Atec Tektronix-1502 User Manual
Page 28
TM 9-4935-601-14-3&P
DETAILED CIRCUIT DESCRIPTION
Pulser
The Pulser consists of three circuits: a Clock Generator, a Logic Programmer, and a Pulse Generator.
These circuits provide a signal for the fast ramp generator called the FAST RAMP START, and provide timing
for firing the tunnel diode, CR1703. A complete programming cycle consists of six equally divided time slots
derived from the 60 kHz Clock Generator output. The Logic Programmer develops the timing pulses from the
Clock Generator output to control bias level timing on the tunnel diode and the duration of the Fast Ramp
Generator.
The Pulser uses a two step process to generate a test pulse. Two steps are used to automatically bias
the tunnel diode. The first step checks the peak current point of the tunnel diode then stores the firing voltage
level on Memory Capacitor C1596. The second step uses the voltage stored on C1596 and an additional pulse
(TD TRIGGER) to fire the tunnel diode and transmit the test pulse to the CABLE connector. This peak current
check is performed for each Programming cycle. Thus, the automatic bias corrects for tunnel diode aging,
thermal effects, and loading variations. Biasing action on the tunnel diode during each time slot is as shown in
Fig. 3-3 and described as follows:
Time Slot 1. All current is removed from the tunnel diode to reset it to its low state. The low state is on
Time Slot 2. Bias current is applied to the tunnel diode so that the current is raised to just below the
minimum peak current point (depending on the tolerance of the tunnel diode characteristics).
Time Slot 3. A current ramp is added to the bias current until the tunnel diode fires. After firing, the
tunnel diode current is reduced to a predetermined amount where it remains for the rest of time slot 3.
Time Slot 4. All current is removed to reset the tunnel diode to the low state. This completes the first
pulse and the firing level of the tunnel diode is now just above the voltage stored in capacitor C1596.
Time Slot 5. This is a waiting period where currents are allowed to stabilize.
Time Slot 6. The tunnel diode is fired by TD TRIGGER from the Pulse Comparator. The fast ramp
makes its sweep and the Sampler takes its sample during this time.
Clock Generator. The Clock Generator consists of two inverters, U1575A and U1575B, which act as an
astable multivibrator. The output signal is a square wave about 60 kHz, as determined by C1572 and R1574.
R1577 protects the input to U1575B. The square wave is applied to a divide-by-six counter in the Logic
Programmer.
Logic Programmer. The Logic Programmer provides the needed logic sequence to allow automatic
tunnel diode biasing and sampler synchronization. It consists of a divide-by-six counter, U1569, and gates
U1575C, U1575D, and U1671. A cycle is completed after every six clock inputs (Fig. 3-3). Q1 starts going
high at the start of the clock pulse train and remains high for three clock pulses, then goes low for the next
three clock pulses.
The outputs (Q1 through Q5) of the counter go to gates U1575C, U1575D, and U1671. The output of
U1575C is called FAST RAMP START and controls the duration of the fast ramp generator. It is in the low
state for the first five clock cycles and in the high state for the sixth clock cycle (see Fig. 3-3b).
The output of U1575D, MEMORY RESET, is used to reset the biasing level of tunnel diode CR1703. By
turning Q1597 on, the charging current to C1596 is shunted to ground, and any voltage on this capacitor is
removed. This occurs during the first 2 time slots (see Fig. 3-3c).
The output of gate U1671D resets the tunnel diode during time slots 1 and 4, accomplished by diverting
the current for the tunnel diode to ground through Q1602. This causes the tunnel diode to turn off. The reset
action occurs each time after the tunnel diode is fired (shown in Fig. 3-3d).
The output of U1671C is used to enable a comparator, Q1688 and Q1695, which senses when tunnel
diode CR1703 goes into the high state during time slot 3 (see Fig. 3-3e).
Pulse Generator. The Pulse Generator consists of the high speed, 20 milliamp tunnel diode, CR1703,
and the tunnel diode bias control circuits. It generates the test output pulses to the CABLE connector.
The voltage pulse across CR1703 is 400 mV with a risetime of about 50 ps. The low impedance of
CR1703, combined with R1701, form a source impedance of 50
Ω
. This circuit provides a 200 mV step pulse
to the CABLE connector.
REV A FEB 1980
3-3