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42
Control section
In normal mode (SWEEP OFF) the input of the amplifier 401.2 is set to ground; so the start potmeter
is not active. As the output of the amplifier is U = 2 Uref-Usweep, where Uref = +5 V, +10 V are applied
to the potmeter 648 for frequency setting. The voltages at the wipers are converted to currents via
resistors 651, 652 and are fed to the summing amplifier 404. The output signal is converted into control
voltages by current determining resistors 656 to 659, 662 and 664 for the 4 different frequency ranges.
For the lowest range of the main oscillator the resistor is reduced
by the factor 10 and the capacitors
in the quadruple switch are not increased; so the size of the capacitors is limited.
Possibly scattered-in spikes via leads of the control section are suppressed via capacitor 531.
In SWEEP mode and stand-by operation the output of +10 V of the integrator of the sweep oscillator
is connected to inverting amplifier 401/2 and to the start potentiometer 646. So O V is applied to the
SWEEP OUTPUT
socket; the (stop) FREQUENCY potmeter 648 is inactive. By this independent setting
of the start and stop frequency for the sweep is achieved.
During the sweep the voltages at the two potmeters are decreasing and increasing in mutual direction
until at the end of the sweep frequency potmeter 648 only is active as in normal mode.
When DUTY CYCLE
is not pressed potmeter 673 is not active. Control voltage is
1: 1 passed via impe-
dance converter OP 405/transistor 312 to the negative current source, comprising amplifier 407 and
transistor 314 and to the auxiliary current source 406. This negative current source is the reference for
the positive current source the control of which is achieved via the symmetry control. The auxiliary
current source 406 generates a current through resistor 667, which is fed to resistor 654. The resulting
voltage generates the basic current for the positive current source, comprising amplifier 408 and tran-
sistor 313.
The symmetry contro! 315 to 318 was introduced to equalize for any inaccuracies caused by distortion
and unbalance of the positive and negative current source and of the 4 transistors of the integrator.
On the virtual earth side of the selected charging capacitor, the capacitor 515 will be charged by any
unbalance, because it is inserted between charging capacitor and earth. This unbalance provides a control
voltage for FET 318. The differential stage operates as a voltage/current converter, the output either
putting current into the current source or taking out of it depending on the state of the positive current
control. By this means, and by transistors with high current gain (i.e. negligible leakage current) in the
quadruple switch circuit, any unbalance of the two loading currents is equalized, so reducing time sym-
metry errors of the signal.
When the DUTY CYCLE button is pressed the ratio of the two charging currents can be varied by means
of the potentiometer DUTY
CYCLE. So the duty cycle of the signal generated by the main oscillator
is varied, At amplifier 405 divider 668/669/671
now is connected to the negative reference potential.
So only a portion of the output voltage of the amplifier is fed back resulting in increased gain of 5.
Furthermore
resistors 672, 674 are switched to the negative reference potential, so that the DUTY
CYCLE potmeter is active.
Variation of the control voltage at potentiometer 673 results in load current variation from 0.2 to 5;
so the duty cycle is adjustable between 10 % and 90 %.
The period time in duty cycle mode is adjusted by 669 to the same value as in normal mode. Symme-
tric arrangement of the duty cycle circuitry guarantees equal period time of the main oscillator, when
the DUTY CYCLE potmeter is turned.
Furthermore in duty cycle mode the symmetry control has to be switched off which is done by connec-
ting the virtual zero of the quadruple switch to ground and by separating the output of the symmetry
control from the positive current source.
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