TTI LD300 Service Manual page 18

When STRT (from the slow start logic) is low, B3 and B0 are both low and IC7 selects 0V.
When SR ("select Remote" from the Control PCB) is low, B3 is low and B0 high and IC7 selects
REMV.
When SB ("select Level B" from the Control PCB) is low, B3 is high and B0 low and IC7 selects
LVLB.
When SA ("select Level A" from the Control PCB) is low, B3 and B0 are both high and IC7
selects LVLA.
When ST ("select Transient Mode" from the Control PCB) is low, B3 is high and B0 follows the
state of IOSC; when this is high IC7 selects LVLA and when low IC7 selects LVLB.
When all of the above signals are high (which occurs when External TTL mode is selected), B3 is
high and B0 follows the state of XOSC from the comparator IC20. When the remote input is below
1·5V, XOSC is high and IC7 selects LVLA; when it is above 1·5V, XOSC is low and IC7 selects
LVLB.
Slew Rate Generator
This circuit produces straight line transitions between any changes of level, rather than the
exponential transitions which would be produced by a CR network. It applies to all changes of
level, whether determined by the transient oscillator or any other cause. It operates by using a
controlled constant current to charge and discharge a capacitor to the required levels.
There are two sets of three slew rate ranges; one set is used in Current mode, the second
(slower) set is used in all other modes. IC4 selects the capacitor for each range and IC3 selects a
fraction of the control setting SLWV through a separate calibration adjustment for each range.
R38, Q1 and IC14-A convert this setting into a bias current for the OTA IC16. This compares the
selected level from IC7 with the value currently stored on the capacitor, and charges or
discharges the capacitor at a rate determined by its bias current, until the two are equal. The
accuracy of this match depends on the input offset voltage of IC16, which varies with bias current
(especially at low values), so the unit is calibrated with the slew rate control at its physical centre
(a position that the user can easily establish).
IC13 presents a high impedance load to the capacitor and drives DMD with a small gain, so the
scaling becomes 2392mV at the rated top of range (80 Amps etc.) – the subsequent calibration
potentiometers reduce this to match the exact scaling of IFB and VFB. Depending on operating
mode, DMD drives either the comparison amplifier or the function multiplier.
Function Multiplier
In the derived operating modes, the function multiplier is used to compute the required current (at
PROD) from the instantaneous terminal voltage (at VFB) and the present control level setting (at
DMD).
In constant power mode I = W÷V.
In constant conductance mode I = V*G
In constant resistance mode, I = V÷R.
In fact, in constant resistance mode, the voltage value used by the multiplier is actually VFB +
VXO. VXO is the negative of VROP, so the equation is effectively I = (VFB-VDROP) ÷ R. When
VFB is less than VDROP the multiplier simply cuts off with the PROD output at zero. The user
can set VDROP to zero, to obtain a simple resistor where I = VFB÷R.
The transistors in IC18, in conjunction with the associated op-amps IC24 and IC25, form a
standard log-antilog analogue multiplier-divider. IC25-B adjusts the base-emitter voltage of
transistor IC18-B until its collector current equals the current through R73 (the first multiplication
input). IC24-A does the same with IC18-C to match the sum of the currents in R78 & R80 (the
second multiplication input). The base-emitter voltage across the output transistor IC18-D is the
sum of these two voltages (which represent the logarithms of their input values), less the voltage
developed by IC25-A across IC18-A (which represents the logarithm of the division input current
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