the resonant circuit to be 180° different.
The 1000 current limiting resistor is left out of Fig. 3-1
to emphasize that it plays no part in the oscillator action.
(Rl and R11 serve only to limit +-20-Volt power supply cur-
rent in case the transistor develops a base-to-collector short
circuit.) Energy to keep the tank circuit RF current as a
steady RMS value is supplied through R4 (and R14) to the
half-voltage point between the two equal tank capacitors.
Transistor current flows approximately one half of each
cycle. As Q1 conducts, positive feedback current (of about
15mA) is supplied to the resonant circuit through R4.
While Q1 current is cut off, the emitter return resitsor (R5)
applies about 6.3mA of negative current to the resonant
3-1