Philips PM 5167 Instruction Manual page 42

41
3.1.2.6. Impedance converter 3 consists of a complementary emitter follower U3/326 and 327 which decouples the
square-wave voltage at U3/649 attenuated to 2 V pp and causes an output impedance of 50
n.
3.1.2.7. The amplitude controi operates at frequencies higher than 3 kHz and compensates the rise of the triangular­
voltage ampl itude at R 1 for. The rise of the triangular·voltage
amplitud~
is due to the short switching intervals
of comparator 1 (ca. 20 ns), during which the tr iangular voltage keeps its momentary rise direction .
At 10M Hz the triangular voltage reaches a rise speed of lOV/50 ns, which wou Id cause an ampl itude increase
of 10 V .20 ns/50 ns
=
4 V.
The amplitude controi reduces the amplitude of the square-wave voltage at Rl by 4 V at 10 MHz, which results
in a linear amplitude response of the triangular voltage. The FET-pair U3/323 performs the high-ohmic
decoupling of the triangular voltage.
One of the two transistors is used for offset· and temperature compensation, the adjustment of which is
achieved by U3/681 11682. The high-pass filter U3/637, 690, 514 suppresses the controi effect below 3 kHz .
In the controller used as integrator (U3/311 to 313 and 336 to 338), the actual value of the triangular voltage
obtained by peak rectifying (diodes U3/403, 410) is compared with the nominal values at the base contacts
of transistors U3/313 and 338. The controller reduces the square-wave amplitude at R 1 by means of the
transistors (U3/309, 310 and 333, 335) used as adjusting elements. The adjustment of the nominal value is
carried out by select-on·test resistors U3/619, 663.
3.1.2.8. The hold-condition is obtained by means of transistors U3/321, 350, 322, 351.
When U3/16 is connected to U3/17 via switch run/hold (807), all transistors mentioned are conductive.
No current passe s then through the switching array. The connected capacitor C(U2/501 to 510 and 512)
remains charged, voltage remains available at connection point 13 and switching array 1 stops oscillating.
Transistors U3/304, 329 and 303,328 enables the single-shot operation to be performed. If -5 V is applied
to connection point U3/20 all transistors mentioned are conductive.
The switching array 2 no longer oscillates. U3/304 is connected in paralIei with U3/306 so that I R is applied
to U3/648 and 649, independent of the position of the switching array . I n the same way U3/329 is connected
in paralIei with U3/334. As a result of this uR becomes -5 V.
The triangular oscillation is held until the triangular voltage uC reaches the potential at R 1. This condition
is stable and the comparator is balanced .
If a voftage more positive than -5 V is applied to transistor U3/339 from potentiometer START LEVEL via
point 8/U3, this voltage determines the voltage at Rl (U3/659
+
660) .
The voltage at C becomes the same potential. In this way any start potential for the t riangular signal is possible .
3.1.3. Controi unit
U5
The controi unit supplies the charging currents for the oscillator.
The controi unit fig. 22 consists of:
- a current source for potentiometer FREQUENCY,
- two pairs voltage·current converters,
- four cu rrent mirrors,
- a symmetry-control stage and
- a single-shot logic .
3. 1.3 . 1. The current supplied by power source U5/311 determines the voltage at potentiometer FREQUENCY . By
means of potentiometer FREQ. OFFSET the emitter resistance U5/646 of power source 311 and, as a result
of this, the voltage at potentiometer FREQUENCY can be changed by ±5
%.
The frequency ranges 1 Hz to
1 M are adjustable via potentiometers U2/601 to 607 .
3.1.3.2. Voltage-current converters U5/351 and ·U5/352 are controlled by the voltage between input SWEEP IN and
potentiometer FREQUENCY. The emitters of transistors 301 and 305 are brought to the same potential
difference by feedback.
The frequency-determining resistor R
=
(U2/608 to 611 and 6141161611628) or (617 to 620) wh ich is selectable
with switch FREQ. MULTIPL., is located between these two emitters.
The last mentioned resistor series applies for the time-asymmetric operation, the switch-over being performed
by Ul/\. Resistor R determines the emitter currents and consequently the collector currents of transistors
U5/301 and U5/305, which correspond to the charging currents IC and IC* (fig . 7).