Manual - 8500A Series Peak Power Meter - Giga-tronics

Theory of Operation
U11 interfaces with the CPU, the data bus, chip select lines, read/write lines, the A1 address line, and the
clock input line. The output of U11 goes directly to the display. U1 is a 69K RAM to store what is being
shown on the display. This means that there is a separate data address and data bus between U11 and the
U1 memory. U2 (the display and keyboard interface) has logic for the selection of the read and write
functions. It has a number of scan lines (outputs) designated SL0, SL1, SL2, and SL3 that go to the
keyboard. Connection is made between one of the lines and one of the return lines (RL) to tell the
circuitry which key was pressed. The scan lines are also connected to the peripheral drive circuit so that,
together with the output lines A0 through B3 (connected to U7), the control of which indicator lamp is to
be turned on is accomplished.
Refer to Sheet 2 of schematic diagram #20196. The circuit receives its input from the stepper motor that
interfaces with the spin knob on the front panel. The output is a signal that corresponds to 16 different
keys in the keyboard interface circuitry. Each one is equal to a speed at which the stepper motor is being
turned. Also, there are two other lines on the interface IC (U17B) that sense that the spin knob is in use
and what direction it is being turned (U12B through RL0 through 7, and SL0 & 1). This signal is input to
U14A & B.
The stepper motor creates two sinusoidal signals that are 90 degrees out of phase with each other. The
voltages of these signals are proportional to the speed of rotation of the spin knob. That is, the higher the
frequency (speed of rotation), the higher the voltage. U14A & B are low pass filters with the cut-off
frequency determined by the slowest speed the spin knob can be turned (a small fraction of a Hertz). As
the frequency increases, the level builds until it reaches a constant amplitude. The amplitude is input to
timers U8A & B and used so that if the input level is above 2/3 of the level of the supply voltage, then
the output will be high. If the input goes below 1/3 of the supply voltage level, it will go to zero. The
function is similar to a Schmitt trigger with levels of 1/3 and 2/3 of the supply voltage.
U8A & B output a train of square waves which are also 90 degrees out of phase with each other. The
square waves go through RC networks, and then give four inputs to U10. Each one of the inputs
represents an up or down-going edge of one of the U8A & B square waves. The output is a pulse train of
short pulses that indicate when the spin knob is being moved. U21A receives the pulse train, and U17A,
B, C & D decode the direction of movement. If the knob is turned in one direction, U21B will have the
same pulse train. When the knob is turned in the other direction, the pulse train will not be present.
Flip flop U18A is a gate controlled by the data input signal. From U12A the signal goes to monostable
oscillator U17A so that when the knob is first turned an approximate 100 ms pulse is generated (read at
TP6). While this pulse is high, the U16 4-bit counter counts the number of pulses from the spin knob.
The more pulses that are counted, the faster the rotation of the knob. When the count reaches 15, a carry
out is generated and U9B stops the counter from starting back at zero. Thus the maximum count of 15 is
preserved, indicating the fastest spin knob rotation. When the U17A 100 ms timer ends, the U17B
monostable outputs a SHIFT signal for approximately 20 ms. This tells the U2 keyboard decoder that the
spin knob was moved. The four bit count from U16 is coded onto RL0 - RL7 by the U5 demultiplexer
and part of PAL U10. PAL U10 gates the scan lines SL0 and SL1 with the highest order bit from U16 to
control U5. If the count is 0 - 7, the three lower order bits from the counter are coded onto R0 - R7 by
U5 when SL0 goes high. If the count is 8 - 15, the three lower order bits are coded out when SL1 goes
high. The state of the CNTRL signal from flip-flop U18B indicates whether the spin knob was turning
clockwise or counterclockwise.
Refer to sheet 3 of schematic diagram #20196. This is the circuitry which supplies power to the display.
Linear regulator U22 regulates the +12 V down from the unregulated +20 V. The higher voltages are
stepped up from the unregulated +20 V by a switching fly-back power supply. U19 is a current mode
switching regulator IC which runs the supply. U19 controls the output voltages by varying the duty cycle
of FET Q1, the power switch. The current through Q1 is limited by U19-3 which monitors the voltage
across sense resistor R44. The rise time of the flyback voltage is controlled by a damping network
composed of C60, R32, and R41. The frequency of U19 operation is fixed at about 100 kHz by timing
components R37 and C53.
Manual No. 20790, Rev C, November 1998 4-25
Superceded by Revision D, March 2009